36.	(a)	Iodine reacts with hot concentrated sodium hydroxide solution according to the equation:

(i)	What is the oxidation number of iodine in
I2 .......................................................................................................................
NaI ....................................................................................................................
NaIO3 ...............................................................................................................
(2)
(ii)	What is the name given to this type of redox reaction?
..........................................................................................................................
(1)
(b)	An experiment was carried out to determine the purity of a sample of sodium iodate, NaIO3, made by the reaction described in (a).
	0.060 g of the sample was dissolved in pure water to make 100 cm3 of solution.
	A 10.0 cm3 portion of this solution was taken and added to an excess of acidified potassium iodide solution.  The iodine liberated was titrated with 0.0100 m sodium thiosulphate solution.
(i)	What piece of apparatus would you use to measure out the 10.0 cm3 sample?
..........................................................................................................................
(1)

(ii)	Name a suitable indicator to use for this titration and give the colour change expected.
Indicator ............................................................................................................
Colour change from .......................................... to ...........................................
(3)
(iii)	The volume of sodium thiosulphate solution required in the titration was 16.7 cm3.
	Calculate the number of moles of sodium thiosulphate used.

(1)
(iv)	The equation for the reaction between iodine, I2, and thiosulphate ions, , is

	Calculate the number of moles of iodine molecules, I2, which reacted with the sodium thiosulphate solution.

(1)
(iv)	The equation for the reaction between sodium iodide and sodium iodate, NaIO3, is

Calculate the number of moles of sodium iodate, NaIO3, in the original sample.

(1)

(vi)	Calculate the mass of sodium iodate, NaIO3, in the original sample and hence calculate the percentage purity of the sample to 2 significant figures.
(Molar mass of NaIO3 = 198 g mol–1)

(2)
(c)	When a similar experiment is repeated, making and analysing potassium iodate, KIO3, a higher percentage purity is obtained.  Suggest a reason for this.
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
(1)
(d)	An acidified solution of sodium iodate, NaIO3, is added to a solution of iron(II) sulphate.  Describe and explain what you would expect to see.
Description .................................................................................................................
....................................................................................................................................
....................................................................................................................................
Explanation ................................................................................................................
....................................................................................................................................
....................................................................................................................................
(2)
(Total 15 marks)

37.	Some organic compounds having molecules containing eight carbon atoms are listed below with their molar masses and boiling points.

(a)	(i)	What type of intermolecular force is present in all of these compounds?
..........................................................................................................................
(1)
(ii)	2,2,4–trimethylpentane has a lower boiling point than its isomer, octane.  
Suggest a reason for this.
You may find it helpful to draw a diagram.

(2)
(b)	(i)	Suggest ONE reason why octanoic acid has a higher boiling point than octan–1–ol.
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
(1)
(ii)	Estimate the boiling point of octanal and justify your estimate.
(Molar mass of CH3(CH2)6CHO = 128 g mol––1).
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
(2)
(iii)	Would you expect octan–1–ol to dissolve readily in water?
Justify your answer.
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
(2)
(Total 8 marks)

38.	Economic and environmental factors must be considered if a new fuel is to be used.
(i)	Do you think the production of hydrogen from steam is likely to cost–effective?  Give a reason for your answer.
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
(1)
(ii)	State and explain one environmental advantage of hydrogen over fossil fuels.
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
(1)
(Total 2 marks)

39.	Cynogen, (CN)2, behaves like the halogens in its chemical reactions.  Its hydride, hydrogen cyanide, HCN, is similar to the hydrogen halides.
(a)	(i)	Draw ‘dot–and–cross’ diagrams of hydrogen chloride, hydrogen cyanide and cyanogen molecules.  You need include only outer shell electrons.
hydrogen chloride

hydrogen cyanide

cyanogen

(3)

(ii)	Draw the displayed formula for cyanogen showing the CCN bond angle.

(2)
(b)	Use the Hess cycle and the data, at 298 K, to calculate the standard enthalpy change of formation of cyanogen, including its sign and unit.
DH e [(CN)2(g)]	=	+2121 kJ mol–1
DH e [C(graphite)]	=	+717 kJ mol–1
DH e [½N2(g)]	=	+473 kJ mol–1

(3)
(c)	Potassium cyanide, KCN, reacts with concentrated sulphuric acid in a similar way to potassium iodide.  Suggest THREE possible gaseous products.
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
(3)
(d)	Chlorine reacts with cold sodium hydroxide solution in a disproportionation reaction.

(i)	State the oxidation number of chlorine in
C12 ....................................................................................................................
NaC1 .................................................................................................................
NaC1O ..............................................................................................................
(2)

(ii)	What is meant by the term disproportionation?
..........................................................................................................................
..........................................................................................................................
(1)
(iii)	Suggest a balanced chemical equation for the reaction between cyanogen and cold sodium hydroxide solution.

(1)
(e)	Suggest a reason why there are no experiments on the chemistry of cyanogen in Advanced level chemistry courses.
....................................................................................................................................
....................................................................................................................................
(1)
(Total 16 marks)

40.	Cynogen, (CN)2, behaves like the halogens in its chemical reactions.  Its hydride, hydrogen cyanide, HCN, is similar to the hydrogen halides.
(i)	Draw ‘dot–and–cross’ diagrams of hydrogen chloride, hydrogen cyanide and cyanogen molecules.  You need include only outer shell electrons.
hydrogen chloride

hydrogen cyanide

cyanogen

(3)
(ii)	Draw the displayed formula for cyanogen showing the CCN bond angle.

(2)
(Total 5 marks)

41.	Use the Hess cycle and the data, at 298 K, to calculate the standard enthalpy change of formation of cyanogen, including its sign and unit.
DH e [(CN)2(g)]	=	+2121 kJ mol–1
DH e [C(graphite)]	=	+717 kJ mol–1
DH e [½N2(g)]	=	+473 kJ mol–1

(3)

42.	Potassium cyanide, KCN, reacts with concentrated sulphuric acid in a similar way to potassium iodide.  Suggest THREE possible gaseous products.
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
(3)

43.	Chlorine reacts with cold sodium hydroxide solution in a disproportionation reaction.

(i)	State the oxidation number of chlorine in
C12 ....................................................................................................................
NaC1 .................................................................................................................
NaC1O ..............................................................................................................
(2)
(ii)	What is meant by the term disproportionation?
..........................................................................................................................
..........................................................................................................................
(1)
(iii)	Suggest a balanced chemical equation for the reaction between cyanogen and cold sodium hydroxide solution.

(1)
(Total 4 marks)

44.	Suggest a reason why there are no experiments on the chemistry of cyanogen in Advanced level chemistry courses.
....................................................................................................................................
....................................................................................................................................
(1)

45.	Butane reacts with chlorine, in a free radical chain reaction, to form 1–chlorobutane as one of the products.  The reaction takes place in a number of steps:

(i)	What condition is needed to promote Step 1?
..........................................................................................................................
(1)
(ii)	What type of bond breaking occurs in Step 1?
..........................................................................................................................
(1)
(iii)	Classify the type of reaction step occurring in Steps 2 and 3.
..........................................................................................................................
(1)
(iv)	Suggest an equation for another possible chain termination step.

(1)
(v)	Give one other example of a reaction with a free radical chain mechanism.
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
(1)
(Total 5 marks)

46.	Read the passage below on GROWING DIAMONDS straight through, and then more carefully, in order to answer the following questions.
(a)	Give TWO reasons why alchemists tried to make gold from copper, rather than diamonds from graphite.
(2)
(b)	(i)	State the physical property of diamond which suggested that high pressure would be needed in its synthesis from graphite.
(1)

(ii)	Explain why high pressure is needed in the synthesis of diamond from graphite
(1)
(iii)	Suggest a reason why high temperatures are needed to make diamond from graphite.
(1)
(iv)	Where do many scientists believe that the conditions of pressure and temperature for making diamonds occur naturally?
(1)
(c)	Explain why large natural diamonds rather than synthetic ones are still used in jewellery.
(1)
(d)	Describe in no more than 100 words how diamonds can be made from graphite.
(8)
	You are not asked to summarise the whole passage, nor to include equations in your summary.
	At the end of your summary state the number of words you have used.
	Credit will be given for answers written in good English, using complete sentences and with correct use of technical words.  Avoid copying long sections from the original text.  Numbers count as one word, as do standard abbreviations and hyphenated words.  Any title you give to your passage does not count in your word total.
(Total 15 marks)
GROWING DIAMONDS
From the fabled Golconda mines of ancient India to Paul Simon’s “Diamonds on the Soles of Her Shoes”, diamonds have been a source of fascination to men and to women.  Had medieval alchemists known that graphite and diamond are merely different forms of the same element, carbon, they might have spent as much time attempting to synthesize diamonds as they did attempting to transmute base metals into gold.  But copper at least looks something like gold, whereas graphite and diamond could scarcely be more dissimilar; one is dull, black, and soft enough to rub off on paper, the other is brilliant, transparent, and hard enough to drill granite.  It was not until 1954 that modern alchemists learned how to synthesize diamonds from graphite.
The first clues toward the successful synthesis of diamond came from theoretical arguments and from considerations of the circumstances under which diamonds are found in nature.  Diamond has a density of 3.51 g cm–3 whereas the density of graphite is only 2.26 g cm–3.  Thus very high pressures are required to squeeze the graphite carbons into the tighter packing arrangement of the diamond crystal.  Many scientists believe that natural diamonds are formed under such conditions, 100 or 200 miles below the surface of the Earth, and are later brought by volcanic action closer to the surface where they can be mined.

Laboratory duplication of these harsh conditions was finally accomplished in 1954 by four General Electric scientists, Francis Budy, Tracy Hall, Herbert Strong, and Robert Wentorf.  The team of General Electric scientists developed a small chamber that could maintain temperatures up to 2500°C and pressures of 1.0 × 105 atm.
Their first attempts not only failed to produce diamond but also failed even to melt the graphite.  The melting temperature of graphite is above 4000 °C, far beyond the chamber’s maximum of 2500°C.
The researchers finally hit upon the idea of adding small amounts of a metal that does melt below 2500°C, in the hope that some of the graphite might dissolve in the molten metal then crystallize out as diamond.  The strategy worked beautifully with many metals:  chromium, manganese, iron, cobalt, nickel, tantalum, ruthenium, rhodium, palladium, iridium, and platinum.  When the mixture reached 1200–2400°C under a pressure of 1 .0 × 105 atm small diamond crystals formed at rates as high as 0.1 per minute.  At last, a practical synthesis of diamond had been achieved.
The crystals – usually yellow or green and less than 1 mm across – were not gem quality, but they were as hard as natural diamond and were useful as an industrial abrasive.

Today, the same method is used.  It is called the high–pressure, high–temperature method (HPHT).  Annual world–wide production of synthetic diamonds is measured by the ton, far outstripping natural diamond production from mines.  Synthetic HPHT diamonds as large as 8 mm across have been made, but the long heating and pressing time they require – seven days – makes them more costly than natural diamonds of the same size.  Therefore, the large diamonds used in jewellery are still natural diamonds.
(550 words)
Adapted from Growing Diamonds by Harold Zaugg and Burning Diamonds and Squeezing Peanuts by Dr. K.A.  Davenport published in Chem Matters, April 1990.

47.	The boiling points of four compounds which have hydrogen bonds between their molecules are shown below:

(a)	Draw a diagram showing the position of a hydrogen bond between two molecules of methanol.

(1)
(b)	By considering the structures and intermolecular forces involved, explain
(i)	why water has a higher boiling point than ethanol,
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
(2)

(ii)	why methanol has a lower boiling point than ethanol.
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
(2)
(c)	Butan–1–ol has an isomer named ethoxyethane, C2H5OC2H5
	Explain why there are  no hydrogen bonds between ethoxyethane molecules.
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
(1)
(d)	Iodine is almost insoluble in water but is more soluble in ethoxyethane.
(i)	What type of force is present between molecules of iodine in a crystal?
..........................................................................................................................
(1)
(ii)	Explain why iodine is more soluble in ethoxyethane than in water.
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
(3)
(Total 10 marks)

48.	This question is concerned with the molecular interactions between the polymer chains in various semi–crystalline plastic materials in common use.

(a)	Complete the right hand column to show the strongest type of bonding present between polymer chains.
(3)
(b)	(i)	Why are the densities of the two types of poly(ethene) different?
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
(1)
(ii)	Why is the melting temperature of high density poly(ethene) greater than that of low density poly(ethene)?
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
(1)
(c)	Why is the melting temperature of bakelite not given?
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
(1)
(d)	Suggest FOUR further pieces of data that you would like to see included in the table if you wished to evaluate the suitability of various polymers for use as a rope on board a boat.
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
(4)
(Total 10 marks)

49.	(a)	4.50 g of water is evaporated by 10.0 kJ of electrical energy.
(i)	calculate the number of moles of water evaporated.

(1)
(ii)	Calculate the enthalpy change of vaporization per mole of water.  
Your answer should include both sign and units.

(1)

(b)	(i)	Draw a diagram to show the principal molecular force between two water molecules.

(2)
(ii)	Explain why the value for the enthalpy change of vaporization of water is much greater than that for the other Group 6 hydrides.
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
(1)
(Total 5 marks)

50.	This question concerns the five hydrocarbons below.

(a)	Which hydrocarbons are isomers of each other?
……………………………………………………………………………………….
 (1)
(b)	Which hydrocarbons would decolorize an acidified solution of potassium manganate (VII)?
……………………………………………………………………………………….
(1)
(Total 2 marks)

51.(a)	Bromobenzene can be prepared from benzene by reaction with bromine and a suitable catalyst.
(i)	Write the balanced equation for the reaction.
..……………………………………………………………………………….
(2)
(ii)	Name a suitable catalyst for this reaction.
..……………………………………………………………………………….
(1)
(b)	This part of the question is about three ways of preparing bromoethane, C2H5Br.
	Each method has a different mechanism or reaction type.
(i)	State the reagent and the condition needed to prepare bromoethane from ethane.
	Reagent ..……………………………………………………………………..
	Condition ..……….…………………………………………..……………….
	State the mechanism and type of this reaction.
	Mechanism ……………………………………………………………………
	Type …………………………………………………………………………..
(4)
(ii)	Bromoethane can also be prepared by an electrophilic addition reaction.
	Write the balanced equation for this reaction.
(2)
(iii)	In a third route bromoethane can also be prepared by adding potassium bromide to concentrated sulphuric acid and ethanol.
	The potassium bromide reacts with the concentrated sulphuric acid to produce hydrogen bromide, which then reacts with the ethanol.
	Name the mechanism and type of the reaction between hydrogen bromide and ethanol.
	Mechanism ……………………………………………………………………
	Type …………………………………………………………………………..
(2)
(c)	When samples of pure hydrogen bromide gas are needed, concentrated sulphuric acid is not an appropriate acid to react with potassium bromide.
(i)	Which acid should be used?
..……………………………………………………………………………….
(1)

(ii)	Why is concentrated sulphuric acid not suitable for preparing a sample of pure hydrogen bromide?
..……………………………………………………………………………….
..……………………………………………………………………………….
(1)
(iii)	Suggest a reason why concentrated sulphuric acid is suitable for making hydrogen bromide in the preparation of bromoethane from ethanol in (b)(iii).
..……………………………………………………………………………….
..……………………………………………………………………………….
(1)
(iv)	Describe and explain what you would observe when a test tube containing hydrogen bromide gas is inverted in water.
	Observation …………………………………………………………………..
..……………………………………………………………………………….
	Explanation …………………………………………………………………..
..……………………………………………………………………………….
(2)
(Total 16 marks)

52.	You have used 1–iodobutane in some of your experiments.
(a)	Construct a Hess cycle for the atomization of 1–iodobutane,and use the data below, at 298 K, to show by calculation that the value of DHat[CH3CH2CH2CH2I(g)] is +4987.6 kJ mol–1.
	DHf   [CH3CH2CH2CH2I(g)]	=   –52.0 kJ mol–1
	DH e [C(graphite)]	= +716.7 kJ mol–1
	DH e []	= +218.0 kJ mol–1
	DH e []	= +106.8 kJ mol–1
(3)
(b)	(i)	Draw a ‘dot–and–cross’ diagram for the electronic structure of 1–iodobutane, showing outer shell electrons only.
(2)

(ii)	Calculate the energy of the C—I bond in 1–iodobutane.
	Use the standard enthalpy change of atomization of 1–iodobutane, and the data below:
	E(C—H) = +413 kJ mol–1
	E(C—C) = +347 kJ mol–1
(3)
(c)	Three drops of 1–iodobutane are added to a test tube containing 2 cm3 of ethanol and 2 cm3 of hot aqueous silver nitrate solution. A reaction occurs immediately.
(i)	What would you observe?
..……………………………………………………………………………….
	Write an ionic equation, including state symbols, for your observation.
(3)
(ii)	Name the organic product of this reaction.
..……………………………………………………………………………….
(1)
(d)	When the experiment in (c) is repeated with 1–chlorobutane it is very slow.
	Explain why the rates of the two reactions are different.
……………………………………………………………………………………….
……………………………………………………………………………………….
……………………………………………………………………………………….
……………………………………………………………………………………….
(2)
(e)	The experiment in (c) is repeated using 2–chloro–2– methylpropane.
(i)	Draw a displayed formula of 2–chloro–2–methylpropane.
(1)
(ii)	Describe what you would expect to see.
..……………………………………………………………………………….
..……………………………………………………………………………….
(1)

(iii)	Explain why this reaction is more rapid than the reaction when 1–chlorobutane is used.
..……………………………………………………………………………….
..……………………………………………………………………………….
..……………………………………………………………………………….
..……………………………………………………………………………….
(2)
(Total 18 marks)

53.	Read the passage below on CHEMICAL BONDING straight through, and then more carefully, in order to answer the following questions.
CHEMICAL BONDING
	Atoms come together to form compounds. By so doing they can pair off electrons and the resulting product is energetically more stable. Not all combinations of atoms, however, are known and some elements such as the rare gases are particularly loath to enter into compound formation. A fruitful avenue to understanding chemical compounds is via chemical bonding.
	The basis of chemical bonding is electron interaction, and in particular electron (spin) pairing. Not all the electrons of an atom are involved in the formation of bonds to other atoms. The electrons in filled inner shells around the nucleus play little or no part. The electrons in the outer, partly filled, shells are the important ones. These are known as the valence or bonding electrons and the shell is called the valence shell. If the atom has a full complement of electrons in its outermost shell, then it shows little inclination to compound formation and for this reason the rare gases, helium, neon, and argon, form no true chemical bonds. Krypton and xenon can form compounds if enough energy is supplied to break into the filled shell and promote some of its electrons to a higher energy level, whereupon bond formation becomes feasible. Even so, only fluorine and oxygen can provide bonds with sufficient energy to do this.
	There are three kinds of strong chemical bond – ionic, covalent, and metallic. The part played by the bonding electrons in each of these is quite different. In ionic (or electrovalent) bonding the electrons in the valence shell of a metal atom are transferred to the valence shell of the non–metal atom. In other words, the bonding electrons become associated with one of the atoms and charge separation occurs. The coulombic attractions between the ions so formed are the real basis of the binding energy of ionic compounds.

	The structure of ionic compounds is determined mainly by the physical parameters of ion size and charge. The result is a lattice of alternate metal cations and non–metal anions in which there is no particular cation associated with a particular anion. This means that there is no particular direction to the bonding – the ions bond equally to neighbouring ions in all directions.

	In covalent bonding the bond is said to be directed. The bonding electrons associated with two atoms occupy the region of space between them. The result of this is that the internuclear repulsion is lowered and the nuclei can approach one another more closely. This is represented in terms of energy in Figure 1. Most covalent bonds are of this type, that is, the bonding electrons are associated with two atoms. There are, in addition, some examples of three–atom covalent bonds in which a pair of bonding electrons serves to hold three nuclei together. These bonds are called three–centre bonds. This ability of electrons to hold several nuclei together reaches its ultimate form in metallic bonding.

	In metals the bonding electrons are associated with no particular atom or group of atoms but are free to move throughout the body of the metallic compound. This freedom is reflected in the properties of metals such as electrical conductance, which is invariably high.

	This method of classifying chemical compounds through their bonding can be illustrated by reference to Figure 2. In this the three corners of the triangle represent ideal forms of each kind of compound, but in reality these are not attained. No bond is purely ionic, covalent, or metallic. Because electrons are not fixed they can spend some of their time in a covalent bond with one of the atoms, so that in effect we have an equilibrium between covalent and ionic forms.

	Similarly, deformation of the electron clouds of ions can produce a covalent contribution to the bonding in ionic compounds.
	Adapted from ‘Chemical Compounds’ an article by John Emsley in the Physical Science Sourcebook published for the Nuffield Foundation by Penguin Books 1974.
(a)	(i)	Why do atoms come together to form compounds?
(1)

(ii)	Suggest a reason why electrons in filled inner shells around the nucleus play little or no part in bonding.
(1)
(iii)	Explain what is meant by the term valence shell.
(1)
(iv)	Explain why krypton and xenon form compounds only with fluorine and oxygen.
(2)
(b)	Why do metals have a high electrical conductance?
(1)
(c)	Why is Figure 2 helpful in understanding real compounds?
(1)
(d)	Summarise the information given in the passage about the three types of chemical bond. Use continuous prose and no more than 100 words.
	You are NOT asked to summarise the whole passage, nor to include equations in your summary.
	At the end of your summary state the number of words you have used.
(8)
	Credit will be given for answers written in good English, using complete sentences and with correct use of technical words. Avoid copying long sections from the original text. Numbers count as one word, as do standard abbreviations and hyphenated words. Any title you give to your passage does not count in your word total.
	There are penalties for the use of words in excess of 100.
(Total 15 marks)

54.(a)	Van der Waals forces are relatively unimportant in water but in poly(ethene) they are much more significant.
(i)	What feature of poly(ethene) molecules makes these forces so significant?
..……………………………………………………………………………….
..……………………………………………………………………………….
..……………………………………………………………………………….
(1)
(ii)	Suggest one physical property of poly(ethene) which results from its strong van der Waals forces.
..……………………………………………………………………………….
(1)
(iii)	What are the strongest intermolecular forces present in water?
..……………………………………………………………………………….
(1)
(iv)	Explain the origin of these strong forces in water. You may find it helpful to draw a diagram.
(2)
(v)	Explain, in terms of intermolecular forces, why poly(ethene) is insoluble in water.
..……………………………………………………………………………….
..……………………………………………………………………………….
..……………………………………………………………………………….
..……………………………………………………………………………….
..……………………………………………………………………………….
(2)
(b)	Ionic solids such as sodium chloride often dissolve readily in water. Complete the following diagram to show how the dissolving process takes place and what is produced.
	You may find it helpful to use    to represent a water molecule.

(2)
(c)	The Hess cycle below can be used to calculate the enthalpy change of solution of lithium chloride.

(i)	Insert the chemical symbols and the appropriate state symbols into the three boxes in the Hess cycle.
(3)
(ii)	Calculate the enthalpy change of solution of lithium chloride, including its sign and units.
(1)
(iii)	Will the temperature rise or fall when lithium chloride dissolves in water?
	Explain the rise or fall in terms of the making and breaking of bonds.
	Temperature change ………………………………………………………….
	Explanation …………………………………………………………………..
..……………………………………………………………………………….
..……………………………………………………………………………….
..……………………………………………………………………………….
..……………………………………………………………………………….
(2)
(Total 15 marks)

55.	Electron density maps for sodium chloride, a hydrogen molecule ion, , and a benzene molecule are shown below.

(a)	(i)	Describe fully the difference between the contours of the electron density maps for sodium chloride and the hydrogen molecule ion, .
….………………………………………………..……………………………
….………………………………………………..……………………………
….………………………………………………..……………………………
….………………………………………………..……………………………
(2)
(ii)	What does the difference in (i) tell you about the distribution of electrons in molecules compared with ionic compounds?
….………………………………………………..……………………………
….………………………………………………..……………………………
….………………………………………………..……………………………
….………………………………………………..……………………………
(2)
(iii)	Draw a `dot–and–cross' diagram for a hydrogen molecule ion, .
(1)
(b)	(i)	What is the typical characteristic of some of the bonding electrons in an arene compound like benzene?
….………………………………………………..……………………………
(1)
(ii)	How does the electron density map for benzene demonstrate this characteristic?
….………………………………………………..……………………………
(1)
(Total 7 marks)

56.(a)	The equation for the reaction between thiosulphate ions and hydrogen ions is

(i)	What is the oxidation number of sulphur in
	thiosulphate ions, ……………………………………………………..
	sulphur dioxide, SO2 …………………………………………………………
	sulphur, S? ….…………………………………………………………..........
(2)

(ii)	State and explain the type of redox reaction represented by the equation.
	Type of reaction ………………………………………………………………
	Explanation …………………………………………………………………...
….………………………………………………..……………………………
….………………………………………………..……………………………
….………………………………………………..……………………………
(2)
(b)	The effect of temperature on the rate of this reaction was investigated.
	Solutions of sodium thiosulphate and hydrochloric acid were heated to the same temperature.
	The time for the reaction to produce a fixed amount of sulphur was measured, and the reaction repeated at different temperatures.
(i)	How could you heat the solutions to ensure that both reactants are at the same temperature?
….………………………………………………..……………………………
….………………………………………………..……………………………
….………………………………………………..……………………………
….………………………………………………..……………………………
(1)
(ii)	How could you measure the time for the reaction to produce a fixed amount of sulphur?
….………………………………………………..……………………………
….………………………………………………..……………………………
….………………………………………………..……………………………
(2)
(iii)	How could you obtain a measure of the rate of reaction from your measurement of time?
(1)

(c)	The results obtained from a series of experiments are tabulated below. Most of the measurements have been converted into values of 1/temperature and ln rate.

(i)	Complete the table by calculating the missing values.
(2)
(ii)	Plot a graph of ln rate on the vertical axis, against 1/temperature on the horizontal axis.

(3)

(iii)	Calculate the activation energy, EA, for the reaction, using your graph and the relationship:
ln rate = constant –
	where R = 8.31 J K–1 mol–1.
	Remember to include a sign and units in your answer.
(3)
(Total 16 marks)

57.	Read the passage below on SIR HUMPHRY DAVY AND THE SAFETY LAMP straight through, and then more carefully, in order to answer the following questions.
SIR HUMPHRY DAVY AND THE SAFETY LAMP
	Today it is not uncommon to find buildings decorated with brightly polished brass miners' lamps. Indeed, display seems to be the major use of these devices. But soon after the safety lamp was developed in late 1815, it was hailed as "the pride of science, the triumph of humanity, and the glory of the age". Over the next 150 years, thousands of the lamps were carried into mines the world over, and an unknown number of miners were saved from the ravages of poisonous and explosive gases.
	Reports of coal miners being overcome by toxic gases and burned in mine explosions appear as early as the sixteenth century. In time, the gas responsible for both catastrophes came to be called fire–damp, but its chemical nature and origins remained unknown. The problem posed by fire–damp was two–fold. To avoid disaster, it was necessary both to devise a method of detecting the gas and to find a way of illuminating the mine without igniting the gas. Canaries and other birds served as early detectors. When they stopped singing and fell off their perches, it was a clear sign for the miners to head for the exit shaft. Illumination posed a greater problem. The open flames of candles and oil lamps could easily ignite explosive mixtures of fire–damp and air. One book reports that attempts were made to light some mines by the dull phosphorescent glow from rotting fish—a method that no doubt had a stronger effect on the sense of smell than the sense of sight. One solution to the detection problem was the flint and steel mill in which a stream of sparks was generated by pressing a piece of flint against a rotating steel wheel. Although fire–damp caused changes in the size and luminosity of the sparks and could thus be detected, the mill could and did ignite explosions.
	The event that ultimately led to the invention of Davy's safety lamp was an explosion at the Felling Colliery near Sunderland on 26 May 1812. A total of 92 miners were killed, and a great public outcry resulted. A group of public–spirited citizens contacted Davy in 1815.

	The development of the Davy lamp, as it soon came to be called, was a blend of scientific research and practical inventiveness. Sir Humphry began by demonstrating that fire–damp is "pure light carburetted hydrogen" consisting of "four portions of hydrogen in weight 4, and one portion of charcoal in weight 11.5". John Dalton's applications of atomic theory to chemistry had been published only 7 years earlier, but Davy must have used atomic ideas to come up with the first correct statement of the formula of the gas we know as methane, CH4. Davy was also able to show that when methane is mixed with 6–14 times its volume of air, an explosive mixture results. Mixtures that are richer in methane extinguish a candle flame; those that are poorer cause the flame to burn more brightly, but do not explode. Fortunately, even explosive mixtures of methane and air proved to require considerably higher temperatures to ignite than do mixtures of hydrogen and oxygen.
	In late October 1815 Davy described three promising preliminary lamp designs, but his real breakthrough was the introduction of a "chemical fire sieve". In a letter dated 1 January 1816 (Davy was working on New Year's Day!) he reported that a candle or oil lamp surrounded with a wire mesh cylinder was absolutely safe in the presence of even potentially explosive mixtures of fire–damp and air. The flame would not propagate through the mesh, and the mesh would diffuse the heat of the lamp so that the methane would not be explosively ignited. In the presence of low concentrations of methane, the size of the flame would increase as the methane burned, much as it does in a Bunsen burner. When methane was present in high concentrations, the flame would be extinguished. Thus, the Davy lamp is both an explosion–proof source of light and a detector of methane.
	The same principle of design has been incorporated in all subsequent lamps, though various refinements have been made. One of the first was the introduction of a metal shield to prevent draughts from blowing the flame horizontally into the mesh, which might thus become hot enough to ignite the gas. A second layer of iron or bronze mesh was soon added to further enhance safety. Modern lamps also have cylindrical glass windows around the flame, and the gauze cylinder, which is situated above the glass, is fully protected by a metal wind shield. Thus, the screen, the key to the safety of the Davy lamp, is not obvious from a casual inspection of the typical decorative lamp.
	Davy appears to have taken great satisfaction from his invention. His contribution was recognised by a valuable silver dinner service, a gift from a group of wealthy mine owners. Davy was made a baronet in 1819, but perhaps he prized even more highly a letter from those who benefited the most from his invention. It reads thus:
	We, the undersigned, miners at the Whitehaven Collieries, belonging to the Earl of Lonsdale, return our sincere thanks to Sir Humphry Davy, for his invaluable discovery of the safe lamps, which are to us life preservers; and being the only return in our power to make, we most humbly offer this, our tribute of gratitude.

	The 82 `signatures' include 47 marks made by miners unable to write their names.

(913 words)
	Adapted from `Sir Humphry Davy and the Safety Lamp' by A. Truman Schwartz, Chemistry Review, March 1996.
(a)	State the two types of catastrophe caused by fire–damp.
(1)
(b)	Why did fire–damp pose a two–fold problem?
(2)
(c)	Show how the formula of methane, CH4, can be deduced from the information "four proportions of hydrogen in weight 4, and one proportion of charcoal in weight 11.5".
(1)
(d)	(i)	Suggest a property of iron which makes it a suitable metal for the wire mesh in a safety lamp.
(1)
(ii)	Suggest another metal that could be used for the mesh.
(1)
(e)	Why was the safety lamp hailed as "the pride of science, the triumph of humanity and the glory of the age"?
(1)
(f)	Write a summary in continuous prose, in no more than 120 words, describing the development of the safety lamp and how it works.
	You are NOT asked to summarise the whole passage, nor to include equations in your summary.
	At the end of your summary state the number of words you have used.
(8)

	Credit will be given for answers written in good English, using complete sentences and with the correct use of technical words. Avoid copying long sections from the original text. Numbers count as one word, as do standard abbreviations and hyphenated words. Any title you give to your passage does not count in your word total.
	There are penalties for the use of words in excess of 120.
(Total 15 marks)

58.	(a)	The standard enthalpy change of formation of tetrachloromethane, CCl4, is –129.6 kJ mol–1. Look up the values for the standard enthalpy change of formation of the chlorides BeCl2, BCl3 and NCl3.
	For each of the four chlorides, calculate the standard enthalpy change of formation per mole of chlorine atoms.
(3)
(b)	Plot a graph of standard enthalpy change of formation per mole of chlorine atoms on the vertical axis, against atomic number of the combining elements, Be to N, on the horizontal axis.
(3)

(c)	(i)	What does your graph tell you about the change in the strength of the bond between the element and chlorine, X–Cl, from beryllium to nitrogen?
(2)
(ii)	Suggest a reason for the trend you describe in (i).
(2)
(iii)	Lithium chloride, LiCl, does not fit the pattern you have found. Suggest a reason for this.
(2)
(d)	Describe and explain the pattern you would expect for the chlorides:
CaCl2, AlCl3, SiCl4 and PCl3.
	You are not expected to carry out further calculations.
(3)
(Total 15 marks)

59.	Steam distillation is often used to obtain chemicals such as lavender oil and limonene from plants.
(a)	(i)	Explain why heating the plants directly, to distil off their oil, is not usually successful.
…………………..…………………………………………………………….
…………………..…………………………………………………………….
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…………………..…………………………………………………………….
(2)
(ii)	How does steam distillation overcome these problems?
…………………..…………………………………………………………….
…………………..…………………………………………………………….
(1)
(b)	(i)	Draw a labelled diagram of a suitable apparatus for steam distillation to obtain an oil from lemon peel.
(3)
(ii)	How would you obtain a pure, dry sample of the oil from the mixture obtained by steam distillation?
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(2)
(c)	(i)	The oil obtained turns dilute acidified potassium manganate(VII) from pink to colourless. Which functional group does this suggest is present in the oil?
…………………..…………………………………………………………….
…………………..…………………………………………………………….
…………………..…………………………………………………………….
(1)

(ii)	The dry oil reacts with sodium to give off hydrogen. Which functional group does this suggest is present in the oil?
…………………..…………………………………………………………….
…………………..…………………………………………………………….
(1)
(Total 10 marks)

60.	This question is about some of the reactions of the halogens.
(a)	Chlorine solution, Cl2 (aq), is added to potassium bromide solution and potassium iodide solution in separate test–tubes.
	To help identify the products an equal volume of hydrocarbon solvent is shaken with the contents of each test–tube.
(i)	State the colour of the hydrocarbon solvent at the end of each reaction with chlorine.
	Potassium bromide reaction ………………………………………………….
	Potassium iodide reaction …………………………………………………….
(2)
(ii)	Why does only one product from each reaction dissolve in the hydrocarbon solvent?
…………………..…………………………………………………………….
…………………..…………………………………………………………….
(1)
(iii)	Write the equation, including state symbols, for the reaction between chlorine solution and potassium bromide solution.
(2)
(b)	Bromine solution reacts with alkanes such as methane, and alkenes such as ethene.
(i)	State the condition needed to promote the reaction between methane and bromine.
…………………..…………………………………………………………….
(1)
(ii)	Name the type and mechanism of the reaction between methane and bromine.
	Type …………………………………………………………………………..
	Mechanism ……………………………………………………………………
(2)
(iii)	Name the product of the reaction between ethene and bromine and draw a ‘dot–and–cross’ diagram of the structure for this product, showing outer electrons only.
	Name ………………………………………………………………………….
(2)
(c)	Concentrations of oxidizing agents which will oxidize iodide ions to iodine can be measured by titrating the resulting iodine with a standard sodium thiosulphate solution.
	10.0 cm3 samples of a solution containing 0.0100 mol dm–3 of iodine, I2, were found to react with exactly 20.0 cm3 of a 0.0100 mol dm–3 aqueous solution of sodium thiosulphate.
	Calculate the number of moles of iodine and sodium thiosulphate that react together and write the equation for the reaction between iodine and sodium thiosulphate.
	Show how your equation is consistent with your calculation.
(3)
(d)	(i)	What is the biological action that is common to all three solutions of chlorine, bromine, and iodine?
…………………..…………………………………………………………….
(1)
(ii)	Give ONE everyday application of one of these solutions acting in this way.
(1)
(Total 15 marks)

61.	(a)	Cyclohexene reacts with hydrogen in the presence of a catalyst to form cyclohexane.
(i)	Draw the displayed formula of cyclohexene, C6H10.
(1)
(ii)	Write a balanced equation for the reaction.
(1)
(iii)	Name a catalyst for the reaction.
…………………..…………………………………………………………….
(1)
(iv)	Calculate the standard enthalpy change, DH f , for the reaction using the data at 298 K.
DH f  [C6H10(l)] = –   38 kJ mol–1
DH f  [C6H10(l)]  = – 156 kJ mol–1

	Your answer should include a sign and units.
(2)
(v)	Give an example of an industrial application of the hydrogenation of alkenes in the food industry.
…………………..…………………………………………………………….
…………………..…………………………………………………………….
(1)

(b)	Benzene also reacts with hydrogen in the presence of a catalyst and under suitable conditions.
(i)	State the conditions for the reaction.
…………………..…………………………………………………………….
…………………..…………………………………………………………….
(2)
(ii)	Kekulé proposed the following structural formula for benzene.

	Calculate a value for the standard enthalpy change of hydrogenation of benzene based on this structure and your answer to (a)(iv).
(1)
(iii)	The true value for the standard enthalpy change of hydrogenation of benzene is less negative than the value calculated in (iii) by nearly 150 kJ mol–1.
	Describe how the electronic structure of benzene accounts for the difference between this value and the value you have calculated based on the Kekulé structure. You should include a diagram in your answer.
…………………..…………………………………………………………….
…………………..…………………………………………………………….
…………………..…………………………………………………………….
…………………..…………………………………………………………….
…………………..…………………………………………………………….
…………………..…………………………………………………………….
(3)
(c)	The reactions of benzene and cyclohexene with hydrogen are addition reactions, though benzene usually reacts with other reagents in a different way.
(i)	What type and mechanism of reaction does benzene usually take part in?
	Type …………………………………………………………………………..
	Mechanism ……………………………………………………………………
(2)
(ii)	Give an example of the usual type of benzene reaction by writing an appropriate equation.
(2)
(Total 16 marks)

62.	Read the passage below on A GOLDEN OPPORTUNITY straight through, and then more carefully, in order to answer the following questions.

A GOLDEN OPPORTUNITY
	“THERE’S GOLD IN THEM THAR HILLS!”
	And plenty of volunteers to help dig for it, it seems. Few people strike it lucky, however; more fortunes were lost than were ever won in the famous 19th century gold rushes of California, Ballarat and the Klondike.
	Gold mining today may not be quite so frenetic, but it is still a risky business. Even after finding reliable deposits, extracting the gold from them is no mean feat. Between 15 and 30 per cent of the world’s gold reserves occur as refractory minerals – microscopic particles of gold encapsulated in a mineral matrix; well known examples include arsenopyrite (FeAsS),iron pyrites (FeS2) and chalcopyrite (FeCuS2).
	Gold is usually obtained by crushing and grinding ore from the mine and separating these refractory minerals from the oxide ore and other non–metallic minerals by froth flotation to produce a sulphide concentrate. This is then roasted to liberate the gold, which is extracted by treating the resulting mixture with an aerated solution of sodium cyanide. The process is not without problems; roasting converts any sulphur in the refractory minerals to sulphur dioxide and any arsenic to arsenic(III) oxide, both of which have undesirable environmental and economic implications. In some cases, roasting traps the gold in fused silicate minerals and fails to liberate all the metal. Cyanidation is also difficult. The mineral matrix acts as an impervious physical barrier and shields most of the gold particles from attack by cyanide ions. Despite being used for over 100 years, cyanidation of refractory ores yields only a fraction of the contained gold.
	And so it might have continued, if it had not been for a chance coincidence that resulted as part of the reorganisation of the University of London. At the time, a team of Chelsea undergraduates was carrying out a final–year project on the mineralogy of a refractory sulphide concentrate produced from a gold deposit at the Clogau St David’s mine in North Wales. The group quickly hit upon a problem. Since roasting the concentrate to liberate the gold was not permitted on the National Park property where the mine was located, they were only able to extract 10 per cent of the gold by conventional cyanidation. As an alternative to roasting, the undergraduates investigated various ways of solubilising the gold in the concentrate by using acidic solutions of thiourea with different oxidants. They showed that gold could be solubilised, but not to an extent that would be economic.
	Instead, the answer to the problem arose from existing interests at Queen Elizabeth College on metal–microbe interactions. For the Chelsea researchers, this work suggested a possible solution and the two groups soon combined forces. The researchers treated the concentrate with the thermophilic (heat–loving) bacterium Sulpholobus acidocalderius.
	This is a bacterium which catalyses two processes. It helps atmospheric oxygen to oxidize sulphide minerals and it helps to make the products of oxidation water soluble. Fresh water contains sufficient dissolved oxygen to carry out the oxidation. The bacteria, concentrate and fresh water were mixed together in stirred tanks. When the oxidation process was complete, the pH of the mixture was adjusted by the addition of lime. Sodium cyanide solution was now added. Gold was then extracted from the resulting solution by reaction with zinc shavings. The extract increased the gold recovery from 10 to 100 per cent. BacTech have patented the process and have been using it at the Youanmi Mine 500 km north–east of Perth in Western Australia.

	The recovery of base metals during gold extraction is also high. BacTech researchers have recovered between 95 to 99 per cent Cu and Ni, 89 to 99 per cent Co and 91 to 96 per cent Zn, all of which contribute to the profitability of plant operations. In fact, recent work has shown that bacterial recovery of the base metals alone, whether or not gold is present in the concentrate, is feasible and economically competitive with conventional processes.
	BacTech has recently managed to raise more funds, which will help to accelerate the development of its technology and establish its presence in the US. The company has also joined forces with the South African company Mintek to pool and jointly market bacterial oxidation technology. Soon, BacTech will be floated on the Toronto stock exchange under the name BacTech Mineralogical Solutions.
(711 words)
	Adapted from A Golden Opportunity by Jack Barrett and Martin Hughes, published in Chemistry in Britain Vol. 33, Number 6 June 1997, and the Royal Society of Chemistry.
(a)	How does gold occur in refractory minerals?
(1)
(b)	Why is cyanidation difficult in the traditional process for extracting gold?
(1)
(c)	State TWO reasons why roasting concentrate is not permitted on National Park property.
(2)
(d)	Why is fresh water needed for the BacTech process for gold extraction?
(1)
(e)	What other factor contributes to the profitability of the BacTech operation, apart from the extraction of gold?
(1)
(f)	Write a summary in continuous prose, in no more than 100 words, describing the BacTech process of extracting gold from its ore.
(9)
	You are not asked to summarise the whole passage, nor to include equations in your summary. At the end of your summary state the number of words you have used.
	Credit will be given for answers written in good English, using complete sentences and with correct use of technical words. Avoid copying long sections from the original text. Numbers count as one word, as do standard abbreviations and hyphenated words. Any title you give to your passage does not count in your word total.
	There are penalties for the use of words in excess of 100.
(Total 15 marks)

63.	(a)	(i)	Propanone, CH3COCH3, is a polar liquid. Draw a structural formula for propanone and use the symbols d + and d – to identify the polar atoms in your formula.
(1)

(ii)	What simple experiment could you carry out to show that propanone is polar?
	You may find it helpful to draw a diagram.
(2)
(iii)	Explain why butanone, CH3CH2COCH3, has a higher boiling point (79 ºC) than propanone (56 ºC).
…………………..…………………………………………………………….
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(2)
(iv)	Would you expect the boiling point of propanal, CH3CH2CHO, to differ from that of propanone?
	Justify your answer.
…………………..…………………………………………………………….
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(2)
(b)	Glucose and cyclohexane are both six–membered ring compounds which can be oxidized to carbon dioxide and water in exothermic reactions. Explain why living organisms are able to use glucose as an energy source but are unable to use cyclohexane. You should refer to the structures of the two compounds in your answer.

(2)
(Total 9 marks)

64.	This question is concerned with boric acid, H3BO3, which is a weak acid.
	When it is dissolved in water several equilibria are established, one of them being
H3BO3(aq)        H+(aq) + H2BO3–(aq)
(a)	(i)	Write the expression for the acid dissociation constant, Ka, for the equilibrium shown above.
(1)
(ii)	Using a pH meter, a student finds the pH of a 0.1 M solution of boric acid to be 5.1.
	Assuming that this equilibrium makes the only significant contribution to the hydrogen ion concentration, calculate the value of  Ka for this equilibrium.
(3)
(iii)	Write equations for TWO other equilibria likely to be established in an aqueous solution of boric acid.
(2)
(iv)	Unlike many acids, boric acid does not react with sodium carbonate to produce carbon dioxide gas. Suggest the reason for this.
…………………..…………………………………………………………….
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(2)
(v)	Suggest the formula of a compound which would make a buffer solution when added to an aqueous solution of boric acid.
…………………..…………………………………………………………….
(1)
(b)	A single boric acid molecule has the structure shown below:

(i)	What are likely to be the most significant intermolecular forces in boric acid crystals?
…………………..…………………………………………………………….
(1)

(ii)	Draw a diagram showing how two boric acid molecules might be linked by these intermolecular forces.
(1)
(Total 11 marks)

65.	This question is about some of the isomers with the molecular formula C4H8O.
(a)	Draw the structural formula of the isomer(A) which is a ketone.

A
(1)
(b)	Draw the displayed formula and give the name of an isomer(B) which would turn Benedict’s solution from blue to red on warming.

Displayed formula of B
	Name of B......................................................
(2)
(c)	Draw the structural formula of an isomer(C) which is both an alkene and an alcohol.

C
(1)
(d)	Draw the structural formulae for two isomers(D and E) which are cyclic alcohols.

D	E
(2)
(e)	(i)	Which of your isomers(A to E) would react with sodium to give off hydrogen?
.................................................................................................................
(1)

(ii)	For ONE of the isomers you listed in (e)(i), write a balanced equation for its reaction with sodium. Show the charges on the ionic product.

(2)
(f)	Which one of the isomers(A to E) will decolourize bromine water?
....................................................................................................................................
(1)
(g)	State TWO reagents that can be used to dehydrate your alcohols D and E. Suggest a structural formula for either of the organic products.
....................................................................................................................................
....................................................................................................................................
(3)
(Total 13 marks)

66.	Silicon tetrachloride is a liquid of formula SiCl4. It reacts with water to form silicon dioxide and hydrochloric acid according to the equation:
SiCl4(l) + 2H2O(l) ® SiO2(s) + 4HCl(aq)
(a)	(i)	Is the formula of silicon tetrachloride consistent with the position of silicon in the Periodic Table?
	Justify your answer.
.................................................................................................................
.................................................................................................................
.................................................................................................................
.................................................................................................................
(2)
(ii)	Would you expect silicon tetrachloride to contain ionic or covalent bonds?
	Which two items of information are there in the question which support your answer?
.................................................................................................................
.................................................................................................................
.................................................................................................................
(3)

(iii)	How could you obtain a pure, dry sample of silicon dioxide from the resulting reaction mixture?
.................................................................................................................
.................................................................................................................
.................................................................................................................
.................................................................................................................
.................................................................................................................
(3)
(b)	Hydrochloric acid is described as a strong acid. What is meant by a strong acid?
	Write an equation to illustrate your answer.
....................................................................................................................................
....................................................................................................................................
(2)
(c)	1.50 g of an impure sample of silicon tetrachloride was weighed out on an accurate balance. It was cautiously added to water and when the reaction was complete, the mixture was made up to 250 cm³ with pure water.
	A 25.0 cm³ portion was titrated, using a suitable indicator, with 0.100 M sodium hydroxide solution. 30.0 cm³ was needed for neutralization.
(i)	What piece of apparatus would you use for
(A)	making up the solution to exactly 250 cm³ ................................
(B)	removing a 25.0 cm³ portion for the titration?
.................................................................................................................
(2)
(ii)	Suggest a suitable indicator for this titration and give the colour change. 
	Indicator
	Colour change from ...................................... to ....................................
(2)
(iii)	The procedure described in (c) is incomplete. Suggest TWO important steps that the student should have taken to ensure an accurate result.
.................................................................................................................
.................................................................................................................
.................................................................................................................
.................................................................................................................
(2)
(iv)	Use the information given in (c) and the equation
SiCl4(l) + 2H2O(l) ––––® SiO2(s) + 4HCl(aq)
	to calculate the percentage purity of the sample of silicon tetrachloride.
(4)
(Total 20 marks)

67.	(a)	Pure samples of hydrogen bromide and hydrogen iodide are prepared by adding phosphoric(V) acid to the corresponding potassium halide.
	Hydrogen chloride is prepared by adding concentrated sulphuric acid to potassium chloride.
(i)	Write a balanced equation for the preparation of hydrogen chloride.
(2)
(ii)	Explain why phosphoric(V) acid is used instead of sulphuric acid in the preparation of pure hydrogen bromide and hydrogen iodide.
.................................................................................................................
.................................................................................................................
.................................................................................................................
.................................................................................................................
(2)
(b)	(i)	The apparatus in the diagram can be used to form hydrochloric acid from hydrogen chloride gas.

	Suggest why it is better to use a funnel rather than a glass tube to prepare hydrochloric acid from hydrogen chloride gas.
.................................................................................................................
.................................................................................................................
.................................................................................................................
(2)

(ii)	Describe what you would see when hydrochloric acid is added to an aqueous solution of silver nitrate and the mixture is allowed to stand in direct sunlight for an hour.
	Initial appearance.....................................................................................
	Final appearance.......................................................................................
(2)
(c)	Write the equation for the reaction between ammonia and hydrogen bromide.
(1)
(d)	A hot wire is plunged into a test-tube containing hydrogen iodide.
(i)	Describe what you would see. ...........................................................
(1)
(ii)	Write a balanced equation for the reaction.
(1)
(e)	The halogens also form salts with high oxidation states. When potassium chlorate(V) is gently heated, it disproportionates to give potassium chlorate(VII) and potassium chloride.
(i)	Write a balanced equation for this reaction.
(2)
(ii)	The potassium chlorate(VII) formed then decomposes on further heating.
	Name the gaseous product of this reaction. .............................................
(1)
(Total 14 marks

68.	This question is about nitrogen trifluoride, NF3, and nitrogen trichloride, NCl3, which are covalent compounds. Van der Waals attractions and permanent dipole–dipole attractions exist between molecules of both compounds in the liquid state.
(a)	(i)	Describe how van der Waals attractions are caused.
............................................................................................................................
............................................................................................................................
............................................................................................................................
............................................................................................................................
(1)
(ii)	In which of the two compounds would you expect there to be greater van der Waals attractions? Justify your answer.
............................................................................................................................
............................................................................................................................
(1)

(iii)	In which of the two compounds would you expect there to be the greater permanent dipole–dipole attractions? Justify your answer.
............................................................................................................................
............................................................................................................................
 (1)
(iv)	The boiling point of nitrogen trichloride is much higher than that of nitrogen trifluoride. How might this be explained in terms of the two types of intermolecular attractions?
............................................................................................................................
............................................................................................................................
(1)
(b)	The standard enthalpy change for the formation of gaseous nitrogen trifluoride is
–125 kJ mol–1.
N2 (g)  +  1F2 (g)  ®  NF3 (g)  DHοf = –125 kJ mol–1
	The standard molar enthalpy changes of atomisation of nitrogen, N2, and of fluorine, F2, are given below.
	ΔHat/kJ mol–1
N2 (g) ® N (g)	+ 473
F2 (g) ® F (g)	  + 79
	This information can be represented on a Hess cycle in the following way, and then used to calculate bond energies.

(i)	Insert formulae, showing the correct quantities of each element, into the appropriate boxes.
(1)
(ii)	Insert arrows between the boxes and write the correct numerical data alongside the appropriate arrows.
(2)
(iii)	Use the cycle to calculate the N ¾ F bond energy in nitrogen trifluoride.

(2)
(Total 9 marks)

69.	(a)	(i)	The gas hydrogen iodide decomposes when heated to form hydrogen and iodine.
	Describe how you would carry out this reaction in the laboratory, if you were provided with a test tube of hydrogen iodide.
............................................................................................................................
............................................................................................................................
(1)
(ii)	What would you expect to see as the reaction takes place?
............................................................................................................................
(1)
(b)	A mixture of hydrogen iodide, hydrogen and iodine (all in the gaseous state) establishes dynamic equilibrium if a constant temperature is maintained.
2HI (g)     H2 (g) + I2 (g)                      ΔH = +9.6 kJ mol–1
(i)	Explain the meaning of the term dynamic equilibrium.
............................................................................................................................
............................................................................................................................
............................................................................................................................
(2)

(ii)	How, if at all, would the proportion of hydrogen iodide present at equilibrium change if the temperature were to be increased? Justify your answer.
............................................................................................................................
............................................................................................................................
............................................................................................................................
............................................................................................................................
(2)
(iii)	The reaction is catalysed by metals such as gold and platinum. How, if at all, would the proportion of hydrogen iodide present at equilibrium change if the reaction were to be catalysed? Justify your answer.
............................................................................................................................
............................................................................................................................
(1)
(c)	Part of an energy profile for this reaction is shown below. It is not intended to be to scale.

	Complete the profile showing:
·	the products;
·	the progress of both uncatalysed and catalysed reactions;
·	labelled arrows to indicate the activation energies of both the uncatalysed and catalysed reactions.
(4)
(Total 11 marks)

70.	The following table was used to record the results of experiments in which aqueous solutions of halogen elements were added to aqueous solutions of potassium halides.

(a)	(i)	The ticked box indicates that a reaction occurred when bromine solution was added to potassium iodide solution.
	Place ticks in appropriate empty boxes to indicate the other experiments in which you would expect reactions to have occurred.
(1)
(ii)	Give ONE observation you would expect to make when bromine solution is added to potassium iodide solution.
............................................................................................................................
(1)
(iii)	Write a balanced ionic equation for the reaction in (ii).
(Omit or delete any spectator ions).

(1)
(iv)	Select ONE of the products of the reaction in (ii).
	Describe a test you could carry out to confirm its identity, indicating the result of the test.
Product ...............................................................................................................
Test .....................................................................................................................
............................................................................................................................
Result..................................................................................................................
............................................................................................................................
(2)

(b)	Iodine and its compounds are quite expensive and it is sometimes worthwhile recovering this element from solutions containing iodide ions. This can be done by adding a solution of potassium dichromate acidified with sulphuric acid. Iodine is then precipitated and can be separated from the mixture.
	An unbalanced ionic equation for the reaction is:
Cr2O72– (aq) + H+ (aq) + I– (aq)   ®   2Cr3+ (aq) + I2 (s) + H2O (l)
(i)	Identify the two elements which show a change of oxidation number during the reaction, giving their initial and final oxidation numbers.
	First element.......................................
	Initial oxidation number..............      Final oxidation number ..............................
	Second element ..................................
	Initial oxidation number..............      Final oxidation number ...............................
(3)
(ii)	Using oxidation numbers (or otherwise), balance the equation:
Cr2O72– + ........H+  + ........I–  ®  2Cr3+ + ........I2 + ........H2O
(1)
(c)	The halogen elements have a wide variety of commercial and industrial uses.
	Bromine, for example, is extracted from sea water and used to make compounds such as bromomethane, CH3Br, an insecticide.
(i)	Why must strict precautions be taken if it is necessary to transport bromine from the extraction plant to another manufacturing site?
............................................................................................................................
............................................................................................................................
(1)
(ii)	Suggest ONE environmental concern arising from the use of gaseous bromomethane to destroy insects in a grain store.
............................................................................................................................
............................................................................................................................
(1)
(Total 11 marks)

71.	Cracking is an important process in the petrochemical industry. Cracking the fraction of crude oil with a boiling range of 200–300 °C produces a number of useful alkanes and alkenes.
(a)	Why does the original fraction of crude oil have a boiling range rather than a single boiling point?
.....................................................................................................................................
.....................................................................................................................................
(1)
(b)	The following equation represents one possible reaction which might occur during cracking.

(i)	Give the name of Product 1.
............................................................................................................................
(1)
(ii)	Give the molecular formula of Product 1.
............................................................................................................................
(1)
(iii)	Product 1 of this reaction is used as a component of petrol. Suggest ONE reason why it is more suitable for this use than the original undecane.
............................................................................................................................
............................................................................................................................
(1)
(iv)	State TWO necessary conditions used when cracking petroleum fractions.
............................................................................................................................
............................................................................................................................
(2)

(v)	Draw a labelled diagram showing suitable apparatus and materials for ‘cracking’ a liquid such as ‘light paraffin’ in the laboratory. You should indicate how a gaseous product of the reaction could be collected.

(4)
(c)	There are several isomers of Product 2. One of them, an alkene, can be made from
2-bromo-2-methylpropane, which has the following formula:

(i)	Draw the structural formula of the alkene produced.

(1)
(ii)	Name the type of reaction involved in the conversion of 2-bromo-2-methylpropane to Product 2.
............................................................................................................................
(1)

(iii)	What reagent and conditions would be used to bring about this conversion?
Reagent ..............................................................................................................
Conditions ..........................................................................................................
(2)
(Total 14 marks)

72.	Read straight through the passage below on “Consolidating chemistry”.
Then read it more carefully, in order to answer the following questions.
	Consolidating chemistry
	Conservation is often thought to be concerned mainly with the natural environment, but the conservation of man-made objects is also important. Such objects may be made from wood, glass or metal and also include documents, paintings, buildings and monuments. Important materials used in conservation work are cleaning materials, adhesives, coatings and consolidants.
	Consolidants are materials that restore structural strength to objects in danger of disintegration. Stone buildings and monuments deteriorate for many reasons, such as frost damage, the effects of acid rain and the internal crystallisation of salts. Wooden objects may have lain in water for prolonged periods or been attacked by woodworm.
	A consolidant must be capable of penetrating the object and then binding it together. Normally it needs to be applied as a liquid or solution but then solidify after application. The ease and extent of penetration will depend on the liquid’s viscosity. The viscosity of a liquid is a measure of its mobility or runniness; water has a low viscosity compared to treacle. Ideally a liquid used as a consolidant should have a low viscosity as this will assist penetration. It is sometimes possible to change the viscosity by using a different solvent or increasing the temperature, but this is often undesirable.
	Another factor affecting penetration is the nature of the material. Both wood and stone have an essentially porous structure, but only a fraction of the pores will be connected to the surface and therefore accessible to the consolidant. This fraction is called the effective porosity of the material.

	One successful consolidant is polyethylene glycol (PEG). This is a long-chain polymer with the following structural formula:

	 being the repeat unit in the formula.
	Different varieties of PEG are identified by their molar masses; for example, PEG 1500 has a molar mass of 1500 g mol–1 and an average of around 34 repeat units per molecule. PEG is a good consolidant for wood, which consists chiefly of cellulose, a polymer having a structure made up of the following repeat unit:

	PEG readily forms hydrogen bonds to cellulose.
	Another method of consolidation uses liquids that react together to form a solid product within the damaged or corroded object. A problem here is that the liquids may be too viscous to penetrate effectively and need to be made into solutions. A more successful approach has been to apply the consolidant as a monomer which polymerises within the object. The monomer is likely to have a much lower viscosity than the polymer.
	A good example is the organosilane group of stone consolidants based on trimethoxymethylsilane. The polymerisation reaction takes place in several stages, but its effect is to convert Si ¾ O ¾ C linkages into Si ¾ O ¾ Si linkages and build up a polymer resembling silica (silicon dioxide).
	Silica is found naturally as quartz and sand. It is very stable and has a giant covalent structure in which each silicon atom is joined tetrahedrally to four oxygen atoms, each oxygen being joined in turn to another silicon atom.
	After the reaction has occurred the solid polymer shrinks back on to the pore walls and binds the object together.
(516 words)
	Adapted from Science for Conservators, published by The Conservation Unit, Museums and Galleries Commission.
(a)	(i)	Why is the viscosity of a consolidant liquid or solution important?
(1)
(ii)	Suggest a reason why some liquids have much higher viscosities than others.
(1)
(b)	Explain what is meant by the term effective porosity of a material.
(1)

(c)	Draw a diagram to show one way in which a hydrogen bond can be formed between a repeat unit in a molecule of polyethylene glycol (PEG) and part of a molecule of cellulose.
(1)
(d)	Carry out a calculation to confirm that there are approximately 34 repeat units in one molecule of PEG 1500.
Use the Periodic Table as a source of data.
(2)
(e)	Draw a diagram to illustrate the structure of silica (silicon dioxide).
(1)
(f)	Describe in no more than 120 words:
·	why consolidants are used on stone objects;
·	what properties consolidants must have;
·	how organosilane consolidants act on stone.
(8)
	You are not asked to summarise the whole passage.
	Credit will be given for answers written in good English, using complete sentences and with the correct use of technical words. Avoid copying long sections from the original text. Numbers count as one word, as do standard abbreviations, units and hyphenated words.
Any title you give your passage does not count in your word total.
	There are penalties for the use of words in excess of 120.
(Total 15 marks)

73.	(a)	A few crystals of potassium bromide were dissolved in water in a test tube. A few drops of aqueous chlorine solution were then added and a colour change was observed.
(i)	What colour are potassium bromide crystals?
..........................................................................................................................
(1)
(ii)	What colour was the solution after the chlorine solution had been added?
..........................................................................................................................
(1)

(iii)	Write a balanced ionic equation for the reaction of bromide ions with chlorine solution.
Do not include spectator ions in your equation.
..........................................................................................................................
(1)
(iv)	What would you expect to see if the contents of the test tube were shaken with a small quantity of a hydrocarbon solvent and then allowed to settle?
..........................................................................................................................
..........................................................................................................................
(2)
(b)	(i)	Hydrogen bromide can be made by bubbling hydrogen sulphide gas into liquid bromine.
The equation for the reaction is:
2H2S(g)  +  3Br2(l)  ®  4HBr(g)  +  S2Br2(l)
	Two elements show changes in oxidation number during this reaction.
Identify these elements, stating their original and final oxidation numbers.
	First element.......................................
	Initial oxidation number.............. Final oxidation number .........................
	Second element ..................................
	Initial oxidation number.............. Final oxidation number .........................
(3)
(ii)	The equation shows that 2 moles of hydrogen sulphide molecules react with 3 moles of bromine molecules. Show how your answers to (i) agree with this ratio.

(1)
(c)	The boiling points of three hydrogen halides are given in the following table:
Compound	Boiling point
/K	
HF	293	
HBr	206	
HI	238	

(i)	Explain why hydrogen iodide has a higher boiling point than hydrogen bromide.
..........................................................................................................................
..........................................................................................................................
(2)
(ii)	Explain why hydrogen fluoride has a higher boiling point than either hydrogen bromide or hydrogen iodide.
..........................................................................................................................
..........................................................................................................................
(1)
(iii)	Predict the boiling point, in K, of hydrogen chloride.
Justify your prediction.
	Predicted boiling point.........................
	Justification ......................................................................................................
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
(2)
(Total 14 marks)

74.	This question is concerned with alkenes including ethene and buta-1,3-diene,
CH2 = CHCH = CH2.
(a)	(i)	The typical reactions of alkenes are addition reactions, for example their reactions with bromine.
	Explain why the reaction of ethene with bromine is described as an addition reaction.
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
(1)
(ii)	Why should this reaction be carried out in the absence of sunlight or ultra-violet radiation?
..........................................................................................................................
..........................................................................................................................
(1)

(iii)	Explain how, in this reaction, the bromine molecule is able to act as an electrophile, even though it is normally non-polar.

(2)
(b)	(i)	When 1 mole of bromine molecules is added to 1 mole of buta-1,3-diene, the principal product is 1,4-dibromobut-2-ene, CH2BrCH = CHCH2Br, a compound which exists as two geometric isomers.
	Draw the displayed formulae of both of these two isomers.

(2)
(ii)	State why geometric isomerism is possible in 1,4-dibromobut-2-ene.
[You may find it helpful to refer to the formulae you have drawn above].
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
(1)
(c)	It has been suggested that cyclohexene, another alkene, could be made by a reaction between ethene and buta-1,3-diene.
	Using molecular formulae, the reaction could be represented by the equation
C2H4(g)   +    C4H6(g)    C6H10(l)        ΔH = –200 kJ mol–1

(i)	Draw the displayed formula of cyclohexene.

(1)
(ii)	Decide whether high or low temperature and pressure would give the higher proportion of cyclohexene at equilibrium. Justify your choice in each case.
Temperature .....................................................................................................
..........................................................................................................................
Pressure ............................................................................................................
..........................................................................................................................
..........................................................................................................................
(2)
(Total 10 marks)

75.	This question is concerned with hydrogen cyanide, HCN.
(a)	(i)	Draw a ‘dot and cross’ diagram for a molecule of hydrogen cyanide, showing outer shell electrons only.

(1)
(ii)	Predict the numerical value of the bond angle in hydrogen cyanide. 
Justify your prediction.
Bond angle ........................................................................................................
Justification .......................................................................................................
..........................................................................................................................
..........................................................................................................................
(2)
(b)	Write a balanced equation for the combustion of hydrogen cyanide in oxygen, assuming that the products are water, carbon dioxide and nitrogen.

(1)
(c)	Hydrogen cyanide is an extremely toxic, volatile liquid that is used to make useful compounds, such as ‘Perspex’. Do you think it is acceptable for it to be used in this way?
	Justify your answer.
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
(1)
(d)	The standard enthalpy change of formation, ΔHfο, for gaseous hydrogen cyanide, HCN(g), is +110 kJ mol–1.
	The standard molar enthalpy changes of atomisation of hydrogen, carbon and nitrogen are given below, in kJ mol–1.
	ΔHοat/kJ mol–1
	½H2(g) ® H(g)	+ 218
C(s, graphite) ® C(g)	+ 717
N2(g) ® N(g)	+ 473
	The C–H bond energy in hydrogen cyanide is + 413 kJ mol–1.
	This information can be represented on a Hess cycle in the following way, and then used to calculate a bond energy.

(i)	Insert formulae, showing the correct quantities of each element, into the appropriate boxes.
(2)
(ii)	Insert arrows between the boxes and write the correct numerical data alongside the appropriate arrows.
(2)
(iii)	Use the cycle to calculate ΔHοat[HCN(g)] and then the carbon to nitrogen bond energy in hydrogen cyanide.

(1)
(Total 10 marks)

76.	One of the most important industrial uses of chlorine is in the production of poly(chloroethene), usually called PVC. A sequence of reactions used to make PVC is set out below.

(a)	(i)	Name compound A.
..........................................................................................................................
(1)
(ii)	Give the structural formula of chloroethene.

(1)

(iii)	It is important that hydrogen chloride gas is not allowed to escape into the atmosphere.
	Suggest a way in which its escape could be prevented.
..........................................................................................................................
..........................................................................................................................
(1)
(b)	(i)	Chloroethene is polymerised by a reaction involving free radicals. Explain what is meant by a free radical.
..........................................................................................................................
..........................................................................................................................
(1)
(ii)	Give a necessary condition for the production of free radicals.
..........................................................................................................................
..........................................................................................................................
(1)
(c)	(i)	Ethane-1,2-diol, CH2OHCH2OH, is a useful compound which could be made from compound A using a nucleophilic substitution reaction.
	Suggest a suitable nucleophile for this reaction.
..........................................................................................................................
(1)
(ii)	The diagram below shows part of the formula of compound A. Use the diagram to show how your suggested nucleophile attacks A.

(2)
(iii)	What is the leaving group in this reaction?
..........................................................................................................................
(1)

(iv)	Suggest a suitable chemical test you could use to confirm the identity of this leaving group.
	You should state the reagent you would use and give the observation expected.
..........................................................................................................................
..........................................................................................................................
(2)
(Total 11 marks)

77.	Read the passage below on EMISSION CONTROL straight through and then more carefully, in order to answer the following questions.
	EMISSION CONTROL
	Photochemical smog, a brown haze just above the horizon, was first observed in Los Angeles in the 1940s. Research showed that the smog was produced by the action of sunlight on air containing a variety of pollutants, many of them produced by burning fuels.
	In a conventional petrol engine, a mixture of fuel and air is ignited by a spark. Petrol is essentially a hydrocarbon mixture so if combustion is complete the reaction products are carbon dioxide and water. The proportions of air and fuel are vitally important; the most efficient is the stoichiometric ratio. This is the ratio of air to fuel which, in theory, leads to complete combustion of the fuel. In practice, incomplete combustion results in the formation of toxic carbon monoxide and also leaves unburnt hydrocarbons which are the main cause of photochemical smogs. There is also reaction between nitrogen and oxygen gases in the engine, producing oxides of nitrogen, usually referred to as NOx.
	‘Lean-burn’ engines use a higher air:fuel ratio. They work at lower temperatures, producing less NOx but more unburnt hydrocarbons. Diesel engines also work at lower temperatures, producing less of all the gaseous pollutants but significantly more solid particles such as carbon.
	Catalytic converters are fitted to vehicle exhaust systems to deal with gaseous pollutants. The earliest catalysts tried were metals such as nickel, copper and cobalt. These were relatively cheap but were adversely affected by high temperatures and were ‘poisoned’ by sulphur and lead compounds in the fuel. Platinum was found to be very much more satisfactory. Very small particles of the metal were supported on a ceramic honeycomb to increase its surface area.
	Getting the right conditions for combustion is not easy. Carbon monoxide and unburnt hydrocarbons need to be oxidised but the nitrogen oxides have to be reduced to nitrogen gas. An ingenious solution is to use the nitrogen oxides to oxidise the other pollutants, but this still requires some oxygen for complete removal of the hydrocarbons and carbon monoxide. Modern three-way catalysts (so-called because they deal with all three types of pollutants) only work effectively if the composition of the original air:petrol mixture is carefully controlled; too much fuel will mean insufficient oxygen in the exhaust fumes. The exhaust system must therefore be fitted with oxygen sensors linked back to an electronically controlled fuel injection system. Efficiency is also improved by incorporating other ‘platinum group’ metals such as rhodium and sometimes palladium.

	Catalysts for diesel-engined vehicles have to include an efficient filter to remove carbon particles. This leads to a further problem because the solid carbon (soot) needs to be burned to allow the filter to continue to work effectively. Soot combustion needs a temperature of 500–600°C and the exothermic reaction with oxygen can raise it to as much as 1000 °C,which can damage the filter. This problem has been tackled by first converting all nitrogen oxides to nitrogen dioxide (NO2) using a platinum catalyst, and then using the nitrogen dioxide to oxidise the carbon, a reaction which takes place at 200 °C.
	The catalyst will only work if it is hot. Platinum needs a temperature of 240°C but rhodium helps by reducing this to 150°C. This means that the catalyst is ineffective when the engine is first started, a major problem under investigation. Unleaded fuel is essential and the introduction of ‘very low sulphur’ fuel has helped to eliminate the bad-egg smell noticed if hydrogen sulphide is produced.
(573 words)
	Adapted from ‘Emission control’ by B. Harrison, Education in Chemistry, September 2000.
(a)	Why is the brown smog first seen in Los Angeles said to be photochemical?
(1)
(b)	Write a balanced equation for the reaction between carbon monoxide and nitrogen monoxide (NO), producing carbon dioxide and nitrogen gas.
(1)
(c)	Explain what is meant by the incomplete combustion of a fuel.
(1)
(d)	Why do conventional petrol engines give higher NOx emissions than ‘lean-burn’ or diesel engines?
(1)
(e)	Three ‘platinum group’ metals used in catalyst systems are mentioned in the passage.
Suggest another metal of this type which might be tried in the search for improved catalysts.
(1)
(f)	Why has it been found necessary to change the composition of motor fuel for engines fitted with converters?
(1)
(g)	Why are catalysts least effective when the engine is first started?
(1)
(h)	Describe, in no more than 120 words, how major exhaust pollutants are dealt with by modern exhaust systems.
(8)

	You are not asked to summarise the whole passage, nor to include equations in your summary.
	At the end of your summary state the number of words you have used.
	Credit will be given for answers written in good English, using complete sentences and using technical words correctly and chemical names rather than formulae. Avoid copying long sections from the original text. Numbers count as one word, as do standard abbreviations, units and hyphenated words. Any title you give your passage does not count in your word total.
	There are penalties for the use of words in excess of 120.
(Total 15 marks)

78.	The flow chart below shows some of the reactions of alcohols, alkanes, alkenes and halogenoalkanes.

(a)	Compound A, C4H9Cl, is a tertiary halogenoalkane.
	Draw the displayed formula for A and state its name.
	Displayed formula

	Name ..........................................................................................................................
(2)
(b)	The halogenoalkane A can be formed from the alcohol D.
(i)	What reagent could be used to bring about this reaction? ...............................
..........................................................................................................................
(1)
(ii)	State the type and mechanism of this reaction.
Type ................................................................................................................
Mechanism ......................................................................................................
(2)

(c)	When Ais heated with concentrated, alcoholic potassium hydroxide, only one alkene, B, can be formed.
(i)	Draw the displayed formula for B and state its name.
	Displayed formula

	Name ................................................................................................................
(2)
(ii)	Can B exist as geometric isomers? Explain your answer.
...........................................................................................................................
...........................................................................................................................
...........................................................................................................................
(1)
(d)	State the type of reaction in the conversion of
B to C .........................................................................................................................
B to E ..........................................................................................................................
(2)
(e)	Draw the displayed formula for E.

(1)
(Total 11 marks)

79.	The reaction between chlorine and methane, in the presence of ultraviolet light, involves the formation of free radicals and includes the following steps:
A	Cl2 ®	2Cl•	ΔΗο = +242 kJ mol–1
B	CH4 + Cl• ®	HCl + CH3•	ΔΗο = +4 kJ mol–1
C	Cl2 + CH3• ®	CH3Cl + Cl•	ΔΗο = –97 kJ mol–1
D	Cl• + Cl• ®	Cl2
E	CH3• + CH3•  ®	CH3CH3
F	Cl• + CH3• ®	CH3Cl	ΔΗο = –339 kJ mol–1
(a)	(i)	What is meant by a free radical? ....................................................................
...........................................................................................................................
...........................................................................................................................
 (1)
(ii)	Draw a ‘dot-and-cross’ diagram, showing outer shell electrons only, for a chlorine free radical.

(1)
(iii)	What type of bond breaking occurs in step A?
...........................................................................................................................
(1)
(b)	Which of the steps, A to F, are chain propagation steps?
...........................................................................................................................
(1)
(c)	(i)	Write the equation for the overall reaction between one mole of chlorine and one mole of methane molecules.

(1)

(ii)	Calculate the standard enthalpy change, ΔΗο, for this reaction.

(2)
(d)	(i)	What is the value of ΔΗο for step D? ................................................................
(1)
(ii)	Would you expect step E to be exothermic or endothermic? Justify your answer.
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
(1)
(e)	The overall reaction was repeated using bromine gas instead of chlorine gas.
	Would you expect step A for bromine to be more or less endothermic than step A for chlorine? Justify your answer.
....................................................................................................................................
....................................................................................................................................
....................................................................................................................................
(2)
(Total 11 marks)

80.	This question is about a mixture, M, which contains 80% of potassium bromide, KBr, 
and 20% of potassium chloride, KCl, by mass.
(a)	When concentrated sulphuric acid was added to mixture M and the mixture gently warmed, four different gases were given off.
(i)	What would happen to moist universal indicator paper when held in this mixture of gases?
..........................................................................................................................
..........................................................................................................................
(1)

(ii)	What would you see if a glass rod, dipped into concentrated ammonia solution, was held in these gases?
..........................................................................................................................
..........................................................................................................................
(1)
(iii)	Which of these gases would be responsible for turning potassium dichromate(VI) solution from orange to green?
..........................................................................................................................
(1)
(b)	(i)	What would you see when silver nitrate solution is added to a solution of mixture M?
..........................................................................................................................
..........................................................................................................................
(1)
(ii)	After adding silver nitrate solution to a solution of mixture M in (b)(i), concentrated ammonia solution is added.
	What would you see? Justify your answer.
..........................................................................................................................
..........................................................................................................................
(2)

(c)	The graph below shows how the solubilities of potassium bromide and potassium chloride vary with temperature.

(i)	What is meant by a saturated solution?
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
(2)

(ii)	125 g of mixture M, containing 80% KBr and 20% KCl by mass, was added to 100 g of boiling water to make a solution.
	Calculate the masses of KBr and KCl present in mixture M.

	Use your calculation and the graph to predict which crystals will form on cooling to 20 °C, and what will be left in the solution.
..........................................................................................................................
..........................................................................................................................
(3)
(Total 11 marks)