equilibria_a2

NT Exampro 1

1. Propanoic acid is a weak acid; explain the term weak.

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2. (i) Give an equation for the dissociation of propanoic acid and hence an expression for its dissociation constant, Ka.

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(ii) At 25°C Ka for propanoic acid is 1.30 × 10–5 mol dm–3. Find the pH of a solution of propanoic acid of concentration 0.0100 mol dm–3. State any assumptions you make.

(iii) Increasing the temperature of the propanoic acid solution causes the pH to decrease. What does this tell you about the enthalpy of dissociation? Justify your answer.

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3. (i) Define pH. .......................................................................................................…….

(ii) Define Kw, the ionic product of water.

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4. (i) What is the principal property of a buffer solution?

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NT Exampro 2

(ii) The dissociation constant for ethanoic acid is 1.80 × 10–5 mol dm–3. Calculate the pH of

a buffer solution which has a concentration of 0.0150 mol dm–3 with respect to ethanoic acid and 0.0550 mol dm–3 with respect to sodium ethanoate.

(Total 5 marks)

5. Phosphoric(V) acid, H3PO4 , is a tribasic acid.

Write the formulae of the potassium salts of this acid.

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6. For the reaction

CaCO3(s) CaO(s) + CO2(g)

the equilibrium total pressure at 1200 K is 4 atm. Determine Kp. (3)

7. (a) State Raoult’s Law for an ideal binary liquid mixture.

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(b) Liquid oxygen and liquid nitrogen form an ideal liquid mixture. At 70 K the vapour pressures are 38.3 kPa for N2 and 6.40 kPa for O2. Find the composition of the vapour in equilibrium with a liquid mixture which at equilibrium is an equimolar mixture of the two elements.

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NT Exampro 3

(c) Show by means of sketches of boiling point/composition diagrams and a brief comment

why ideal mixtures of liquids of similar boiling temperature are more difficult to separate by fractional distillation than those with boiling temperatures more widely separated.

(3)

(d) The vapour pressure/composition diagram at constant temperature for a mixture of two liquids A and B which shows a strong deviation from Raoult’s Law is shown below. (x(A) = mole fraction of A).

x( ) = 1A

vapourpressure

x( ) = 0Amole fraction

(i) Sketch on the axes below the boiling point/composition diagram for this system, showing the curves for both liquid and vapour.

x( ) = 1A

boilingtemperature


(ii) Consider a mixture for which the mole fraction of A is initially 0.75. Use your diagram to explain what happens when this mixture is fractionally distilled, making clear the nature of the distillate and of the residue.

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(Total 16 marks)

8. State Raoult’s Law for an ideal binary liquid mixture.

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NT Exampro 4

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9. Liquid oxygen and liquid nitrogen form an ideal liquid mixture. At 70 K the vapour pressures are 38.3 kPa for N2 and 6.40 kPa for O2. Find the composition of the vapour in equilibrium with a liquid mixture which at equilibrium is an equimolar mixture of the two elements.

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10. Show by means of sketches of boiling point/composition diagrams and a brief comment why ideal mixtures of liquids of similar boiling temperature are more difficult to separate by fractional distillation than those with boiling temperatures more widely separated.

(3)

11. The vapour pressure/composition diagram at constant temperature for a mixture of two liquids A and B which shows a strong deviation from Raoult’s Law is shown below. (x(A) = mole fraction of A).

x( ) = 1A

vapourpressure

x( ) = 0Amole fraction www.

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NT Exampro 5

(i) Sketch on the axes below the boiling point/composition diagram for this system, showing

the curves for both liquid and vapour.

x( ) = 1A

boilingtemperature


(ii) Consider a mixture for which the mole fraction of A is initially 0.75. Use your diagram to explain what happens when this mixture is fractionally distilled, making clear the nature of the distillate and of the residue.

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12. (a) Ammonia is manufactured by direct synthesis in the Haber Process:

N2(g) + 3H2(g) 2NH3(g) ∆H = –92 kJ mol–1

(i) Write an expression for the equilibrium constant, Kc, for this reaction and give its units.

(ii) When 3 mol of hydrogen and 1 mol of nitrogen were allowed to reach equilibrium in a vessel of 1 dm3 capacity at 500°C and 1000 atm pressure, the equilibrium mixture contained 0.27 mol of N2, 0.81 mol of H2 and 1.46 mol of NH3.

Calculate Kc at this temperature.

(4)

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NT Exampro 6

(b) Predict and explain the effect of an increase in temperature on:

(i) the value of Kc;

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(ii) the rate of the forward reaction.

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(Total 10 marks)

13. When ammonium salts are dissolved in water, the following reaction occurs.

NH4+ (aq) + H2O(1) NH3(aq) + H3O+ (aq)

(a) Identify the acid/base conjugate pairs in this reaction by writing appropriate symbols under each of the species in the equation above.

(b) Write an expression for the dissociation constant, Ka, for NH4+ (aq).

(c) Calculate the pH of a solution of ammonium chloride of concentration 0.100 mol dm–3 at 298 K, the Ka value for NH4+ being 5.62 × 10–10 mol dm–3 at this temperature.

(Total 5 marks)

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NT Exampro 7

14. Benzene and methylbenzene may be separated by fractional distillation. Sketch the general

form of the boiling point/composition diagram for such a mixture and use it to explain the basis on which fractional distillation rests.

0 1

b.p. of benzene

temperature/ºC

b.p. of methylbenzene

mole fraction of methylbenzene

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15. (a) When ethanoic acid is dissolved in water, the following equilibrium is established:

CH3CO2H + H2O CH3CO2– + H3O+ I

When hydrogen chloride dissolves in ethanoic acid, the equilibrium established is:

CH3CO2H +HCI CH3CO2H2+ + Cl– II

Comment on the role of the ethanoic acid in:

(i) equilibrium I; .................................................................................................

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(ii) equilibrium II; ................................................................................................

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(b) What is the relationship between the species CH3CO2H2+ and CH3CO2H?

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NT Exampro 8

(c) The value of Ka for ethanoic acid at 298 K is 1.74 × 10–5 mol dm–3 and for methanoic

acid, HCO2H, it is 1.60 × 10–4 mol dm–3 at the same temperature.

(i) Write an expression for Ka for CH3CO2H.

(1)

(ii) Hence calculate the pH of a 0.100 mol dm–3 solution of CH3CO2H at 298 K.

(2)

(d) The pH of a 0.050 mol dm–3 solution of HCO2H is 2.55.

Using this, together with the data in (c) and your answer to (c)(ii):

(i) state which of the two acids is the stronger;

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(ii) comment on the relative pH values of the two acids.

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(e) (i) Sketch with reasonable accuracy, on the axes below, how the pH changes during the titration of 20.0 cm3 of a 0.100 mol dm–3 solution of methanoic acid with 0.050 mol dm–3 sodium hydroxide solution.

Volume of NaOH solution/cm

12

10

8

6

4

2

00 40 45 505 10 15 20 25 30 35 55 60

pH

3 (3)

(ii) Select using the data below a suitable indicator for this titration. Give a brief

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NT Exampro 9

reason for your choice based on the curve drawn in (e)(i).

Indicator pH Range

Bromocresol green 3.5 – 5.4

Bromothymol blue 6.0 – 7.6

Phenol red 6.8 – 8.4

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(Total 15 marks)

16. This question concerns redox behaviour, and the following data will be found useful.

E /V Fe3+(aq) + e– Fe2+(aq) +0.77 ½Cl2(g) + e– Cl–(aq) +1.36 MnO4– (aq) + 8H+ (aq) + 5e– Mn2+(aq) + 4H2O(1) +1.52

(a) In potassium manganate(VII) titrations, the solutions are acidified with dilute sulphuric acid.

(i) Using the data above, explain why dilute hydrochloric acid is not used for this purpose.

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(ii) Why is potassium manganate(VII) usually placed in the burette, despite the difficulties it presents in reading the burette?

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(b) A test for Mn2+ ions in solution is to react them with sodium bismuthate(V), NaBiO3, in the presence of nitric acid. A purple colour will develop owing to the formation of MnO4

– ions in the solution. The ionic half equation for the reduction of BiO3– ion is:

BiO3– + 6H+ + 2e– Bi3+ + 3H2O

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NT Exampro 10

(i) Use this half equation and the data above to write an ionic equation for the

oxidation of the Mn2+ ion.

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(ii) Suggest, qualitatively, how the E value for BiO3– |Bi3+ compares with that for

MnO4– |Mn2+

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(c) An alloy contains iron and manganese only. On warming with dilute nitric acid 2.30 g of this alloy gave a solution containing iron(III) ions and manganese(II) ions. Treatment of this solution with excess sodium bismuthate(V) completely oxidised all the Mn2+ ions present to MnO4

– ions.

The excess bismuthate(V) ions were then completely destroyed and the solution made up to 250 cm3 with distilled water and thoroughly shaken.

Titration of 25.0 cm3 portions of this solution required 25.0 cm3 of standard 0.100 mol dm–3 iron(II) sulphate solution.

(i) Write the equation for the reaction occurring during the titration.

.......................................................................................................................... (2)

(ii) Calculate the percentage of manganese present in the alloy.

(5)

(Total 15 marks)

17. Hydrogen and iodine react together to give an equilibrium:

H2(g) + I2(g) 2HI(g)

(a) Write an expression for Kp for this equilibrium, giving consideration to its units.

(2)

(b) When 0.50 mol of I2 and 0.50 mol of H2 were mixed in a closed container at 723 K and 2 atm pressure, 0.11 mol of I2 were found to be present when equilibrium was established.

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NT Exampro 11

(i) Calculate the partial pressures of I2, H2 and HI in the equilibrium mixture.

(3)

(ii) Hence calculate the value of Kp at 723 K.

(2)

(c) In an experiment to establish the equilibrium concentrations in (b), the reaction was allowed to reach equilibrium at 723 K and then quenched by addition to a known, large volume of water. The concentration of iodine in this solution was then determined by titration with standard sodium thiosulphate solution.

(i) Write an equation for the reaction between sodium thiosulphate and iodine.

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(ii) What indicator would you use? Give the colour change at the end point.

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(iii) In this titration and in titrations involving potassium manganate(VII), a colour change occurs during reaction. Why is an indicator usually added in iodine/thiosulphate titrations but not in titrations involving potassium manganate(VII)?

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(Total 13 marks)

18. (a) Hydrogen peroxide reacts both as an oxidising agent and as a reducing agent, depending on the conditions. Hydrogen peroxide reduces potassium dichromate(VI) in acidic solution to chromium(III) ions. Hydrogen peroxide will oxidise chromium(III) ions back to chromate(VI) ions in alkaline solution.

Cr2O72–(aq) + 14H+(aq) + 6e– → 2Cr3+(aq) + 7H2O(l) E = +1.33 V

O2(g) + 2H+(aq) + 2e– → H2O2(aq) E = +0.68 V

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NT Exampro 12

Deduce the overall equation for the reaction between hydrogen peroxide and

dichromate(VI) ions using the data above.

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(b) In alkaline solution the reaction of hydrogen peroxide with chromium(III) ions is

2Cr3+(aq) + 10OH–(aq) + 3H2O2(aq) → 2CrO42–(aq) + 8H2O(l)

This reaction is used to prepare potassium dichromate(VI) from chromium(III) chloride and hydrogen peroxide in potassium hydroxide solution. The mixture is boiled until it is bright yellow. Boiling is continued until excess hydrogen peroxide has been destroyed. The solution is then cooled, and acidified with ethanoic acid.

(i) The reaction occurring on acidification with ethanoic acid is:

2CrO42–(aq) + 2H+(aq) → Cr2O72– (aq) + H2O(l)

Show that this is not a redox reaction.

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(ii) Hydrogen peroxide on heating reacts as follows:

2H2O2(aq) → 2H2O(l) + O2(g)

Suggest how you would know when all the hydrogen peroxide has been destroyed in the reaction mixture.

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(iii) Why is it essential to destroy all the hydrogen peroxide in the mixture before it is acidified?

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NT Exampro 13

(iv) Derive the two half equations which together give the overall equation for the

reaction between hydrogen peroxide and chromium(III) ions in alkaline solution.

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(Total 10 marks)

19. (a) At high temperatures phosphorus pentachloride is a gas which dissociates as follows

PCl5 (g) PCl3 (g) + Cl2 (g)

(i) Write an expression for the equilibrium constant KP for this equilibrium. (1)

(ii) At a given temperature 39% of an original sample of the PCl5(g) is dissociated at equilibrium. If the total equilibrium pressure is 2 atm, calculate the values of the equilibrium partial pressures of:

PCl5

PCl3 (4)

(iii) Hence calculate the value of KP and give its units. (2)

(b) Aluminium chloride occurs in the anhydrous state and in the hydrated state. You may regard the structure of the anhydrous state as having the formula AlCl3. When water is added to solid anhydrous aluminium chloride, steamy acidic fumes are seen.

(i) What are the steamy fumes?

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(ii) Write an equation for the reaction occurring.

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(iii) Explain by reference to the structure of AlCl3 how the first step of this reaction occurs.

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NT Exampro 14

(c) (i) When water is added to hydrated aluminium chloride no steamy fumes are seen.

What does this indicate about the bonding in hydrated aluminium chloride?

………………………………………………………………………………… (1)

(ii) Hydrated aluminium chloride dissolves in water to give a solution which is acidic. Suggest an explanation for the fact that the solution is acidic. Give an equation to illustrate your answer.

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(Total 14 marks)

20. (a) At high temperatures phosphorus pentachloride is a gas which dissociates as follows

PCl5 (g) PCl3 (g) + Cl2 (g)

(i) Write an expression for the equilibrium constant KP for this equilibrium. (1)

(ii) At a given temperature 39% of an original sample of the PCl5(g) is dissociated at equilibrium. If the total equilibrium pressure is 2 atm, calculate the values of the equilibrium partial pressures of:

PCl5

PCl3 (4)

(iii) Hence calculate the value of KP and give its units. (2)

(Total 7 marks)

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NT Exampro 15

(iv) Derive the two half equations which together give the overall equation for the

reaction between hydrogen peroxide and chromium(III) ions in alkaline solution.

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(Total 10 marks)

21. This question concerns the lead-acid battery.

The following data will be required.

E /V

PbO2(s) + 4H+(aq) + –24SO (aq) + 2e– PbSO4(s) + 2H2O(1) +1.69

PbSO4(s) + 2e– Pb(s) + –24SO (aq) –0.36

(a) Give the equation for the overall cell reaction during discharge.

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(b) (i) State the essential requirement for the rusting of iron in water.

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(ii) Explain why corrosion of iron results in deep pitting of the metal surface.

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(iii) Explain why sheet iron which has been fabricated to a particular shape, sometimes under high pressure, is more likely to corrode than a single strip of pure iron.

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NT Exampro 16

(iv) An underground iron pipe is less likely to corrode if bonded at intervals to

magnesium stakes. Give a reason for this. Explain why aluminium would be a poor substitute for the magnesium.

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(Total 8 marks)

22. Iron is extracted from its oxides in a blast furnace. Carbon, in the form of coke, is one of the raw materials mixed with the iron ore.

(a) The reaction between iron(III) oxide and carbon monoxide to produce iron occurs in many stages; one stage might be represented by the equation

Fe2O3(s) +CO(g) 2FeO(s) + CO2(g)

Write the expression for the equilibrium constant, Kp, for this reaction. (1)

(b) When an acidified solution of potassium dichromate(VI), K2Cr2O7, is added to a solution of an iron(II) compound the dichromate(VI) ions are reduced to chromium(III) ions and the iron(II) ions are oxidised to iron(III) ions.

(i) Write an ionic half equation for the reduction of dichromate(VI) ions in acidic solution.

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(ii) Write an equation for the reaction between dichromate(VI) and iron(II) ions.

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NT Exampro 17

(c) A 0.204 g sample of steel was reacted with excess dilute sulphuric acid. The resulting

solution required 27.4 cm3 of 0.0220 mol dm–3 potassium dichromate(VI) solution for complete reaction.

Calculate the percentage of iron in the sample of steel.

(4)

(Total 8 marks)

23. (a) Use Brønsted-Lowry theory of acid/base behaviour and acid/base conjugate pairs to explain how sulphuric acid shows its acidic nature when added to water.

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(b) If the enthalpy of neutralisation ∆Hneut is measured for hydrobromic acid, HBr, or for nitric acid, HNO3, both with sodium hydroxide solution, the value is –57.6 kJ mol–1; for the reaction of ethanoic acid, CH3COOH, with sodium hydroxide solution it is –55.2 kJ mol–1.

(i) Why is ∆Hneut the same for both HBr and HNO3 with NaOH?

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(ii) Give one reason why ∆Hneut for the reaction of ethanoic acid with sodium hydroxide solution is less exothermic than for hydrobromic acid with sodium hydroxide solution.

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NT Exampro 18

(c) (i) What is the function of a buffer solution?

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(ii) When sodium hydrogensulphate, Na+HSO4–, is added to a solution of sodium sulphate the mixture behaves as a buffer solution. Show how it functions by writing equations for the reactions which occur on the addition of acid or base:

acid: .........................................................……………………………………..

base: .........................................................…………………………………….. (2)

(d) Sulphuric acid is used in the production of soapless detergents.

(i) Why is sulphuric acid used in the production of these materials?

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(ii) Briefly indicate the advantages of soapless detergents over soap itself.

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(Total 15 marks)

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NT Exampro 19

24. Use the following redox potential data to answer the questions that follow

Electrode reaction E /V

Fe(OH)3 +e– Fe(OH)2 + OH– –0.56

Fe2+ +2e– Fe –0.44

Fe3+ +3e– Fe –0.04

H+ +e– 1/2H2 0.00

1/2O2+H2O +2e– 2OH– +0.40

Fe3+ +e– Fe2+ +0.77

1/2Cl2 +e– Cl– +1.36

Co3+ +e– Co2+ +1.82

(a) (i) Explain why the reaction of iron metal with dilute, aqueous hydrochloric acid gives iron (II) chloride and not iron (III) chloride.

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(ii) Suggest how an aqueous solution of iron (III) chloride could be made from iron metal. Justify your answer.

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(b) Suggest, in outline, the stages in the rusting of iron, using the data above.

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(Total 8 marks)

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NT Exampro 20

25. (a) (i) Calculate the concentration, in mol dm–3, of a solution of hydrochloric acid, HCl,

which has a pH of 1.13.

(1)

(ii) Calculate the concentration, in mol dm–3 , of a solution of chloric(l) acid, HOCl, which has a pH of 4.23. Chloric(l) acid is a weak acid with Ka = 3.72 × 10–8 mol dm–3.

(4)

(b) The pH of 0.100 mol dm–3 sulphuric acid is 0.98.

(i) Calculate the concentration of hydrogen ions, H+, in this solution.

(1)

(ii) Write equations to show the two successive ionisations of sulphuric acid, H2SO4, in water.

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(iii) Suggest why the concentration of hydrogen ions is not 0.20 mol dm–3 in 0.100 mol dm–3 sulphuric acid.

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NT Exampro 21

(c) Many industrial organic reactions produce hydrogen chloride as an additional product.

This can be oxidised to chlorine by the Deacon process:

4HCl(g) + O2(g) 2Cl2(g) + 2H2O(g) ∆H = –115 kJ mol–1.

0.800 mol of hydrogen chloride was mixed with 0.200 mol of oxygen in a vessel of volume 10.0 dm3 in the presence of a copper(I) chloride catalyst at 400 ºC. At equilibrium it was found that the mixture contained 0.200 mol of hydrogen chloride.

(i) Write an expression for the equilibrium constant Kc.

(1)

(ii) Calculate the value of Kc at 400 ºC.

(4)

(d) State and explain the effect, if any, on the position of equilibrium in (c) of:

(i) decreasing the temperature;

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(ii) decreasing the volume;

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NT Exampro 22

(iii) removing the catalyst.

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(Total 20 marks)

26. (a) The acid dissociation constant, Ka for ethanoic acid is 1.80 × 10–5 mol dm–3.

(i) Write an equation for the dissociation of aqueous ethanoic acid.

...................................................................................................................... (1)

(ii) Give the expression for the acid dissociation constant, Ka, for aqueous ethanoic acid.

(1)

(iii) Calculate the pH of 2.00 mol dm–3 ethanoic acid.

(3)

(b) When ethanoic acid and propan-1-ol are heated together in the presence of a little concentrated sulphuric acid an ester is produced.

(i) Draw the structural formula of the ester produced in this reaction.

(2)

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NT Exampro 23

(ii) Give the name of the ester.

...................................................................................................................... (1)

(iii) State the function of the sulphuric acid.

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(c) 6.00g of ethanoic acid was mixed with 6.00g of propan-1-ol and 0.00100mol of concentrated sulphuric acid was added. The mixture was heated under reflux until no further reaction was detectable. The mixture was then cooled rapidly to room temperature and titrated with 1.00 mol dm–3 sodium hydroxide solution. 35.0 cm3 of the 1.00 mol dm–3 sodium hydroxide solution was required for complete neutralisation.

(i) Calculate the volume of sodium hydroxide solution that would have been needed if only the 0.00100mol of concentrated sulphuric acid had been in the flask.

(2)

(ii) Calculate the volume of sodium hydroxide solution that must have been needed to react with the ethanoic acid remaining in the cooled mixture.

(1)

(iii) Calculate the amount (in moles) of ethanoic acid remaining.

(1) ww

w.ch

emac

tive.

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NT Exampro 24

(iv) Hence, calculate the amount (in moles) of propan-1-ol, ester and water in the final

cooled mixture.

(3)

(v) Calculate the value of the equilibrium constant under these conditions. The volume of the reaction mixture should be taken as 100 cm3.

(2)

(d) The reverse of esterification is hydrolysis. In experiments to study the hydrolysis of this ester with aqueous sodium hydroxide, the following data were obtained.

Experiment

Concentration of ester

/mol dm–3

Concentration of sodium hydroxide

/mol dm–3

Rate of reaction /mol dm–3 s–1

1 0.1 1.0 6 × 10–4

2 0.1 2.0 1.2 × 10–3

3 0.2 2.0 2.4 × 10–3

(i) Determine the order of the reaction with respect to the:

ester;.............................................................................................................. (1)

sodium hydroxide........................................................................................... (1)

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NT Exampro 25

(ii) Calculate the value of the rate constant, k, for this reaction including its units.

(3)

(Total 23 marks)

27. (a) (i) Define pH ………….……………………………………………………... (1)

(ii) Define the term ‘weak acid ’ as applied to methanoic acid, , HCOOH.

….………………………………………………………………………….

….…………………………………………………………………………. (2)

(b) Calculate the pH of the following solutions: (The ionic product of water, Kw = 1.00 × 10–14 mol2 dm–6 at 25 °C. The acid dissociation constant for methanoic acid is 1.78 × 10–4 mol dm–3).

(i) a solution of hydrochloric acid of concentration 0.152 mol dm–3;

(2)

(ii) a solution of sodium hydroxide of concentration 0.747 mol dm–3;

(3)

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NT Exampro 26

(iii) a solution of methanoic acid of concentration 0.152 mol dm–3.

(4)

(c) (i) What is the principal property of a buffer solution?

….………………………………………………………………………….

….…………………………………………………………………………. (2)

(ii) The acid dissociation constant, Ka, for ethanoic acid is 1.80 × 10–5 mol dm–3. Calculate the pH of a buffer solution which has a concentration of 0.105 mol dm–3

with respect to ethanoic acid and 0.342 mol dm–3 with respect to sodium ethanoate.

(3)

(Total 17 marks)

28. (a) Write an expression for Kp for each of the following equilibria, giving the units in each case.

(i) N2O4 (g) 2NO2 (g)

Kp =

(2)

(ii) CaCO3 (s) CaO(s)+CO2 (g)

Kp =

(2)

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NT Exampro 27

(b) (i) With reference to the equilibrium in (a) (i), calculate the equilibrium partial

pressures of N2O4 and NO2 at 60 °C and 1.2 atm pressure, given that 81%of the initial N2O4 is dissociated at this temperature.

(5)

(ii) Calculate Kp at this temperature.

(1)

(c) The equilibrium reaction

2SO2(g) + O2(g) ↔ 2SO3(g) ∆H = –196 kJ mol–1

is used in the manufacture of sulphuric acid and uses vanadium(V) oxide as the catalyst.

(i) Write the expression for Kc for this equilibrium.

(1)

(ii) What effect does the catalyst have on the position of equilibrium in this reaction?

….…………………………………………………………………………….

….……………………………………………………………………………. (1)

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NT Exampro 28

(iii) A steel vessel of volume 2.0 dm³ has introduced into it 0.20 mol of SO3, 0.040 mol

of SO2 and 0.010 mol of O2. By calculation of the apparent value of Kc show that this mixture is not at equilibrium, and explain in which direction the system will move in order to achieve equilibrium at a temperature of 800K. The value of Kc at this temperature is 1.7 × 106 mol–1 dm³.

(3)

(Total 15 marks)

29. (a) What is meant by the term weak acid?

....................................................................................................................................

....................................................................................................................................

.................................................................................................................................... (2)

(b) A weak acid, represented by HA, dissociates in water according to the equation:

HA(aq) + H2O(l) H3O+(aq) + A–(aq)

Write an expression for the dissociation constant, Ka, for HA.

(1)

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NT Exampro 29

(c) 25 cm3 of 1.00 mol dm–3 aqueous HA, was titrated with 1.00 mol dm–3 aqueous sodium

hydroxide and the pH measured throughout. The titration curve is shown below.

14

12

10

8

6

4

2

0 10 20 30 40 50Volume of sodium hydroxide added / cm

pH

3

Use the titration curve to find:

(i) the value of the pH at the end point of the titration.

............................................................................................................................ (1)

(ii) the pH of an aqueous solution of the salt NaA.

............................................................................................................................ (1)

(iii) the value of pKa for the acid HA and, hence the value Ka.

pKa ...................................................................................................................

Ka ..................................................................................................................... (2)

(d) Some of the solutions made during this titration would act as buffer solutions.

(i) What is meant by the term buffer solution?

............................................................................................................................

............................................................................................................................

............................................................................................................................ (2)

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NT Exampro 30

(ii) Use the titration curve to find:

the range of pH values over which this mixture acts as a buffer;

from .................................................... to .................................................... (1)

the pH of the most efficient buffer solution.

............................................................................................................................ (1)

(e) Suggest, with reasoning, whether methyl orange or phenolphthalein would be the better indicator for this titration.

Choice ........................................................................................................................

Reasoning ..................................................................................................................

....................................................................................................................................

.................................................................................................................................... (3)

(f) Explain why, as the titration proceeds, the flask becomes warm but not as warm as it would in a similar titration using 1.00 mol dm–3 solutions of hydrochloric acid and sodium hydroxide.

....................................................................................................................................

....................................................................................................................................

.................................................................................................................................... (3)

(g) A different monobasic weak acid has a dissociation constant of 1.8 × 10– 5 mol dm–3.

(i) Define pH.

............................................................................................................................ (1)

(ii) Calculate the pH of a 1.00 mol dm–3 aqueous solution of this acid.

(3)

(Total 21 marks)

30. In the vapour phase sulphur trioxide dissociates:

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NT Exampro 31

2SO3(g) 2SO2(g) + O2(g)

(a) (i) Write an expression for Kp for this dissociation.

(1)

(ii) At a particular temperature, 75% of the sulphur trioxide is dissociated, producing a pressure of 10 atm. Calculate the value of Kp at this temperature paying, attention to its units.

(5)

(b) Solid vanadium(V) oxide, V2O5, is an effective catalyst for this reaction. State the effect of using double the mass of catalyst on:

(i) the position of the equilibrium;

............................................................................................................................

............................................................................................................................ (1)

(ii) the value of Kp.

............................................................................................................................

............................................................................................................................ (1)

(Total 8 marks)

31. (a) Methane reacts with steam in a reversible reaction. In industry this reaction, carried out at a pressure of 30 atm, is used to produce hydrogen for the manufacture of ammonia

CH4(g) + H2O(g) CO(g) + 3H2(g) ∆H = +210 kJ mol–1

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NT Exampro 32

(i) Define the term partial pressure as applied to a gas mixture.

............................................................................................................................

............................................................................................................................ (1)

(ii) Write an expression for the equilibrium constant, Kp, for this reaction.

(1)

(iii) State and explain the effect of increasing the total pressure on the position of this equilibrium;

............................................................................................................................

............................................................................................................................

............................................................................................................................

............................................................................................................................

............................................................................................................................ (2)

(b) State the effect on the value of Kp for this equilibrium of the following.

(i) Increasing the total pressure.

............................................................................................................................ (1)

(ii) Increasing the temperature.

............................................................................................................................ (1)

(iii) Adding a catalyst.

............................................................................................................................ (1)

(c) There is a theory that methane, CH4, constantly leaks from the earth’s crust. This is not noticeable on land but at the bottom of a cold sea, such as off the Canadian coast, the methane is trapped in a solid cage of water molecules.

CH4(g) + 6H2O(s) [CH4(H2O)6](s) methane hydrate

At –29 °C the equilibrium pressure of the methane is 101.3 kPa.

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NT Exampro 33

(i) Write an expression for Kp for this equilibrium.

(1)

(ii) Deduce the value of Kp at –29 °C, stating its units.

(1)

(iii) At 0 °C the equilibrium pressure of methane rises to 2600 kPa. What does this tell you about the effect of temperature change on the position of equilibrium and about the enthalpy change for this reaction?

............................................................................................................................

............................................................................................................................

............................................................................................................................ (2)

(iv) Some people have suggested collecting the methane hydrate from the bottom of the sea and allowing it to warm up to 0 °C on board a ship. Comment on whether this would be a useful method for collecting methane.

............................................................................................................................

............................................................................................................................

............................................................................................................................ (1)

(Total 12 marks)

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NT Exampro 34

32. Ethanoic acid, CH3COOH, is a weak acid which can be used, with its salts, to make buffer

solutions.

(a) Explain what is meant by the term weak acid.

....................................................................................................................................

.................................................................................................................................... (1)

(b) Explain what is meant by the term buffer solution.

....................................................................................................................................

.................................................................................................................................... (2)

(c) An aqueous solution of ethanoic acid of concentration 1.00 mol dm–3 has a pH of 2.8. Sketch, with care, how the pH changes during the titration of 25.0 cm3 1.00 mol dm–3 aqueous ethanoic acid with aqueous sodium hydroxide of the same concentration.

14

12

10

8

6

4

2

00 10 20 30 40 50

Volume of alkali added/cm

pH

3 (4)

(d) Indicate on your sketch the portion of the curve where the mixture is behaving as a buffer. (1)

(e) (i) Write an expression for the dissociation constant, Ka, for ethanoic acid.

(1)

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NT Exampro 35

(ii) Explain how the pKa of ethanoic could be found from the graph.

............................................................................................................................

............................................................................................................................ (2)

(Total 11 marks)

33. The following standard electrode potentials will be required in answering this question.

E /V

Zn2+ + 2e– Zn –0.76

V3+ + e– V2+ –0.26

SO –24 + 2e– + 2H+ SO –2

3 + H2O +0.17

VO2+ + 2H+ + e– V3+ + H2O +0.34

VO +2 + 2H+ + e– VO2+ + H2O +1.00

(a) Give the colours of the following ions:

VO +2 VO2+

V3+ V2+

(2)

(b) (i) Derive the overall equation for the reduction of VO +2 to VO2+ by zinc in acidic

solution.

............................................................................................................................

............................................................................................................................ (2)

(ii) Give the formula of the vanadium species that would result if VO +2 ions were to be

reduced by sulphite ions, SO –23 . Explain your answer.

............................................................................................................................

............................................................................................................................

............................................................................................................................

............................................................................................................................ (3)

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NT Exampro 36

(c) Disproportionation requires an atom to have at least three oxidation states.

(i) Explain why this is so.

............................................................................................................................

............................................................................................................................ (1)

(ii) Derive an equation for the disproportionation of VO2+ into VO +2 and V3+.

............................................................................................................................

............................................................................................................................

............................................................................................................................ (2)

(Total 10 marks)

34. (a) The bombardier beetle Metrius contractus persuades potential predators to disappear by firing a boiling mixture of irritants at them. The reaction producing this ammunition is a redox reaction, H2O2 being the oxidising agent.

The two half-reactions involved are:

OH O

OH O

+ 2H + 2e + 0.70+ –

+ –

E /V

+1.77H O + 2H + 2e 2H O2 2 2

(i) Write the overall equation for the reaction and show that the reaction is feasible. (3)

(ii) The beetle makes use of an enzyme catalyst in the reaction. Explain in general terms how catalysts increase the rate of a chemical reaction using a graph of the Maxwell-Boltzmann distribution of molecular energies.

(5)

(iii) The reaction is highly exothermic; in principle its enthalpy of reaction could be found by using average bond enthalpies. By a consideration of the structure and bonding in the compounds involved, suggest why the use of the average bond enthalpies for C==O, CC, C==C and OH would give a highly inaccurate answer for the enthalpy of reaction.

(2)

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NT Exampro 37

(b) On heating hydrogen peroxide decomposes according to the equation

2H2O2 → 2H2O + O2

Hydrogen peroxide is marketed as an aqueous solution of a given ‘volume strength’. The common 20-volume solution gives 20 dm3 of oxygen from 1 dm3 of solution. What is the concentration in g dm–3 of such a solution? (Molar volume of any gas at the temperature and pressure of the experiment is 24 dm3.)

(3)

(c) Hydrogen peroxide, H2O2, can also act as a reducing agent.

The rapid oxidation of hydrogen peroxide was used in World War II to generate steam to launch the V1 ‘flying bomb’. H2O2 (100 volume) was reacted with acidified potassium manganate(VII) solution.

(i) Write the half-equation for the oxidation of hydrogen peroxide to oxygen, O2. (1)

(ii) The MnO –4 ions are reduced to Mn2+ during the reaction. Derive the overall

equation for the reaction between H2O2 and acidified KMnO4. (2)

(iii) Suggest in terms of the collision theory of chemical kinetics why 100-volume hydrogen peroxide (this gives l00 dm3 of oxygen from 1 dm3 of hydrogen peroxide when it decomposes to water and oxygen) was used rather than the more common 20-volume solution.

(2) (Total 18 marks)

35. (a) The first stage in the manufacture of sulphuric acid is the Contact Process.

2SO2 + O2 2SO3

At 400 °C the equilibrium constant Kp = 3.00 × 104 atm–1. A catalyst of vanadium(V) oxide is used. In a particular equilibrium mixture at 400 °C the partial pressures of sulphur dioxide and of oxygen were 0.100 atm and 0.500 atm respectively. Show that the yield of SO3 is about 95% of the equilibrium mixture.

(5)

(b) (i) Pure sulphuric acid is a viscous liquid with a high boiling temperature of 338 °C. It has the structure:

O

S

OHOHO

Suggest in terms of the intermolecular forces in sulphuric acid why it has such a high boiling temperature.

(3)

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NT Exampro 38

(ii) Sulphuric acid dissolves in water in a highly exothermic reaction

May her rest be long and placid, She added water to the acid; The other girl did what we taught her, And added acid to the water.

Suggest why sulphuric acid must always be added to water to dilute it rather than the other way round.

(2)

(c) Sulphuric acid dissociates in water according to the equations:

H2SO4 + H2O → H3O+ + HSO –4

HSO –4 + H2O H3O+ + SO –2

4

The dissociation constant for the first dissociation is very large; that for the second is 0.01 mol dm–3 at 25 °C.

(i) Calculate the pH of an aqueous solution containing 0.200 mol dm–3 hydrogen ions. (1)

(ii) The pH of 0.100 mol dm–3 sulphuric acid is 0.98. Explain why this is so close to the pH of 0.100 mol dm–3 HCl which is 1.0.

(3)

(d) Sulphuric acid is used as the electrolyte in the lead-acid battery found in cars. The electrodes are made from lead and from lead(IV) oxide. As the cell discharges, the lead and the lead(IV) oxide are both converted to lead(II) sulphate, and the sulphuric acid concentration falls.

(i) Use the information above to deduce the two half equations occurring in the lead acid battery.

(3)

(ii) Hence write an equation to represent the overall process taking place as the cell discharges.


36. (a) Define the following terms.

(i) pH ..................................................................................................................... (1)

(ii) Kw ...................................................................................................................... (1)

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NT Exampro 39

(b) Explain the meaning of the term strong, as applied to an acid or a base.

………………...……………….……………………………………………………..

………………...……………….…………………………………………………….. (1)

(c) Calculate the pH of the following solutions.

(i) HCl(aq) of concentration 0.200 mol dm–3.

(1)

(ii) NaOH (aq) of concentration 0.800 mol dm–3 (Kw = – 1.00 × 10–14 mol2 dm–6).

(2)

(d) HA is a weak acid with a dissociation constant Ka = 5.62 × 10–5 mol dm–3.

(i) Write an expression for the dissociation constant, Ka, of HA.

(1)

(ii) Calculate the pH of a 0.400 mol dm–3 solution of HA.

(3)

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NT Exampro 40

(e) A buffer solution contains HA(aq) at a concentration of 0.300 mol dm–3, and its sodium

salt, NaA, at a concentration of 0.600 mol dm–3. Calculate the pH of this buffer solution.

(3)

(Total 13 marks)

37. Consider the following equation:

2SO2 + O2 2SO3

2.0 moles of SO2 and 1.0 mole of O2 were allowed to react in a vessel of volume 60 dm3. At equilibrium 1.8 moles of SO3 had formed and the pressure in the flask was 2 atm.

(a) (i) Write the expression for Kc for this reaction between SO2 and O2.

(1)

(ii) Calculate the value of Kc, with units.

(3)

(b) The reaction between SO2 and O2 is exothermic. State the effect on the following, if the experiment is repeated at a higher temperature:

(i) Kc …………………………………………………………………………….. (1)

(ii) the equilibrium position .................................................................................... (1)

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NT Exampro 41

(c) State the effect of a catalyst on:

(i) Kc …………………………………………………………………………….. (1)

(ii) the equilibrium position .................................................................................... (1)

(d) (i) Write the expression for Kp for the reaction between SO2 and O2.

(1)

(ii) Calculate the mole fractions of SO2, O2 and SO3 at equilibrium.

(2)

(iii) Calculate the partial pressures of SO2, O2 and SO3 at equilibrium.

(1)

(iv) Calculate the value of Kp, with units.

(2)

(Total 14 marks)

38. (a) The electronic configuration of a cobalt atom can be written as [Ar]3d74s2. Give the electronic configuratin of the Co3+ ion.

………………...……………….…………………………………………………….. (1)

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NT Exampro 42

(b) (i) By reference to the standard electrode potentials given below, suggest a reducing

agent which might reduce aqueous Co3+ ions to cobalt metal. Give your reasoning.

E / V

Zn2+(aq) + 2e– Zn(s) –0.76

Fe2+(aq) + 2e– Fe(s) –0.44

Co2+(aq) + 2e– Co(s) –0.28

Sn2+(aq) + 2e– Sn(s) –0.14

O2(g) + 2H+(aq) + 2e– H2O2(aq) +0.68

Co3+(aq) + e– Co2+(aq) +1.82

Suitable reducing agent ..……………………………………………………..

Reasoning ..…………….……………………………………………………..

………………...……………….………………….………………………….. (3)

(ii) Suggest two factors that might prevent a reducing agent from being as effective as the electrode potentials might seem to suggest.

………………...……………….………………….…………………………..

………………...……………….………………….…………………………..

………………...……………….………………….…………………………..

………………...……………….………………….………………………….. (2)

(c) (i) Write the formula of the hexaaquacobalt(II) ion.

………………...……………….………………….………………………….. (1)

(ii) Give an equation, involving the hexaaquacobalt(II)ion, to illustrate the process of ligand exchange.

………………...……………….………………….………………………….. (2)

(Total 9 marks)

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NT Exampro 43

39. A compound D, CH3.CH(OH).COOH, may be prepared from C3H7OH by the following series

of reactions.

C H OH C H O

C H O Cl

3 3

3

7 6

5

2

2 2 7 2 4

2

2

A

D

B

C

K Cr O (aq)/H SO (aq)

Cl (g)/uv light

(i) NaOH(aq)

(ii) HCl(aq)

(a) (i) Identify compounds A, B and C. (3)

(ii) Classify the reaction with chlorine. (2)

(iii) D has a boiling temperature of 186°C whereas C, C3H5O2Cl, boils at 122°C. Draw a boiling temperature/composition diagram applicable to a mixture of C and D and use it to show how fractional distillation of a mixture of the two compounds could produce a sample of D.

(5)

(b) Assuming the percentage yield for each step in sequence to be 80%, calculate the mass of D that could be made from 60 g of C3H7OH.

(3)

(c) An aqueous solution of D of concentration 0.100 mol dm–3 has a pH value of 2.04.

(i) Calculate the value of the dissociation constant, Ka, for D. (3)

(ii) Suggest, with reasoning, whether D or propanoic acid, CH3CH2COOH, Ka = 1.3 × 10–5 mol dm–3, would be more exothermic in reaction with aqueous sodium hydroxide solution of concentration 0.1 mol dm–3.


40. (a) Define the term partial pressure.

...............................................................................................................................

...............................................................................................................................

............................................................................................................................... (1)

(b) If phosphorus pentachloride is heated in a sealed tube the following equilibrium is set up

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NT Exampro 44

PCl5(g) PCl3(g) + Cl2(g)

(i) Write an expression for the equilibrium constant, Kp, for the above reaction.

(1)

(ii) At a given temperature T a sample of phosphorus pentachloride is 40.0% dissociated, the total equilibrium pressure being 2.00 atm. Calculate the partial pressures of each of the components of the equilibrium mixture, to three significant figures. Hence calculate the value of Kp under these conditions.

(5)

(c) At a higher temperature, T + t, the equilibrium mixture in (b) contains a greater proportion of phosphorus trichloride and chlorine.

(i) State, with a reason, whether the dissociation of phosphorus pentachloride is exothermic or endothermic.

......................................................................................................................

......................................................................................................................

...................................................................................................................... (2)

(ii) How does Kp change when the temperature is increased?

...................................................................................................................... (1)

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NT Exampro 45

(d) If calcium carbonate is heated in a sealed vessel in the absence of air the following

equilibrium is set up:

CaCO3(s) CaO(s) + CO2(g)

(i) Write the expression for Kp for this equilibrium.

...................................................................................................................... (1)

(ii) At 1030 °C the total pressure in the vessel is 16 atm. What is the value of Kp?

...................................................................................................................... (1)

(Total 12 marks)

41. Propanoic acid is a weak acid which dissociates according to

CH3CH2COOH(aq) + H2O(l) CH3CH2COO–(aq) + H3O+(aq)

(a) (i) Indicate, in the space provided below the equation, the two acid/base conjugate pairs.

(2)

(ii) Write the expression for the acid dissociation constant, Ka, for propanoic acid.

(1)

(iii) Calculate the pH of a 0.100 mol dm–3 solution of propanoic acid, for which Ka = 1.3 × 10–5 mol dm–3.

(3)

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NT Exampro 46

(iv) Calculate the concentration of hydroxide ions, OH–, in this same solution of propanoic acid. Kw = 1.00 × 10–14 mol2 dm–6 at the temperature of the solution.

(3)

(b) If sodium propanoate is dissolved in water, the pH of the resulting solution is not 7, but is near to 8. By writing the equation for the reaction occurring suggest why this is so.

................................................................................................................................

................................................................................................................................

................................................................................................................................ (2)

(c) A mixture of sodium propanoate and propanoic acid behaves as a buffer solution.

(i) What is meant by a buffer solution?

......................................................................................................................

......................................................................................................................

...................................................................................................................... (2)

(ii) Calculate the pH of a buffer solution that is made by mixing equal volumes of 0.0500 mol dm–3 propanoic acid and 0.100 mol dm–3 sodium propanoate.

(3)

(Total 16 marks)

42. (a) (i) A fluoride of phosphorus, Y, contains 24.6% by mass of phosphorus and has a

molar mass of 126 g mol–1. Deduce the molecular formula of Y.

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NT Exampro 47

(4)

(ii) Draw the shape of a molecule of compound Y. Show the values of the bond angles on the diagram.

(3)

(iii) Compound Y was converted into the ion PF6–. Draw and name the shape of PF6– and suggest a value for the bond angles.

(3)

(b) (i) Consider the following data, which shows the formulae and boiling temperatures of the Group 7 hydrides.

Group 7 hydride Boiling temperature/°C

HF 19

HCl –85

HBr –68

HI –35

Suggest why hydrogen fluoride, HF, has the highest boiling temperature of the Group 7 hydrides.

(3)

(c) When hydrogen fluoride is dissolved in water a solution of a weak acid is formed. Write the expression for the acid dissociation constant, Ka, for hydrogen fluoride. Calculate the value of Ka, with units, given that a 0.150 mol dm–3 solution of hydrogen fluoride has a pH of 2.04.


43. (a) Consider the following equilibrium, which illustrates one industrial method used to produce hydrogen:

CH4(g) + 2H2O(g) CO2(g) + 4H2(g)

In a certain experiment, 10 g of methane, CH4, and 54 g of water, H2O, were heated in a container of volume 4 dm3. At equilibrium, 2.0 moles of hydrogen, H2, had formed. Write an expression for the equilibrium constant, Kc, for the system, and use the data to calculate a value for Kc, with units.

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NT Exampro 48

(b) The following table shows some data for enthalpies of formation, ∆Hf.

Substance ∆H f /kJ mol–1

CH4(g) –76

H2O(g) –242

CO2(g) –394

Use these data to calculate the enthalpy change for the reaction in (a). (3)

(c) In practice, the industrial production of hydrogen by this method is conducted at the moderately high pressure of 30 atm, and the high temperature of 750 °C, in the presence of a nickel catalyst. Suggest why these conditions are used, considering the factors of rate and yield.


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equilibria_a2

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