Equilibria Questions and Answers For A2 Chemistry

1. For the reversible reaction

X Y

which of the following could represent the change in the concentrations of X and Y with time, starting with a mixture of both X and Y? Equilibrium is reached at time teqm.

(Total 1 mark)

2. (a) In the catalytic converter of a car engines exhaust system, the following reaction occurs.

2NO(g) + 2CO(g) N2(g) + 2CO2(g) H = 745 kJ mol1

The temperature in a catalytic converter is high.

(i) State the effect, if any, on the position of equilibrium if the temperature is lowered. Give a reason for your answer.

Effect.................................................................................................................

Reason...............................................................................................................

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(2)

Sri Lankan School 1

(ii) The gases from the engine are not cooled before entering the converter. Explain why this is so.

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(2)

(iii) State the effect, if any, on the position of equilibrium if the pressure on the reactinggases is increased. Give a reason for your answer.

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Reason...............................................................................................................

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(2)

(b) Nitrogen monoxide, NO, is formed when nitrate ions, 3NO , in acidic solution are

reduced by silver metal.

(i) Calculate the oxidation number of nitrogen in NO and in 3NO

In NO...............................................................................................................

In 3NO .............................................................................................................

(2)

(ii) Balance the half-equation for the reduction of nitrate ions, 3NO , in acidic solution.

3NO + .............H+ + ........e NO + ........H2O

(2)

(iii) Write the half-equation for the oxidation of silver metal, Ag, to silver ions, Ag+.

(1)

(iv) Hence deduce the full ionic equation for the reaction between silver metal and nitrate ions in acidic solution. State symbols are not required.

(2)(Total 13 marks)

Sri Lankan School 2

Sri Lankan School 3

3. In which reaction is water acting as a Brnsted-Lowry acid?

A H2O + HCl H3O+ + Cl

B H2O + SO3 H2SO4

C H2O + NH3 NH4+ + OH

D H2O + CO2 H2CO3

(Total 1 mark)

4. Which of the following solutions has the lowest pH?

A 0.010 mol dm3 hydrochloric acid.

B 0.100 mol dm3 hydrochloric acid.

C 0.010 mol dm3 ethanoic acid.

D 0.100 mol dm3 ethanoic acid.

(Total 1 mark)

5. Which of the following solutions, when mixed, would make a buffer with pH more than 7?

A Methanoic acid and sodium methanoate.

B Sodium hydroxide and sodium chloride.

C Ammonia and ammonium chloride.

D Ammonium chloride and ammonium ethanoate.

(Total 1 mark)

Sri Lankan School 4

6. This question is about the equilibrium reaction

N2(g) + 3H2(g) 2NH3(g) H = 92 kJ mol1

Which statement is not correct?

A The units of Kp are atm2.

B Kp increases as temperature is decreased.

C Kp increases when the pressure increases.

D Kp increases when the total entropy change, Stotal, increases.

(Total 1 mark)

7. 1,2-dichloroethane decomposes in the presence of a catalyst.

CH2ClCH2Cl(g) CH2=CHCl(g) + HCl(g) H = +51 kJ mol1

Which of the following would result in an increase in the equilibrium yield of chloroethene?

A Increasing the temperature.

B Increasing the pressure.

C Increasing the surface area of the catalyst.

D Changing the catalyst to a more efficient one.

(Total 1 mark)

Sri Lankan School 5

8. Ethanoic acid can be manufactured by the following reaction, which is carried out between150 C and 200 C.

CH3OH(g) + CO(g) CH3COOH(g)

(a) A mixture of 50.0 mol of methanol and 50.0 mol of carbon monoxide reaches equilibriumat a pressure of 32.0 atm. At 175 C, the equilibrium partial pressure of ethanoic acid is 22.2 atm.

(i) Write the expression for the equilibrium constant in terms of pressure, Kp, for this reaction.

(1)

(ii) Calculate the partial pressures of methanol and carbon monoxide at equilibrium.

Methanol...............................................................................................

Carbon monoxide................................................................................

(2)

(iii) Calculate the value of Kp for this reaction at 175 C. Include a unit in your answer and give your answer to three significant figures.

(2)

(b) Another sample of 50.0 mol of methanol and 50.0 mol of carbon monoxide was allowed to reach equilibrium at the same pressure of 32.0 atm, but at a lower temperature. 93.6% of the methanol was converted at equilibrium.

(i) Complete the table below to show the number of moles of each species in the equilibrium mixture.

CH3OH CO CH3COOH

Number of moles atstart 50.0 50.0 0

Number of moles atequilibrium

(2)

(ii) Calculate the partial pressure of ethanoic acid in the equilibrium mixture.

(1)

(iii) Is the reaction exothermic or endothermic? Explain your answer.

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Sri Lankan School 6

(1)

Sri Lankan School 7

(c) How, if at all, does the addition of methanol to the equilibrium mixture affect the following? Justify your answers.

CH3OH(g) + CO(g) CH3COOH(g)

(i) The equilibrium constant for the formation of ethanoic acid.

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(1)

(ii) The equilibrium yield of ethanoic acid.

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(1)

(d) In industry, catalysts are used even though they are often expensive.

State and explain ONE benefit to the environment resulting from the use of catalysts in industrial processes.

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(2)(Total 13 marks)

Sri Lankan School 8

9. Vinegar is used as a food preservative. It is an acidic solution containing ethanoic acid, CH3COOH.

(a) A titration was carried out to measure the concentration of ethanoic acid in a sample of vinegar. 25.0 cm3 of a vinegar solution was titrated with a solution of sodium hydroxide, concentration 0.250 mol dm3. The concentration of the ethanoic acid in the vinegar solution was found to be 0.125 mol dm3.

(i) Calculate the pH of 0.250 mol dm3 sodium hydroxide at 298 K.

[Kw = 1.00 1014 mol2dm6 at 298 K.]

(2)

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

(1)

(iii) Calculate the pH of 0.125 mol dm3 ethanoic acid at 298 K.

[Ka for ethanoic acid is 1.7 105 mol dm3 at 298 K.]

(2)

Sri Lankan School 9

(iv) When half the ethanoic acid is neutralized, the concentration of the remaining ethanoic acid equals the concentration of the sodium ethanoate which has formed. What is the pH of the mixture at this point? Justify your answer.

pH.....................................................................................................................

Justification.......................................................................................................

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(2)

(v) On the axes below, sketch the titration curve for this reaction when 30 cm3 of the sodium hydroxide is added to 25.0 cm3 of the vinegar solution.

(3)

Sri Lankan School 10

(vi) The only indicators which were available for this titration were methyl yellow (in ethanol) and thymolphthalein. Explain which indicator is more suitable for this titration and why the other is unsuitable. You will need to refer to your data booklet.

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(2)

(b) In the food industry, ethanoic acid is described as an acidity regulator, additive number E260.

Ethanoic acid can neutralize alkalis. What substance could be mixed with ethanoic acid so that it regulates pH as a buffer in foodstuffs?

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(1)(Total 13 marks)

10. The electrode potential for a cell can be used to calculate the equilibrium constant for the cell reaction. This is because

A Ecell is proportional to lnK.

B Ecell is proportional to K.

C ln Ecell is proportional to lnK.

D In Ecell is proportional to K.

(Total 1 mark)


11. Magnesium nitrate, Mg(NO3)2, decomposes when it is heated. One product is the brown gas, nitrogen dioxide.

(a) (i) Write an equation for this reaction. State symbols are not required.

(2)

(ii) Calcium nitrate decomposes in a similar way to magnesium nitrate, but at a higher temperature.

Explain why the two nitrates have different stability to heat.

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(2)

(b) Sodium nitrate decomposes to give different products to magnesium nitrate. Write an equation for the decomposition of sodium nitrate. State symbols are not required.

(1)

(c) A student suggested that the structure of the nitrate ion, NO3 , is

Scientists have found that the bonds between nitrogen and oxygen in the nitrate ion are allthe same length. Is the students suggestion supported by this evidence? Explain your answer.

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(1)


(d) Nitrogen dioxide gas can dimerize to dinitrogen tetroxide, N2O4, a very pale yellow gas, as shown in the equation below.

2NO2(g) N2O4(g) H = 58 kJ mol1

(i) What would you see when an equilibrium mixture of these gases is warmed gently?Explain your answer.

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(2)

(ii) Explain why an equilibrium mixture of these gases eventually becomes paler in colour when the pressure on it is increased.

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(2)


(e) Two Maxwell-Boltzmann distributions showing the energy of particles in a gas at different temperatures, T1 and T2, are shown below. The activation energy for the reactionis labelled EA.

Use the distributions to explain why gases react faster when the temperature is increased.

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(2)(Total 12 marks)


12. Consider the equilibrium

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

Which of the following is true when the total pressure of the system is increased at constant temperature?

Value of Kp Mole fraction of PCl5(g)

A decreases decreases

B unaltered increases

C decreases increases

D unaltered unaltered(Total 1 mark)

13. In which of the following reactions is nitric acid acting as a base?

A HNO3 + NaOH NaNO3 + H2O

B HNO3 + H2O H3O+ + NO3

C HNO3 + H2SO4 H2NO3+ + HSO4

D HNO3 + NaHCO3 NaNO3 + H2O + CO2

(Total 1 mark)

14. Why does phenolphthalein, which is colourless in acidic solutions, turn pink in alkaline solutions?

A It is oxidized to a pink compound by hydroxide ions.

B It forms a pink anion by loss of H+ ions.

C It forms a pink anion by gain of H+ ions.

D It forms a pink cation by gain of H+ ions.

(Total 1 mark)


15. The dissociation of ethanoic acid in aqueous solution is represented by

CH3COOH(aq) + H2O(l) H3O+(aq) + CH3COO

(aq)

Which of the following statements is true for this equilibrium?

A CH3COOH is an acid and its conjugate base is CH3COO.

B H2O is an acid and its conjugate base is OH.

C At equilibrium, the concentrations of each substance are the same.

D At equilibrium, the reaction from left to right and the reaction from right to left have stopped.

(Total 1 mark)

16. Why are aqueous solutions of sodium ethanoate slightly alkaline?

A The sodium ions react with water to give an alkali.

B The ethanoate ions react with water to give hydroxide ions.

C All sodium salts give alkaline solutions.

D The sodium ethanoate is fully ionized in solution.

(Total 1 mark)

17. A solution of a weak acid cannot be titrated with a weak base using an indicator to find the end-point because

A the pH change is too gradual close to the equivalence point.

B there are too few H+ ions to affect the indicator.

C there are too few OH ions to affect the indicator.

D the pH change occurs outside the range of any indicator.

(Total 1 mark)


18. Iron and steam at high temperature react in a closed vessel to give an equilibrium mixture

3Fe(s) + 4H2O(g) Fe3O4(s) + 4H2(g)

Which of the following is the correct expression for Kp?

A OH2H

p2

PP

K

B4

OH3

Fe

4HOFe

p2

243

PP

PPK

C4

OH

4H

p2

2

P

PK

D4

Hp 2PK

(Total 1 mark)

19. At 100 C, pure water has a pH of 6, whereas at 25 C it has a pH of 7. This is because

A the dissociation of water is endothermic, so the concentration of hydrogen ions is lower at100 C than it is at 25 C.

B the dissociation of water is exothermic, so the concentration of hydrogen ions is lower at 100 C than it is at 25 C.

C the dissociation of water is endothermic, so the concentration of hydrogen ions is higher at 100 C than it is at 25 C.

D at 100 C, water has a higher concentration of hydrogen ions than of hydroxide ions.

(Total 1 mark)

20. (a) (i) Define pH.

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(1)

(ii) Calculate the pH of 0.0100 mol dm3 hydrochloric acid, which is a strong acid.

(1)


(b) Ethanoic acid is a weak acid with an acid dissociation constant, Ka, of value 1.75 105

mol dm3 at 25 C.

(i) Calculate the pH of 0.0100 mol dm3 ethanoic acid at 25 C, stating any ONE assumption that you have made.

(4)

(ii) The pH of hydrochloric and of ethanoic acid at two different concentrations is given in the table.

pH of 0.00100 mol dm3

solutionpH of 0.000100 mol dm3

solution

Hydrochloric acid 3.0 4.0

Ethanoic acid 3.9 4.4

In the case of hydrochloric acid, dilution by a factor of 10 increases the pH by one unit. Suggest why ethanoic acid behaves differently.

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(2)


(c) Orange marmalade usually contains sodium citrate as a preservative. Together with the fruit in the marmalade, it forms a buffer solution which, at a suitable pH, inhibits mould growth.

(i) Define the term buffer solution.

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(2)

(ii) What is the substance in the fruit that produces a buffer with sodium citrate?

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(1)

(iii) Explain how a buffer solution works using this system or any other of your choice. Support your explanation with equations.

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(4)(Total 15 marks)


21. Hydrogen is used in very large quantities as a fuel, as a reducing agent, and in the production of ammonia. Hydrogen is manufactured by steam reforming of methane from natural gas. Two reactions are involved, both being in equilibrium in closed systems.

Reaction I CH4(g) + H2O(g) CO(g) + 3H2(g) H = + 210 kJ mol1

Reaction II CO(g) + H2O(g) CO2(g) + H2(g) H = 42 kJ mol1

(a) Write the expression for the equilibrium constant, Kp, for reaction I.

(1)

(b) Reaction I occurs at a temperature of 1000 K and a pressure of 30 atm over a nickel catalyst.

(i) State and explain the effect, if any, on the value of Kp of increasing the pressure onthe reaction.

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(1)

(ii) Explain, in terms of your answers to (a) and (b)(i), why an increase in the pressure leads to a decrease in yield in reaction I.

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(2)


(iii) Increasing the pressure on this heterogeneously-catalysed reaction I has very little effect on the rate of the reaction. Suggest why this is so.

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(2)

(c) The expression for Kp for reaction II is

OHCO

HCOp

2

22

PPPP

K

At a particular temperature and 30 atm pressure, a mixture of equal amounts of carbon monoxide and steam react to give an equilibrium mixture where 75% of the CO has reacted.

Calculate the value of Kp showing your working.

(3)

(d) Carbon dioxide and hydrogen are separated by washing the gas with potassium carbonate solution to give potassium hydrogencarbonate solution, leaving hydrogen in the gas stream. Potassium carbonate is expensive and is regenerated by heating the potassium hydrogencarbonate and liberating the carbon dioxide.

(i) Hydrogen is often claimed to be a non-polluting fuel as it only produces water on burning.

Explain why its manufacture using reactions I and II does not support this claim.

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(1)


(ii) Write the equation for the thermal decomposition of potassium hydrogencarbonate.

State symbols are not required.

(1)

(e) Although industrial processes are often discussed in terms of equilibria, they are rarely allowed to reach equilibrium.

Suggest why, apart from insufficient reaction time, this is so.

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(1)(Total 12 marks)

22. In the equilibrium below, what effect would the changes described have on the system?

2H2S(g) + SO2(g) 3S(s) + 2H2O(g) H is negative

(a) Increase in temperature

A increase rate, decrease yield

B increase rate, increase yield

C decrease rate, decrease yield

D decrease rate, increase yield

(1)

(b) Decrease in pressure

A increase rate, decrease yield

B increase rate, increase yield

C decrease rate, decrease yield

D decrease rate, increase yield

(1)(Total 2 marks)


23. Iodine can react with sodium hydroxide solution to form NaIO3(aq), according to the equation below.

3I2(aq) + 6NaOH(aq) 5NaI(aq) + NaIO3(aq) + 3H2O(l)

Which of the statements about the reaction is false?

A The oxidation number of some iodine atoms goes up.

B At high temperatures NaIO(aq) also forms.

C Sodium ions are spectator ions.

D The oxidation number of some iodine atoms goes down.

(Total 1 mark)

24. In the reaction between Ag+(aq) ions and Fe2+(aq) ions, what would be the effect of increasing the concentration of Ag+(aq) ions?

Ag+(aq) + Fe2+(aq) Fe3+(aq) + Ag(s)

A Rate of reaction increases, yield of Fe3+(aq) stays the same.

B Rate of reaction increases, yield of Fe3+(aq) decreases.

C Rate of reaction decreases, yield of Fe3+(aq) stays the same.

D Rate of reaction increases, yield of Fe3+(aq) increases.

(Total 1 mark)

25. Which one of these reactions is not a disproportionation reaction?

A 2H2O2(aq) O2(g) + 2H2O(l)

B S2O3 2(aq) + 2H+(aq) SO2(g) + S(s) + H2O(l)

C Cl2(aq) + 2Br(aq) 2Cl(aq) + Br2(aq)

D 2Cu+(aq) Cu(s) + Cu2+(aq)

(Total 1 mark)


26. Almost two thirds of the worlds ethanoic acid is made using the following equilibrium reaction,with the aid of an iridium complex as a catalyst.

CH3OH(l) + CO(g) CH3COOH(l) DH = 135 kJ mol1

Which of the following changes in conditions would increase the equilibrium yield of ethanoic acid?

A increase pressure

B decrease pressure

C increase temperature

D add a catalyst

(Total 1 mark)

27. For the equilibrium,

N2(g) + 3H2(g) 2NH3(g)

Which is the correct expression for Kp?

A3

22

23

g)](g)][H(N[)]g(NH[

B g)(NHg)(Hg)(N

3

22

PPP

C g)(H3

g)(N

g)(NH2

22

3

PPP

D g)(NH2

g)(H3

g)(N

3

22

P

PP

(Total 1 mark)


28. The expression for Kc for the equilibrium 2SO2(g) + O2(g) 2SO3(g) is

Kc = g)](O[g)](SO[

g)]([SO

22

2

23

What are the units of Kc in this equilibrium expression?

A mol dm3

B mol2 dm6

C dm3 mol1

D atm1

(Total 1 mark)

29. For the equilibrium

2NO2(g) N2O4(g) DH = 57.2 kJ mol1

which one of the following changes would result in a different value of the equilibrium constant?

A an increase in temperature

B a decrease in pressure

C an increase in pressure

D an increase in the concentration of NO2(g)

(Total 1 mark)


30. Solutions of concentration 0.1 mol dm3 of iron(II) ions and silver(I) ions were mixed at room temperature and allowed to reach equilibrium.

Fe2+(aq) + Ag+(aq) Fe3+(aq) + Ag(s)

Which one of the following statements is true?

A as the equilibrium position was approached, the forward reaction became slower until it stopped.

B at the equilibrium position, no more Ag(s) reacted with Fe3+(aq).

C at the equilibrium position, the rate of the forward reaction equalled the rate of the backward reaction.

D no Fe3+(aq) reacted with Ag(s) until the equilibrium position was reached.

(Total 1 mark)

31. This question concerns four solutions, A to D. They were prepared by mixing equal volumes of 0.2 mol dm3 solutions of two different substances. The substances were

A HCl(aq) and NaOH(aq)

B HCl(aq) and NaCl(aq)

C NH3(aq) and NH4Cl(aq)

D CH3COOH(aq) and CH3CO2Na(aq)

Select, from A to D, the mixture which would:

(a) have the lowest concentration of hydrogen ions

A

B

C

D

(1)


(b) act as a buffer of pH about 5

A

B

C

D

(1)

(c) have a chloride ion concentration of 0.2 mol dm3.

A

B

C

D

(1)(Total 3 marks)

32. This question concerns the titration of a solution of sodium hydroxide with a solution of hydrochloric acid. As the titration proceeds the pH of the mixture changes.

(a) What was the pH when 24.95 cm3 of 1.00 mol dm3 NaOH(aq) had been added to 25 cm3

of 1.00 mol dm3 HCl(aq)?

A 3

B 6

C 8

D 11

(1)


(b) What was the pH when 25.05 cm3 of 1.00 mol dm3 NaOH(aq) had been added to 25 cm3

of 1.00 mol dm3 HCl(aq)?

A 3

B 6

C 8

D 11

(1)

(c) Which one of the following indicators would be most suitable to use to determine the endpoint of this titration?

pH range

A methyl violet 01.6

B universal indicator 311

C thymolphthalein 8.310.6

D alizarin yellow R 10.113.0

(1)(Total 3 marks)

33. This question is about the pineapple flavouring used in sweets. It is an ester with the formula C3H7COOCH3, which can be broken down into butanoic acid and methanol when mixed with hydrochloric acid.

The following equilibrium is set up:

C3H7COOCH3(l) + H2O(l) C3H7COOH(l) + CH3OH(l)

(a) Give the name of this ester.

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(1)


(b) Why does the ester have a comparatively low boiling point compared to the other three substances in the equation?

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(1)

(c) What is the name given to this type of reaction?

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(1)

(d) Suggest the reasons why manufacturers choose to use the chemically manufactured pineapple flavouring rather than the natural product and why consumers might prefer to choose the natural product.

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(4)


(e) In an experiment, 10.2 g (0.10 mol) of the ester was mixed with 18 cm3 of 1.0 mol dm3 hydrochloric acid and left until equilibrium had been reached. The hydrochloric acid acts as a catalyst and contains 18 g (1 mol) of water. At equilibrium, 4.4 g of butanoic acid was found to be present.

Molar mass of butanoic acid = 88 g; assume the total volume at equilibrium is 30 cm3.

Give the expression for the equilibrium constant, Kc, for this equilibrium and calculate its value. Explain why it has no units.

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(5)(Total 12 marks)

34. Methane reacts with steam in an endothermic reaction.

CH4(g) + H2O(g) CO(g) + 3H2(g)

(a) State the effect on the value of the equilibrium constant of an increase in temperature.

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(1)


(b) Use your answer to (a) to explain the effect of this change on the position of equilibrium.

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(2)(Total 3 marks)

35. This question is about the reaction of magnesium with hydrochloric acid which takes place rapidly at room temperature.

Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g) DH = 467 kJ mol1

(a) Rewrite the equation omitting spectator ions.

(1)


(b) Suggest the sign of the following entropy changes for this reaction. Justify each of your answers.

(i) DSsystem

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(2)

(ii) DSsurroundings

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(2)

(iii) DStotal

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(1)

(c) A student carried out this experiment at five different temperatures in order to calculate the activation energy of the reaction. The students laboratory record is shown below.

MethodClean a strip of magnesium weighing 0.100 g with sand paper. Measure the temperature of 20 cm3 of 1.00 moldm3 hydrochloric acid in a 100 cm3 beaker. Add the magnesium ribbon, stir continuously, and time how long it

takes for the magnesium to disappear. Repeat the experiment at four other temperatures.Assumption: the initial rate of reaction is proportional to 1/time.

Results

Temperature/C

Temperature/K

1/T/K1

time/s

1/time/s1

ln 1/time

24 297 3.37 103 45 0.0222 3.81

33 306 3.27 103 25 0.0400 3.22

45 318 3.14 103 11 0.0909 -2.40

56 329 3.04 103 6 0.1667 -1.79

10 283 3.53 103 122 0.0082 -4.80


The Arrhenius equation is ln k = Ea/R (1/T) + constant

ln 1/time is proportional to ln k and so a graph of ln 1/time will have the same gradient as that of the Arrhenius plot of ln k against 1/Temperature

The student plotted the graph of ln 1/time against 1/Temperature and from this the activation energy, EA, was calculated as + 51.3 kJ mol

1.

(i) Suggest the reason for cleaning the magnesium ribbon with sand paper.

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(1)


(ii) Calculate the number of moles of hydrochloric acid used up when all the magnesium reacts in one experiment. Hence comment on whether the change in concentration during the reaction will have a significant effect on the validity of theassumption that the initial rate of reaction is proportional to 1/time. How would you overcome this potential error?

[Take the relative atomic mass of magnesium as 24 in this and subsequent calculations.]

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(5)


(iii) Use the value of DH and other information given in the question to calculate the temperature change in an experiment assuming no energy is lost to the surroundings. Hence comment on whether this change in temperature will have a significant effect. How would you overcome this potential error?

[DH = 467 kJ mol1.

heat produced = mass specific heat capacity change in temperature.

Assume that the specific heat capacity of the solution is 4.18 J K1 g1]

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(4)


(iv) The most difficult thing to measure accurately is the time it takes for the magnesium to disappear and the time measured can be up to 2 seconds out.Assuming this error, calculate the shortest time at 56 C and the longest time at 10 C for this reaction.

Complete the table for these times. Plot the two points on the grid below and join them with a straight line. From the gradient, which equals EA/R, of this line calculate another value for the activation energy.

Temperature/ C

Temperature/K

1/T/K1

time/s

1/time/s1

ln 1/time

56 329 3.04 103

10 283 3.53 103

1 / T ( K 1 )0 . 0 0 3 0 0 . 0 0 3 1 0 . 0 0 3 2 0 . 0 0 3 3 0 . 0 0 3 4 0 . 0 0 3 5 0 . 0 0 3 6

1 . 0

1 . 5

2 . 0

2 . 5

3 . 0

3 . 5

4 . 0

4 . 5

5 . 0

ln1/

t


(v) If the reaction mixture is not stirred, the magnesium tends to float on the surface of the acid.

Suggest how this would affect the measurements of the rate of the reaction.

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(1)

(vi) Suggest two other improvements the student could do to this experiment to improve the accuracy or validity of the results.

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(2)

(vii) If ethanoic acid of the same concentration and at the same temperature is used instead of hydrochloric acid, explain how the rate would differ.

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(1)(Total 24 marks)


36. One step in the production of nitric acid is the oxidation of ammonia.

4NH3 + 5O2 4NO + 6H2O

This is carried out at 900 C over a platinum-rhodium catalyst and is an example of heterogeneous catalysis.

Explain in terms of collision frequency and collision energy how the rate would change if the temperature were increased, and which of these causes the greater effect.

What is the difference between a heterogeneous and a homogeneous catalyst? Suggest one advantage of using a heterogeneous catalyst in processes such as this.

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(Total 6 marks)


37. In a standard hydrogen electrode

A the hydrogen gas is at one atmosphere pressure

B a solution of 1 mol dm3 sulfuric acid is used

C a temperature of 273 K is maintained

D a piece of shiny platinum foil is used

(Total 1 mark)

38. For a redox reaction to be thermodynamically feasible, Ecell must be

A positive

B negative

C greater than +0.3 V

D more negative than 0.3 V

(Total 1 mark)

39. The standard electrode potential for the electrode system based on the equation below is +1.51 V.

MnO4(aq) + 8H+(aq) + 5e Mn2+(aq) + 4H2O(l)

Which of the following statements about the electrode system is correct?

A the electrode potential at pH 5 is +1.51 V.

B Mn2+(aq) is acting as an oxidising agent.

C changing the concentration of Mn2+(aq) would cause a change in the electrode potential.

D the electrode used in this half-cell is made of manganese.

(Total 1 mark)


40. Which of the following is always proportional to Ecell for a chemical reaction?

A DHr

B DSsystem

C DSsurroundings

D DStotal

(Total 1 mark)

41. Methanol can be vigorously oxidised with an acidified solution containing dichromate(VI) ions, Cr2O7

2, to form methanoic acid and chromic(III) ions.

(a) What are the oxidation numbers of carbon in methanol and methanoic acid?

MethanolMethanoic acid

A 1 +1

B 2 +2

C +1 1

D +2 2

(1)

(b) How many moles of methanol react with one mole of dichromate(VI) ion, Cr2O72?

A 1

B

C 1

D 3

(1)(Total 2 marks)


42. (a) (i) Give the electron configuration of:

Fe [Ar] .........................................................................................................

Fe2+ [Ar] .........................................................................................................

(1)

(ii) Draw the structure of the hexaaquairon(II) ion, [Fe(H2O)6]2+, clearly showing its

shape.

(1)

(iii) Give the equation for the complete reaction of hydroxide ions with a solution of hexaaquairon(II) ions.

(1)

(iv) State what you would see if the product mixture in (iii) is left to stand in air.

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(1)


(b) Consider the equation for the half reaction

Fe2+ + 2e Fe E = 0.44 V

(i) Define the term standard electrode potential with reference to this electrode.

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(3)

(ii) Explain why the value of E suggests that the iron will react with an aqueous solution of an acid to give Fe2+ ions and hydrogen gas.

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(2)

(iii) State why E values cannot predict that a reaction will occur, only that it is possible.

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(1)(Total 10 marks)


43. This question is about the manufacture of sulphuric acid, H2SO4.

(a) The first stage in the manufacture of sulphuric acid is the combustion of sulphur.

The following equation shows the reaction taking place when the standard enthalpy of combustion of sulphur is measured.

S(s) + O2(g) SO2(g)

Define the term standard enthalpy of combustion.

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(3)


(b) In the second stage of the manufacture of sulphuric acid, sulphur dioxide is oxidised to sulphur trioxide as shown in the following equation:

2SO2(g) + O2(g) 2SO3(g) H = 196 kJ mol1

(i) Explain, in terms of collision theory, why the rate of a reaction is increased by increasing the temperature and by the addition of a catalyst.

Temperature

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Catalyst

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(4)

(ii) State and explain the effect, if any, of increasing the temperature on the equilibriumyield of sulphur trioxide.

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(2)


(iii) State and explain the effect, if any, of an increased pressure on the equilibrium yield of sulphur trioxide.

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(2)

(c) The following equation represents the overall reaction for the manufacture of sulphuric acid from sulphur, oxygen and water.

2S(s) + H2O + 3O2 2H2SO4(l)

Use the data below to calculate the enthalpy change for this reaction.

Substance Hf

/ kJ mol1

H2O (l) 286

H2SO4 (l) 814

(2)(Total 13 marks)


44. Consider the equilibrium

N2O4(g) 2NO2(g) H = +58 kJ mol1

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

(1)

(b) (i) An equilibrium mixture contains a mole fraction of dinitrogen tetroxide,N2O4 = 0.20, and nitrogen dioxide, NO2 = 0.80. The total pressure of this mixture is 1.1 atm.

Calculate Kp at this temperature, stating its units.

(3)

(ii) Calculate the total pressure required to reduce the mole fraction of N2O4 to 0.10.

(3)

(c) (i) What is the effect on Kp, if any, of raising the temperature?

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(1)


(ii) Use your answer to (c)(i) to explain the effect of increasing the temperature on the position of equilibrium.

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(2)(Total 10 marks)

45. (a) The values of the ionic product of water, Kw, at two different temperatures are shown in the table below.

Temperature /C Kw/ mol2 dm6

25 1.00 1014

50 5.48 1014

(i) Write an equation to represent the ionisation of water.

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(1)

(ii) Write the expression for Kw.

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(1)

(iii) Define the term pH.

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(1)


(iv) Calculate the pH of pure water at 50 C.

(2)

(v) Explain why pure water at 50 C is neutral despite the fact that its pH is not 7.

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(1)

(b) The pH curve shown below was obtained when a 0.100 mol dm3 solution of sodium hydroxide was added to 25.0 cm3 of a 0.100 mol dm3 solution of ethanoic acid.

1 4

1 2

1 0

8

6

4

2

00 1 0 2 0 3 0 4 0

p H

V o l u m e 0 . 1 0 0 m o l d m N a O H / c m 3 3


(i) What volume of sodium hydroxide solution is required to neutralise half of the ethanoic acid in this reaction?

Volume added = ..............................................................cm3

(1)

(ii) Use the graph to determine the pH when the volume of sodium hydroxide you havestated in part (i) has been added.

pH is .....................................................................................

(1)

(iii) Write an expression for the acid dissociation constant, Ka, of ethanoic acid, CH3COOH.

(1)


(iv) Use your answers to parts (ii) and (iii) to determine the value of Ka for ethanoic acid at the temperature of the titration. Give your answer to two significant figures.

(2)

(c) Phenolphthalein is a suitable indicator for a titration between ethanoic acid and sodium hydroxide solutions whereas methyl orange is not a suitable indicator.

Explain why this is so.

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(2)(Total 13 marks)

46. (a) Define the term standard electrode potential, making clear the meaning of standard.

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(2)


(b) Explain why a reference electrode is needed whenever a standard electrode potential is measured.

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(1)

(c) Hydrogen peroxide decomposes in a disproportionation reaction.

(i) Explain the meaning of disproportionation.

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(2)

(ii) Use the following information to derive the equation for the disproportionation of hydrogen peroxide.

Calculate Ecell and explain whether the reaction is thermodynamically feasible.

O2 + 2H+ + 2e H2O2 E

= + 0.68 VH2O2 + 2H

+ + 2e 2H2O E = + 1.77 V

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(3)


(iii) Explain why your answer to (ii) does not necessarily show that hydrogen peroxide will disproportionate under standard conditions.

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(1)(Total 9 marks)

47. (a) The conversion of butan-2-ol to 2-bromobutane can be performed as outlined below:

Butan-2-ol is heated with a mixture of 50 % aqueous sulphuric acid and sodium bromide for 45 minutes.

The crude 2-bromobutane is distilled off.

The crude 2-bromobutane is shaken with pure water, which removes the sulphuric acidand some of the butan-2-ol that contaminates the product.

The organic layer is separated and then shaken with concentrated hydrochloric acid to remove residual butan-2-ol.

The organic layer is then shaken with dilute sodium carbonate solution.

Anhydrous calcium chloride is added to the organic layer and allowed to stand for some hours.

The organic layer is then redistilled in a dry apparatus.

(i) Explain, in terms of kinetic factors, why the mixture is heated for a significant amount of time.

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(1)

(ii) Why is sulphuric acid necessary in the reaction mixture?

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(2)


(iii) Suggest why butan-2-ol, which is only partially miscible with water, is much more soluble in concentrated hydrochloric acid.

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(2)

(iv) Why is the organic layer shaken with dilute sodium carbonate solution?

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(1)

(v) What is the purpose of the anhydrous calcium chloride?

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(1)

(vi) How would you heat the mixture safely? Explain your choice of method.

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(2)


(b) Both 2-bromobutane and butan-2-ol are chiral molecules.

If one optical isomer of 2-bromobutane is used to make butan-2-ol by reaction with aqueous hydroxide ions, the product mixture is not optically active.

The mechanism for the reaction is either SN1 or SN2; these are given below

S 1N C H C H

H C

2 3

3

H

B rC

C H C H

H C

2 3

3

H

+ : B rC+

C H C H

H C

2 3

3

H

+ : O HC+

C H C H

H C

2 3

3

H

O HC

C H C H

H O

2 3

C H 3

HC + : B r

S 2N C H C H

H C

2 3

3

H

B rC

H O :

C H C H

H O

2 3

H

B rC

H C3


Explain which one of these mechanisms is consistent with the lack of optical activity in the product mixture.

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(3)

(c) The oxidation of butan-2-ol with hot potassium dichromate(VI) in acidic solution produces butanone, CH3COCH2CH3.

(i) What would you see as the reaction proceeds?

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(1)

(ii) The dichromate(VI) ion is reduced under these conditions to chromium(III) ions.

The half-equation for the oxidation of butan-2-ol to butanone is

CH3CH(OH)CH2CH3 CH3COCH2CH3 + 2H+ + 2e

Write the ionic half-equation for the reduction of dichromate(VI) ions, and hence derive the overall equation for the oxidation of butan-2-ol.

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(2)


(iii) The IR spectra of butan-2-ol and of the organic product from its oxidation with dichromate(VI) ions are given below.

S p e c t r u m o f b u t a n - 2 - o l

S p e c t r u m o f t h e o r g a n i c p r o d u c t f r o m t h e o x i d a t i o n o f b u t a n - 2 - o l

4 0 0 0 3 0 0 0 2 0 0 0 1 5 0 0 1 0 0 0 5 0 0

1 0 0

5 0

0

W a v e n u m b e r / c m 1

Tran

smitt

ance

/%

4 0 0 0 3 0 0 0 2 0 0 0 1 5 0 0 1 0 0 0 5 0 0

1 0 0

5 0

0

W a v e n u m b e r / c m 1

Tran

smitt

ance

/%

Bond Wavenumber/cm1 Bond Wavenumber/cm1

CH (alkanes) 28503000 CO (alcohols,esters) 10001300

CH (alkenes) 30003100 OH (hydrogen-bonded alcohols) 32303550

C=O (aldehydes,ketones, carboxylic

acids)

16801750 OH (hydrogen-bonded carboxylic

acids)

25003300


What evidence is there from the spectra that the reaction in part (ii) has occurred?

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(2)

(d) When potassium dichromate(VI) is dissolved in water, the following equilibrium is set up

Cr2O72(aq) + H2O(l) 2CrO4

2(aq) + 2H+(aq)

(i) If a solution of barium ions is then added to this solution, solid barium chromate, BaCrO4, is precipitated; it is sparingly soluble in water, so the equilibrium given below also exists in the solution

Ba2+(aq) + CrO42(aq) BaCrO4(s)

Explain what happens to the pH when the barium ions are added.

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(2)


(ii) If a solution of lead(II) ions is added instead of barium ions, solid PbCrO4 is precipitated. This is almost completely insoluble in water so all chromate(VI) ions are removed from solution

Pb2+(aq) + CrO42(aq) PbCrO4(s)

State how the pH of this solution differs from your answer in part (i).

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(1)(Total 20 marks)

48. Phosphorus(V) chloride dissociates as follows:

PCl5(s) PCl3(l) + Cl2(g) H = + 123.8 kJ mol1

Substance Standard entropy, S

/ J mol1 K1

PCl5(s) + 166.5

PCl3(l) + 217.1

Cl2(g) + 165.0

(a) (i) Explain why the entropy of solid phosphorus(V) chloride, PCl5, is smaller than the entropy of liquid phosphorus(III) chloride, PCl3?

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(1)

(ii) Calculate Ssystem for the forward reaction. Include a sign in your answer.

(1)


(iii) Is the sign of Ssystem as you would expect? Fully justify your answer.

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(2)

(b) Calculate Ssurroundings for the forward reaction at 298 K. Include a sign and units in youranswer.

(2)

(c) (i) Use your answers to calculate Stotal for the forward reaction at 298 K. Include a sign in your answer.

(1)

(ii) Comment on the position of equilibrium at 298 K.

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(1)


(d) In an experiment to investigate this equilibrium, 41.7 g of phosphorus(V) chloride (molar mass 208.5 g mol1) was heated in a closed vessel at 150 C until equilibrium was established. The final pressure was found to be 4.32 atm and 0.15 moles of phosphorus(V) chloride remained. At this temperature all of the reactants and products are gaseous.

(i) Give the expression for the equilibrium constant, Kp, and its units at this temperature.

(2)

(ii) Complete the table

Substance Moles at start Moles at equilibrium Partial pressure atequilibrium, peq

/atm

PCl5(g) 0.15

PCl3(g) 0

Cl2(g) 0

Total number of moles at equilibrium(3)

(iii) Calculate Kp.

(1)


(iv) How would you expect the value of Kp to change, if at all, if the following changes were made? Justify each of your answers.

A Only 20.85 g of phosphorus(V) chloride had been used.

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B The temperature had been increased to 250 C.

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(2)(Total 16 marks)

49. Human blood plasma is a buffer solution. It partly owes its buffer properties to carbon dioxide, produced by respiration, dissolving in the blood for transportation to the lungs.

Carbon dioxide dissolves in water establishing the equilibrium

CO2(aq) + H2O(l) H+(aq) + HCO3

(aq)

(a) (i) Write the expression for Ka for this equilibrium and give its units.

Units ........................

(2)

(ii) What is the relationship between pKa and Ka?

(1)


(b) Explain what is meant by a buffer solution.

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Documents

Equilibria Questions and Answers For A2 Chemistry