HKCEE & HKAL Past Paper Questions: Part 10 Chemical Equilibrium 1. HKCEE 1987 I Q1b The following equation represents the equilibrium system of bromine water:

Br2(aq) + 2H2O(l) H3O+(aq) + Br-(aq) + HOBr(aq)

About 2 cm3 of orange-coloured bromine water are placed in each of two test-tubes A and B, and the following tests are carried out:

Test-tube A Test-tube B Add a few drops of NaOH(aq) and shake; then, add concentrated HCl dropwise, with shaking, until the solution is acidic.

Add a few drops of AgNO3(aq) and shake; then, allow to stand in sunlight.

(i) Describe and explain the observations in test-tube A in terms of chemical equilibrium. (ii) What would be observed in test-tube B? Explain your observation.

(9 marks)

2. HKCEE 1990 I Q5c When sulphur dioxide gas reacts with water, the following equilibrium is established:

SO2(g) + H2O(l) 2H+(aq) + SO32-(aq) [H is negative]

(i) Sulphur dioxide gas is bubbled separately into cold water and hot water until an equilibrium is established in each case.

Compare the acidity of the resulting solutions. Explain your answer.

(ii) Sulphur dioxide gas is a common pollutant found in exhaust fumes from factories, and it can be removed by using aqueous sodium hydroxide.

(1) Why is sulphur dioxide gas present in the exhaust fumes? (2) Give TWO reasons why sulphur dioxide gas should be removed from the exhaust fumes? (3) Explain, in terms of the concept of chemical equilibrium, why aqueous sodium hydroxide can be used

to remove sulphur dioxide gas from the exhaust fumes. (7 marks)

3. HKCEE 1991 I Q5a Ammonia can be produced by the reaction between nitrogen and hydrogen in a closed container according to

the following equation:

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

The graph below shows the amounts of nitrogen and hydrogen at different times after mixing 1 mole each of the two gases in the container until equilibrium has been reached.

(i) Which gas is consumed faster before equilibrium is reached? Explain your answer. (ii) (1) When does the system reach equilibrium? Explain your answer. (2) Suggest how the time required for the system to reach the above equilibrium can be shortened. Explain

your answer. (iii) At the above equilibrium, what is the number of moles of (1) nitrogen consumed? (2) hydrogen consumed? (3) ammonia formed? (iv) Do you agree with the statement the yield of ammonia will be increased by adding water to the system?

Explain your answer. (v) The above reaction is used industrially to manufacture ammonia. (1) Name the industrial process. (2) What is the importance of this process?

(13 marks)

4. HKCEE 1992 I Q1d Fixed amounts of sulphur dioxide and oxygen are mixed in a container and heated to 750oC. After some time,

the following equilibrium is established:

2SO2(g) + O2(g) 2SO3(g) H = -197 kJ mol-1

(i) Suggest one method by which the amount of sulphur trioxide in the above equilibrium can be raised. Explain your suggestion.

(ii) In applying the above reaction to the industrial preparation of sulphur trioxide, the following two sets of conditions are proposed:

Reactants Temperature (oC) Pressure (atm) Catalyst Case A SO2 + excess air 1000 100 V2O5 Case B Excess SO2 + air 500 2 Pt

In each of the above cases, identify TWO conditions which are NOT favourable in industry. Explain your answer.

(6 marks)

5. HKCEE 1994 I Q4 The manufacture of methanol can be represented by the following equation.

CO(g) + 2H2(g) CH3OH(g) H = 128 kJ mol-1

The yield of methanol is 45% when the following set of operating conditions is used: Pressure : 200 atm Temperature : 400oC Catalyst : ZnO With the intention of increasing the yield of methanol, a student suggested three other sets of conditions as

tabulated below: Suggested set of conditions Pressures / atm Temperature / oC Catalyst

I 100 400 ZnO II 200 200 ZnO III 200 400 Cr2O3

For each suggested set of conditions, decide and explain whether it can increase the yield of methanol. (6 marks)

6. [HKAL 1987 I Q1a] Below is a graphical plot of the change in concentration with time, of three gases A, B and AB involved in a

reversible reaction. A(g) + B(g) AB(g) H is negative. Changes to the system were made at the 30th minute and the 50th minute.

a. Suggest what was done to the system at the 30th minute. b. Calculate Kc between the 40th and 50th minute. c. Calculate Kc between the 60th and 70th minute. d. Suggest what was done to the system at the 50th minute. e. What change would the introduction of a catalyst have made to the system if it had been added to

the mixture at the 50th minute? (5 marks)

(i) some B has been added [0.015 mol dm-3 of B has been added Or concentration of B has been increased from 0.02 to 0.035 mol dm-3

1

(ii) -1 3[ ] 0.02 16.67 mol dm[ ][ ] 0.04 0.03ABKc

A B= = =

1

(iii) -1 3[ ] 0.01 5 mol dm[ ][ ] 0.05 0.04ABKc

A B= = =

1

(iv) increase in temperature 1 (v) Same equilibrium reached prior to the 50th minute (or faster rate) 1

7. [HKAL 1988 I Q4b] Outline a method to determine the equilibrium constant at 298K of

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

(You should mention all the solutions and apparatus required and outline the necessary experimental steps. Chemical equations should be given wherever appropriate.)

(10 marks)

The method requires the use of known volumes and known concentrations of Ag+(ag) and Fe2+(aq) for the reaction and the determination of the equilibrium concentration of Ag+(aq) by titration with standard KSCN solution.

Procedures: 1. 25.0 cm3 of 0.10M Fe(NO3)2 solution is added to 25.0 cm3 of 0.10 M AgNO3 solution in a

conical flask. 2. Put the mixture in fume cupboard and wait for 10 hours to allow the reaction to reach

equilibrium.

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

3. Filter the mixture. 4. Determine the [Ag+(aq)]eqm by titrating with a standardized solution of KSCN (about

0.1M) Ag+(aq) + SCN-(aq) AgSCN(s) Fe3+(aq) + SCN-(aq) [FeSCN]2+(aq) blood red complex The end point of the titration is indicated by the first appearance of red colour.

5. [Ag+(aq)]eqm= [Fe2+(aq)]eqm = x mol dm-3 [Fe3+(aq)]eqm = Initial concentration of Ag+(aq) x mol dm-3

6. Kc = 3

2[ ( )]

[ ( )] [ ( )]Fe aq eqm

Fe aq eqm Ag aq eqm

+

+ +

1 1 1

1

1

1

1

1 1

1 ------

10

8. [HKAL 1988] Consider the following reversible reaction:

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

a. A 1 dm3 reaction vessel at a given temperature was found to contain 0.11 mole of SO2, 0.12 mole of SO3 and 0.05 mole of O2. What is the equilibrium constant, Kc, of the closed system?

b. Another 1 dm3 reaction vessel contains 64 g of SO2 at the same temperature. How many moles of O2 must be added to this vessel in order that, at equilibrium, half of the SO2 will be oxidized to SO3?

(Relative atomic masses: O, 16.00 ; S, 32.06.) (7 marks)

9. [HKAL 1994 II Q1b] A colorimetric method can be used to provide data for the determination of the equilibrium constant of the

following reaction.

Fe3+(aq) + SCN-(aq) [FeSCN]2+(aq)

Outline such a method for the determination of the equilibrium constant Kc of the above equilibrium.

(4 marks)

10. [HKAL 1995 I Q1e] The reaction between ethanoic acid and ethanol can be represented by the following equation:

CH3COOH(l) + C2H5OH(l) CH3COOC2H5(l) + H2O(l)

12.01 g of ethanoic acid are treated with 4.61 g of ethanol in the presence of a catalyst. When the reaction reaches equilibrium at 298K, 5.04 g of ethanoic acid are found to have reacted.

(i) Name a suitable catalyst for this reaction in the forward direction. (ii) Calculate the equilibrium constant, Kc, for the reaction at 298K. (iii) What additional mass of ethanol would be required in order to use up a further 0.60 g of ethanoic acid? (iv) Would the addition of more of the same catalyst affect the value of Kc? Explain.

(7 marks)

11. [HKAL 1998 I Q2c] At 4200 K, the equilibrium constant for the following reaction is 1.2 10-2. N2(g) + O2(g) 2NO(g)

1.0 mol of O2(g) and 2.0 mol of N2(g) are allowed to react in a 2.0 dm3 closed container. Calculate the concentration of N2(g), in mol dm-3, in the equilibrium mixture at 4200 K.

(3 marks)

12. [HKAL 1999 II Q4a] In the Haber process, ammonia is synthesized by the exothermic reaction of nitrogen and hydrogen at around

723 K.

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

In a simulation of the process, a mixture of nitrogen and hydrogen was placed in a closed container. The initial concentrations of nitrogen and hydrogen were 0.50 mol dm-3 and 1.50 mol dm-3 respectively. When the equilibrium was attained at 723 K, 25.0 % of the original nitrogen was consumed.

(i) Calculate the respective concentrations of nitrogen, hydrogen and ammonia in the equilibrium mixture. (ii) Calculate Kc for the reaction at 723 K. (iii) (I) State, with explanation, the effect of temperature on Kc for the reaction. (II) Explain why the Haber process is not operated at temperatures much higher or much lower than

723 K. (8 marks)

13. [HKAL 2002 II Q3c] At 298 K, the equilibrium constants, Kc , for the reactions (1) and (2) below are 1.8 107 mol-2 dm6 and

2.010-10 mol2 dm-6

respectively.

Ag+ (aq) + 2NH3(aq) Ag(NH3)2+ (aq) (1) AgCl(s) Ag+(aq) + Cl- (aq) (2)

a. For each of the reactions (1) and (2), write an expression for its Kc. b. Calculate the Kc at 298K for the following reaction:

AgCl(s) + 2NH3(aq) Ag(NH3)2+ (aq) + Cl- (aq)

#c. Using your result in (b), calculate the solubility, in mol dm-3, of AgCl(s) in 0.10 M NH3(aq) at 298K.

14. [HKAL 2000 I Q2c] Cerium(III) iodate(V), Ce(IO3)3, is a sparingly soluble ionic compound. (i) Explain why in a solution saturated with cerium(III) iodate(V), the ionic product [Ce3+(aq)][IO3-(aq)]3

is a constant at a given temperature.

(ii) At 298K, the solubility of cerium(III) iodate(V) in water is 1.87 10-3 mol dm-3. For a saturated solution of cerium(III) iodate(V), calculate the ionic product [Ce3+(aq)][IO3-(aq)]3 at 298K.

(4 marks)

15. [HKAL 2007 I Q2a] A brand of sugar-free chewing gum contains urea, CO(NH2)2, as an additive.

(i) Urea reacts with H + (aq) to give ammonium ions and carbon dioxide. Write the chemical equation for this reaction.

(ii) Each piece of the chewing gum contains 1.5 mg of urea. Calculate the number of moles of H + (aq) that can be neutralised by chewing 2 pieces of the gum.

(iii) Tooth enamel consists mainly of hydroxyapatite, Ca5(PO4)3OH, which undergoes continuous mineralisation and demineralisation according to the following equation:

With reference to the above information, suggest why the manufacturer of this brand of sugar-free chewing gum claimed that chewing such gums after meals can help prevent tooth decay.

(5 marks)