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1. The reaction between solid barium hydroxide and solid ammonium chloride can be represented by the equation below.
Ba(OH) 2 (s) + 2NH 4 Cl(s) → BaCl 2 (s) + 2NH 3 (g) + 2H 2 O(l) Δ H ο^ = +51.1 kJ mol –
The standard entropies, at 298 K, for the reactants and products are:
S ο[Ba(OH) 2 (s)] = + 99.7 J mol –1^ K–
S ο[NH 4 Cl(s)] = + 94.6 J mol –1^ K–
S ο[BaCl 2 (s)] = + 123.7 J mol –1^ K–
S ο[NH 3 (g)] = + 192.3 J mol –1^ K–
S ο[H 2 O(l)] = + 69.9 J mol –1^ K–
(a) Why is the standard entropy of ammonia more positive than the standard entropy of barium chloride?
(1)
(b) Use the values given to calculate the standard entropy change, Δ S οsystem, for this reaction. Include the sign and units in your answer.
(2)
(c) Calculate the standard entropy change of the surroundings, Δ S οsurroundings , at 298 K for
this reaction.
(2)
(d) Use your answers to (b) and (c) to show that this reaction is feasible at 298 K.
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(1)
(e) Calculate the minimum temperature, in kelvin, at which the reaction is spontaneous.
(2) (Total 8 marks)
(b) Calculate the entropy change for the surroundings, ∆ S οsurroundings, at 298 K, showing your
method clearly.
(2)
(c) (i) Calculate the total entropy change for this reaction, ∆ S οtotal, at 298 K.
(1)
(ii) What does the result of your calculation in (c)(i) indicate about the natural direction of this reaction at 298 K?
Justify your answer.
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(1)
(d) (i) Write an expression for the equilibrium constant, K p , for this reaction.
(1)
(ii) State how you would alter ONE condition to increase the yield of carbon dioxide from this equilibrium reaction.
Justify your answer.
(2) (Total 11 marks)
3. When dinitrogen tetroxide, N 2 O 4 , dissociates, the following equilibrium is established.
N 2 O 4 (g) 2NO 2 (g)
(a) State a property which could be measured to follow the progress of this reversible reaction.
(1)
(b) Write an expression for the equilibrium constant, K c, for this reaction.
(1)
(e) The reaction was repeated at the original pressure, but the temperature was increased to 75 °C. The value of K c was approximately twenty times greater.
How does this information show that the reaction is endothermic?
(1)
(f) Predict the sign of Δ S system for the reaction, giving a reason for your answer.
(2)
(g) Write the equation for the relationship between Δ S surroundings and Δ H for the reaction.
(1)
(h) The magnitude of Δ S system for the reaction is greater than the magnitude of Δ S surroundings.
Explain why this must be the case.
(2) (Total 13 marks)
4. The equation below shows a possible reaction for producing methanol.
CO(g) + 2H 2 (g) → CH 3 OH(l) Δ H ο^ = −129 kJ mol –
(a) The entropy of one mole of each substance in the equation, measured at 298 K, is shown below.
Substance
S ο /J mol−^1 K−^1
CO(g) 197.
H 2 (g) 130.
CH 3 OH(l) (^) 239.
(i) Suggest why methanol has the highest entropy value of the three substances.
(1)
(ii) Calculate the entropy change of the system, Δ S ο system , for this reaction.
(2)
(iii) Is the sign of Δ S ο system as expected? Give a reason for your answer.
(1)
(ii) The catalyst used in this reaction is heterogeneous. Explain this term.
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(1)
(iii) Write an expression for the equilibrium constant in terms of pressure, K p , for this
reaction.
CO(g) + 2H 2 (g) CH 3 OH(g)
(1)
(iv) In the equilibrium mixture at 200 atmospheres pressure, the partial pressure of carbon monoxide is 55 atmospheres and the partial pressure of hydrogen is 20 atmospheres.
Calculate the partial pressure of methanol in the mixture and hence the value of the equilibrium constant, K p. Include a unit in your answer.
(2)
(c) The diagram below shows the distribution of energy in a sample of gas molecules in a reaction when no catalyst is present. The activation energy for the reaction is E A.
(i) What does the shaded area on the graph represent?
............................................................................................................................. (1)
(ii) Draw a line on the graph, labelled E C , to show the activation energy of the catalysed reaction. (1) (Total 17 marks)
5. The reaction between nitrogen and hydrogen can be used to produce ammonia.
N 2 (g) + 3H 2 (g) 2NH 3 (g) Δ H ο^ = – 92.2 kJ mol –
Standard entropies are given below
S ο^ [N 2 (g)] = +191.6 J mol –1^ K–
S ο^ [H 2 (g)] = +130.6 J mol –1^ K–
S ο^ [NH 3 (g)] = +192.3 J mol –1^ K–
(ii) Is this reaction feasible at 298 K? Justify your answer.
...............................................................................................................................
(1)
(d) In industry the reaction is carried out at about 700 K using an iron catalyst and high pressures.
(i) The yield of ammonia produced at equilibrium is less at 700 K than at 298 K, if the pressure remains constant. In terms of entropy, explain why this happens.
(1)
(ii) Higher pressures increase the yield of ammonia at equilibrium. Suggest a reason why pressures greater than 300 atmospheres are not routinely used.
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(1)
(iii) Iron is a heterogeneous catalyst. Explain what is meant by heterogeneous.
(1) (Total 9 marks)
6. (a) The distribution of the energy of particles in a gas at temperature T1 is shown below.
(i) On the diagram above, draw the distribution of energy of particles at a lower temperature, T2. (2)
(ii) Use the diagram to explain why the rate of a reaction increases with an increase in temperature.
(3)
(b) The fermentation of glucose is an exothermic reaction and is catalysed by enzymes in yeast.
C 6 H 12 O 6 (aq) → 2C 2 Η 5 ΟΗ(aq) + 2CO 2 (g)
The reaction is slow at room temperature.
(i) Describe, with the aid of a diagram, an experiment you could do to follow the progress of this reaction at different temperatures.
(4)
(ii) Would you expect ∆ S system to be positive or negative for this reaction? Justify your
answer with TWO pieces of evidence.
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(2)
(iii) Deduce the sign of ∆ S surroundings. Show your reasoning.
(2) (Total 15 marks)
(iv) Deduce the direction of this reaction at 323 K. Justify your answer.
................................................................................................................................
(1)
(b) (i) Write the expression for the equilibrium constant, K p , for this reaction.
(1)
(ii) 100 moles of gaseous carbon monoxide is mixed with excess solid nickel at 323 K in a vessel kept at 1.00 atmosphere pressure. At equilibrium, 1.00 mole of the carbon monoxide has reacted.
Complete the table below and then calculate the value of K p at this temperature. Include the units of K p in your answer.
Substance Moles at start Moles at equilibrium
Partial pressure, p eq /atm
Ni(CO) (^4 )
CO 100 99.
(4)
(iii) As K p has such a small value, suggest THREE ways in which this industrial process could be improved to increase profitability. Justify each of your suggestions.
(3)
(c) The second stage of this process is to recover the nickel from the nickel tetracarbonyl, Ni(CO) 4. By considering your calculations of the entropy changes, suggest how this could be done. Justify your suggestion.
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(2) (Total 16 marks)
