A Level Official Exam Material
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Unit 5: Transition Metals and
Organic Nitrogen Chemistry
Topic 12: Entropy and Energetics
For A Level students only
Q1. Which of these changes has the largest increase in the entropy of the system?
A H2O(s) → H2O(l)
B Hg(l) → Hg(g)
C H2O(l) + HCl(g) → H3O+ (aq) + Cl– (aq)
D C(graphite) → C(diamond)
(Total for question = 1 mark)
Q2. Answer the question with a cross in the box you think is correct .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
The entropy change of the surroundings, ΔSsurroundings, and the entropy change of the system, ΔSsystem , for four different reactions are given.
Which of these is thermodynamically feasible?
A reaction P only
B reactions P and Q only
C reaction R only
D reactions R and S only
(Total for question = 1 mark)
Q3. Answer the question with a cross in the box you think is correct .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
The decomposition of calcium carbonate is an important reaction in the manufacture of cement.
CaCO3(s) → CaO(s) + CO2(g) = +178 kJ mol−1
What are the signs of the entropy change of the system, ΔSsystem, and of the entropy change of the surroundings, ΔSsurroundings?
Q4. Answer the question with a cross in the box you think is correct .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
Which of these gases would have the greatest standard molar entropy?
A NH3
B H2
C N2
D SO2
(Total for question = 1 mark)
Q5. Answer the question with a cross in the box you think is correct .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
The standard enthalpy change of solution of potassium chloride, KCl, is +17 kJ mol−1 .
The solubility of potassium chloride in water at 298 K is 359 g dm−3 .
Which of these explains the solubility of potassium chloride in water?
A the hydration enthalpy of K+ and the lattice energy of KCl are exothermic
B the hydration enthalpy of K+ and the lattice energy of KCl are endothermic
C the total entropy change when KCl dissolves is positive
D the total entropy change when KCl dissolves is negative
(Total for question = 1 mark)
Q6. The standard enthalpy change of solution for ammonium nitrate, NH4NO3 , is +25.7 kJ mol–1 .
(a) Calculate the value for the standard entropy change in the surroundings, ΔSsurroundings, when ammonium nitrate dissolves in water at 298 K.
Include a sign and units with your answer. (2)
(b) Explain what can be deduced from your answer in (a) about the sign and the value of the standard entropy change in the system, ΔSsystem , when NH4NO3 dissolves. (3) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(Total for question = 5 marks)
Q7. Answer the question with a cross in the box you think is correct .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
Which of these has the highest standard molar entropy at 298 K and 1 atm pressure?
A carbon dioxide, CO2
B copper, Cu
C ethanol, C2H5OH
D hydrogen, H2
(Total for question = 1 mark)
Q8 Answer the question with a cross in the box you think is correct .
If you change your mind about an answer, put a line through the box and then mark your new answer with a cross .
The standard molar entropy of a substance
A is not affected by changes of state or changes in temperature
B only changes when the temperature changes
C only changes when the substance changes state
D changes when the temperature changes and when the substance changes state
(Total for question = 1 mark)
Q9. Ammonia is manufactured in the Haber Process.
(ii) Explain, using ideas about disorder, whether the sign of your answer to (a)(i) is as expected. (2) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(iii) At 700 K, the enthalpy change for this reaction is ΔH = −110.2 kJ mol−1 .
Calculate the entropy change of the surroundings, ΔSsurroundings , at 700 K.
Include a sign and units in your answer. (2)
(iv) At 700 K, the total entropy change, ΔStotal = −78.7 J K −1 mol−1 .
Calculate ΔSsystem for this reaction at 700 K. Include a sign and units in your answer. (1)
*(v) Explain one advantage and one disadvantage of using a temperature higher than 700 K in the Haber process.
Consider the effect of an increase in temperature on the rate of reaction, on the values of ΔSsurroundings and ΔStotal and on the equilibrium constant. (6)
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(b) (i) Write the expression for the equilibrium constant, Kp , for the reaction in the Haber Process.
(ii) A mixture of nitrogen, hydrogen and ammonia is at equilibrium at 255 atm and 700 K.
The partial pressure of nitrogen is 25 atm and that of hydrogen is 150 atm.
Calculate the equilibrium constant, Kp , for the formation of ammonia, at 700 K. Include units in your answer. (3)
(iii) The pressure of the system is doubled at constant temperature.
Explain the effect of this on the yield of ammonia by considering the change in the Kp expression and hence the position of equilibrium. (3)
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(Total for question = 20 marks)
Q10. Barium oxide is an ionic compound that reacts with water to form barium hydroxide.
(a) A Born-Haber cycle for barium oxide, BaO, is shown. Some of the detail is missing.
The letters R, W and Z represent some missing information.
R ...........................................................
W ........................................................... (2)
(ii) Use the following data to calculate a value for the quantity ΔZ shown on the Born-Haber cycle. Include a sign and units in your answer. (3)
(iii) The table gives some information about the lattice energies of barium oxide and magnesium iodide and shows the % difference between the theoretical and experimental values.
(b) The table gives some information about Group 2 ions, M2+, and their hydroxides.
(i) Calculate the enthalpy change of solution, ΔsolH, of Ba(OH)2 using a fully-labelled Hess's cycle.
[The hydration enthalpy of the hydroxide ion, OH– = –460 kJ mol–1 .] (4)
(ii) Explain why strontium hydroxide is slightly soluble in water, even though the enthalpy change of solution is endothermic. (2)
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(Total for question = 14 marks)
Q11. This question is about halides.
(a) Silicon tetrachloride, SiCl4 , is used in the manufacture of optical fibres.
It can be made by the reaction of silicon carbide, SiC, with hydrogen chloride using a catalyst of nickel(II) chloride.
(ii) In industry, the reaction is carried out at 700 °C.
By considering entropy and other relevant factors, justify the use of this temperature. (3) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(iii) Use your answer from (a)(i) to calculate the equilibrium constant for the reaction at 298 K. (2)
(b) The following data can be used in a Born-Haber cycle for calcium iodide, CaI2
(ii) Use the data to calculate the value for the enthalpy change of formation of calcium iodide. (2)
(iii) The value for the lattice energy of calcium iodide determined experimentally by using the BornHaber cycle differs significantly from the theoretical calculated value.
Explain why the Born-Haber and the theoretical values for the lattice energies are similar for calcium fluoride but significantly different for calcium iodide. (4) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(c) The electron affinity of iodine is –295.4 kJ mol–1 .
Explain how the electron affinity of chlorine differs from that of iodine. (3) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(Total for question = 22 marks)
Q12. The mechanism for the rapid decomposition of hydrogen peroxide, H2O2, in the presence of iodide ions, is:
(a) Write the overall equation for the reaction and hence state the role of the iodide ions. (2) ............................................................................................................................................. .............................................................................................................................................
(ii) Although the oxidation of thiosulfate ions (S2O3 2– ) by hydrogen peroxide is thermodynamically favourable, it does not take place in this clock reaction. Suggest a reason for this. (1) ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. ............................................................................................................................................. .............................................................................................................................................
(Total for question = 3 marks)
Q19. This question is about the thermodynamics of the reaction:
(a) Calculate the entropy change for the reaction, using the information in the table.
Include a sign and units in your answer. (2)
(b) Calculate the enthalpy change of formation, ΔfH, of N2O4 (g) at 298 K, using the information in the table and the enthalpy change of the reaction. Include a sign and units in your answer. (2)
(c) Calculate the entropy change of the surroundings, ΔSsurroundings, at 298 K.
Give your answer to an appropriate number of significant figures.
Include a sign and units in your answer. (3)
(d) (i) Use your answers to parts (a) and (c) to calculate the total entropy change, ΔStotal, for this reaction at 298 K. (1)
(e) (i) Write the expression for the equilibrium constant, Kp, for this reaction, including the units, if any. (2)
(ii) In an experiment, 10 mol of N2O4(g) was placed in a closed container at 50 °C. At equilibrium, 27% of the N2O4(g) had dissociated, and the pressure in the container was 4.0 atm.
Calculate the value of Kp at 50 °C. (4)
(iii) The total pressure is doubled to 8.0 atm. State the effect on Kp. (1) ............................................................................................................................................. .............................................................................................................................................
(iv) The total pressure is doubled to 8.0 atm at constant temperature.
Explain the change in the percentage dissociation of N2O4(g) by considering the effect on the partial pressures of NO2(g) and N2O4(g). (3)
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(Total for question = 20 marks)
If you need the markscheme, including the examiners' reports for each question, and more exam and teaching materials on this topic, contact me through the contact page