Chapter 19: Thermodynamics

1. Distinguish between a spontaneous process and a nonspontaneous process, and give an example of each.

2. Which of the following processes are spontaneous, and which are nonspontaneous:

a)  Freezing water at 2˚C

b)  Corrosion of iron metal

c)  Expansion of a gas to fill the available volume

d)  Separation of an unsaturated aqueous solution of potassium chloride into solid KCl and liquid water

e)  Dissolution of sugar in hot coffee

f)  Decomposition of NaCl to form solid sodium and gaseous chlorine at 25˚C and 1atm pressure

g)  Uniform mixing of bromine vapor and nitrogen gas

h)  Boiling of gasoline at 25˚C and 1atm pressure

3. Define entropy, and give an example of a process in which the entropy of a system increases.

4. Predict the sign of the entropy change in the system for each of the following processes:

a)  A solid sublimes

b)  A liquid freezes

c)  AgI precipitates from a solution containing Ag+1 and I-1 ions

d)  Gaseous CO2 bubbles out of a carbonated beverage

5. Predict the sign of ΔS in the system for each of the following reactions:

a)  PCl5(g) à PCl3(g) + Cl2(g)

b)  CH4(g) + 2O2(g) à CO2(g) + 2H2O(l)

c)  2H3O+(aq) + CO3-2(aq) àCO2(g) + 3H2O(l)

d)  Mg(s) + Cl2(g) à MgCl2(s)

6. What is meant by the standard molar entropy of a substance? Explain how standard molar entropies are used to calculate standard entropies of solution.

7. Which substance in each of the following pairs would you expect to have the higher standard molar entropy? Explain your reasoning in each case.

a)  C2H2(g) or C2H6(g)

b)  CO2(g) or CO(g)

c)  I2(s) or I2(g)

d)  CH3OH(g) or CH3OH(l)

8. Use the standard molar entropies in Appendix B to calculate ΔS˚ at 25˚C for each of the following reactions. Account for the sign of the entropy change in each case.

a)  2H2O2(l) à 2H2O(l) + O2(g)

b)  2Na(s) + Cl2(g) à 2NaCl(s)

c)  2O3(g) à 3O2(g)

d)  4Al(s) + 3O2(g) à 2Al2O3(s)

9. Use the S˚ values in Appendix B to calculate ΔS˚ at 25˚C fir each of the following reactions. Suggest a reason for the sign of ΔS˚ in each case.

a)  2S(s) + 3O2(g) à 2SO3(g)

b)  SO3(g) + H2O(l) à H2SO4(aq)

c)  AgCl(s) à Ag+(aq) + Cl-(aq)

d)  NH4NO3(s) à N2O(g) + 2H2O(g)

10. When heat is added to the surroundings, the entropy of the surroundings increases. How does ΔSsurr depend on the temperature of the surroundings? Explain.

11. Use the data in Appendix B to calculate ΔSsystem, ΔSsurr, and ΔStotal at 25˚C for the reaction:

N2(g) + 2O2(g) à N2O4(g)

Is this reaction spontaneous under standard state conditions?

12. Copper metal is obtained by melting copper (I) sulfide ores:

Cu2S(s) + O2(g) à 2Cu(s) + SO2(g)

Use the data in Appendix B to calculate ΔSsystem, ΔSsurr, and ΔStotal at 25˚C for this reaction. Is this reaction spontaneous under standard state condition at 25˚C?

13. Describe how the signs of ΔH and ΔS determine whether a reaction is spontaneous or Nonspontaneous at constant temperature and pressure.

14. What determines the direction of spontaneous reaction when ΔH and ΔS are both positive or both negative? Explain.

15. Calculate the melting point of benzoic acid (C6H5COOH) given the following data: ΔHfusion = 17.3kJ/mol and ΔSfusion = 43.8J/(K mol).

16. Calculate the enthalpy of fusion of naphthalene (C10H8) given that its melting point is 128˚C and its entropy of fusion is 47.7J/(K mol).

17.What is meant by the standard state of a substance?

18. Use the data in Appendix B to calculate ΔH˚ and ΔS˚ for each of the following reactions. From the values of ΔH˚ and ΔS˚, calculate ΔG˚ at 25˚C, and predict whether each reaction is spontaneous under standard state conditions.

a)  N2(g) + 2O2(g) à 2NO2(g)

b)  2KClO3(s) à 2KCl(s) + 3O2(g)

c)  CH3CH2OH(l) + O2(g) à CH3COOH(l) + H2O(l)

19. Use the standard free energies of formation in Appendix B to calculate ΔG˚ at 25˚C for each reaction in problem 8.

20. The oxidation of iron metal, 4Fe(s) + 3O2(g) à 2Fe2O3(s), is a spontaneous process, even though ΔS˚ for the reaction is negative. Is this consistent with the second law of thermodynamics?

21. The entropy change for a certain nonspontaneous reaction at 50˚C is 104J/K.

a)  Is this reaction endothermic or exothermic?

b)  How does this process illustrate the second law of thermodynamics?

22. Calculate the normal boiling point of ethanol (CH3CH2OH) given that its enthalpy of vaporization is 38.6kJ and its entropy of vaporization is 110J/K.

23. Indicate whether the following processes are spontaneous or Nonspontaneous:

a)  Heat transfer from a block of ice to a room maintained at 25˚C

b)  Evaporation of water from an open beaker

c)  Conversion of iron (II) oxide to iron metal and oxygen

d)  Uphill motion of a automobile

24. Chloroform (CHCl3) ha a normal boiling point of 61˚C and an enthalpy of vaporization of 29.24kJ. What are its values of ΔGvap and ΔSvap at 61˚C.

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