(a) A sample of Dry Ice (solid CO2) at -78ºC or CO2 vapor at 0ºC
(b) Sugar, as a solid or dissolved in a cup of tea
(c) Two 100-mL beakers, one containing 1 mol of pure water and the other
containing 1 mol of pure alcohol, or a beaker containing a mixture of the
water and alcohol
7. Which substance has the higher entropy in each of the following pairs?
(a) A sample of pure silicon (to be used in a computer chip) or a piece of
silicon containing a trace of some other atoms, such as B or P
(b) An ice cube or liquid water, both at 0ºC
(c) A sample of pure solid I2 or iodine vapor, both at room temperature
28. Calculate the entropy change, DSº, for each of the following changes and comment on the sign of the change:
(a) NH4Cl(s)
NH4Cl(aq)(b) C2H5OH(l)
C2H5OH(g)(c) CCl4(g)
CCl4(l)21. Cyclohexane is produced by adding hydrogen gas to benzene.
C6H6(l) + 3 H2(g)
C6H12(l)benzene cyclohexane
The enthalpies of formation are
DHºf[C6H6(l)] = + 49.0 kJ/mol
DHºf[C6H12(l)] = - 156.4 kJ/mol
Is this reaction likely to be product-favored or reactant-favored? Explain your
reasoning briefly.
23. Classify each of the reactions according to one of the four reaction types
summarized in Table 20.2.
(a) C6H12O6(s) + 6 O2(g)
6 CO2(g) + 6 H2O(l)DHº = -673 kJ DSº = 60.4 J/K
(b) MgO(s) + C(graphite)
Mg(s) + CO(g)DHº = 49.118 kJ DSº = 197.67 J/K
28.Using values of DHºf and Sº, calculate the standard molar free energy of formation,
DGºf, for each of the following compounds.
(a) CS2(g)
(b) N2H4(l)
(c) COCl2(g)
Compare your calculated values of DGºf with those listed in Appendix K. Which
reactions are predicted to be product-flavored?
31.Hydrazine is used to remove dissolved oxygen from the water in hot-water
heating systems.
N2H4(l) + O2(g)
2 H2O(l) + N2(g)What is the value of DGºrxn when 1.00 mol of N2H4 is oxidized? What is the
value of DGºrxn for the oxidation of 1.00 kg of hydrazine?
39. Methanol, CH3OH, is now widely used as a fuel in race cars such as those that complete
in the Indianapolis 500. The liquid fuel can be formed using the reaction
C(graphite) + 1/2 O2(g) + 2 H2(g)
CH3OH(l)Calculate Kp for the formation of methanol at 25ºC, Comment on the connection
between the sign of DGº and the magnitude of Kp.
40. Ethylene (ethene) reacts with hydrogen to produce ethane.
H2C=CH2(g) + H2(g)
H3C-CH3(g)(a) Using the data in these tables, calculate DGº for the reaction at 25ºC. Is the reaction
predicted to be product-favored under standard conditions.
(b) Calculate Kp from DGºrxn. Comment on the connection between the sign of
DGº and the magnitude of Kp.
41. Use the data in these tables to calculate DGº and Kp at 25ºC for the reaction
2 HBr(g) + Cl2(g)
2 HCl(g) + Br2(l)Comment on the connection between the sign of DGº and the magnitude of Kp.