## Thermodynamics

The following problems are taken from T. L. Brown, H. E. Lemay, B. E. Bursten, & J. R. Burdge, Chemistry: The Central Science, Ninth Edition, Chapter 19. To see the solution to a problem or to return from the solution, click on the number of the problem.

38)  Calculate ∆S° values for the following reactions. In each case explain the sign of  ∆S°:

(a) N2H4(g) + H2(g) 2NH3(g)

(b) Al(s) + 3Cl2(g) 2AlCl3(s)

(c) Mg(OH)2(s) + 2HCl(g) MgCl2(s) + 2H2O(l)

(d) 2CH4(g) C2H6(g) + H2(g)

42) A certain reaction has ∆H° = -19.5 kJ and ∆S° = +42.7 J/K.

(a) Is the reaction exothermic or endothermic?

(b) Does the reaction lead to an increase or decrease in the disorder of the system?

(c) Calculate ∆G° for the reaction at 298 K.

(d) Is the reaction spontaneous at 298 K?

46) Calculate the change in Gibbs free energy for each of the following reactions. In each case indicate whether the reaction is spontaneous under standard conditions.

(a) H2(g) + Cl2(g) 2HCl (g)

(b) MgCl2(s) + H2O(l) MgO(s) + 2HCl(g)

(c) 2NH3(g) N2H4(g) + H2(g)

(d) 2NOCl(g) 2NO(g) + Cl2(g)

50) From the values given for ∆H° and ∆S°, calculate ∆G° for each of the following reactions at 298 K. If the reaction is not spontaneous under standard conditions at 298 K, at what temperature (if any) would the reaction become spontaneous?

(a) 2PbS(s) + 3O2(g) 2PbO(s) + 2SO2(g)                ∆H° = -844 kJ, ∆S° = -165 J/K

(b) 2POCl3(g) 2PCl3(g) + O2(g)                               ∆H° = 572 kJ, ∆S° = 179 J/K

64) Consider the reaction H2(g) + F2(g)   2HF(g).

(a) Calculate ∆G° at 298 K.

(b) Calculate ∆G at 298 K if the reaction mixture consists of 8.0 atm of H2, 4.5 atm of F2, and 0.36 atm of HF.

66) Write the equilibrium-constant expression and calculate the value of the equilibrium constant for each of the following reactions at 298 K.

(a) NaHCO3(s) NaOH(s) + CO2(g)

(b) 2HBr(g) + Cl2(g) 2HCl(g) + Br2(g)

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

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