Physical Chemistry Third Edition

7.3 Chemical Equilibrium in Solutions 315

7.16 a.Find∆G◦at 298.15 K for the gas-phase reaction

2SO 2 (g)+O 2 (g)
2SO 3 (g)

b.FindKfor the reaction at 298.15 K.

c.If a stoichiometric mixture of SO 2 and O 2 is allowed

come to equilibrium at 298.15 K and 1.000 bar, find the

partial pressure of SO 2.

7.17 a. Find the equilibrium constant at 298.15 K for the
reaction

N 2 (g)+3H 2 (g)
2NH 3 (g)

b. Find the equilibrium composition of a system

originally consisting of 0.250 mol of N 2 and 0.750 mol

of H 2 , maintained at 298.15 K and 1.000 atm.

7.18 a.Find∆G◦andKfor each of the following reactions at
298.15 K:

N 2 (g)+O 2 (g)
2NO(g)

2NO(g)+O 2 (g)
2NO 2 (g)

b.Using the results of part a, find∆G◦andKfor the

reaction at 298.15 K:

N 2 (g)+2O 2 (g)
2NO 2 (g)

c.Find∆G◦andKfor the reaction at 298.15 K:

2NO 2 (g)
N 2 O 4 (g)

7.19 Find∆G◦andKat 298.15 K for each of the gas-phase

reactions:

a.N 2 O+O
2NO

b.H 2 +CO 2 
H 2 O+CO

c.O 2 +O
O 3

7.20 Find∆G◦andKat 298.15 K for each of the gas-phase

reactions:

a. H 2 O+

7

2

H 2 S+3NO 2 
3NH 3 +

7

2

SO 2

b. 3O 2 
2O 3

7.21 Find the total pressure such that at equilibrium at 298.15 K

the partial pressure of NO 2 is twice as large as the partial

pressure of N 2 O 4. Assume that the gases are ideal.

7.3 Chemical Equilibrium in Solutions

The activity of a solute is given in one of several different ways, as described in

Chapter 6. In discussing chemical equilibrium, one must specify which description is

being used, since activities and standard states are different for different descriptions,

and equilibrium constants have different values in different descriptions. We will usu-

ally use the molality description for aqueous solutions, but the molarity description and

convention II can also be used. In the molality description, the activity of a solute is

given in Eq. (6.3-37):

aiγimi/m◦ (7.3-1)

wheremiis the molality of substance numberi,m◦is exactly equal to 1 mol kg−^1 , and

γiis the activity coefficient of substancei. We will usually omit superscripts on the

activity coefficients and rely on the context to tell which description is being used. The

activity of the solvent is given by Eq. (6.3-46) for any description:

a 1 γ 1 x 1 (7.3-2a)

In a dilute solution, both the activity coefficient and the mole fraction of the solvent

are nearly equal to unity, so that the activity of the solvent is frequently approximated

by unity:

a 1 ≈1 (solvent in a dilute solution) (7.3-2b)