Physical Chemistry Third Edition

316 7 Chemical Equilibrium

The Gibbs energy change for a standard-state solution reaction can be calculated from

tables of the standard-state Gibbs energy changes of formation, using Eq. (7.1-14).

Table A.8 of Appendix A includes some values of∆fG◦for some solids, liquids, gases,

and solutes (using the molality description).

EXAMPLE 7.6

Using tabulated∆fG◦values, find∆G◦at 298.15 K for the reaction

2 CO(aq)+O 2 (aq)
2CO 2 (aq)

Solution

From values for the molality description in Table A.8 of Appendix A,

∆G◦ 2 ∆fG◦(CO 2 )+(−2)∆fG◦(CO)−∆fG◦(O 2 )

2(− 385 .98 kJ mol−^1 )+(−2)(− 119 .90 kJ mol−^1 )

+(−1)(16.4kJmol−^1 )

− 548 .56 kJ mol−^1

This value compares with− 514 .452 kJ mol−^1 for the gas-phase reaction at the same

temperature.

Exercise 7.5

Find the value of∆G◦for the reaction of Example 7.6 using convention II instead of the molality

description.

The equilibrium constant for a reaction involving only solutes is given in the molality

description by

KQeq

∏c

i 2

(γimi,eq

m◦

)vi

(only solutes in reaction) (7.3-3)

The equilibrium constant for a reaction involving the solvent is given by

KQeq(γ 1 x 1 )v^1

∏c

i 2

(γ

imi,eq

m◦

)vP (solvent and

solutes in reaction)

(7.3-4)

In a dilute solution,x 1 andγ 1 are both nearly equal to unity, so that the(γ 1 x 1 )v^1 factor

can be omitted to a good approximation.

The value of the equilibrium constant for any reaction can be calculated from the

value of∆G◦. Once the equilibrium constant is evaluated, the equilibrium composition

can be calculated for any particular case, if information about activity coefficients is

available from experimental data or from theoretical estimates. If a solution is dilute