Page 163 - Modern physical chemistry

1.11 Gibbs Energy of Reaction 155

The first four tenus on the right give the standard Gibbs energy change,

/)"Go =lG£ +m~ -aG1-bag =l/),.GPL +m/),.GPM -a/),.GPA -MGPB, [7.49]
while the last four tenus combine to give

RTlnJtP~ =RTlnQp. [7.50]
PtP~
Thus, equation (7.48) has the form

/),.G = /)"Go + RTlnQp. [7.51 ]
Expression Qp is called the reaction quotient.
For a general homogeneous region, formula (7.42) replaces (7.40). Then equation
(7.48) is replaced with
I m
/),.G = /)"Go + RTln aLaM = /)"Go + RTlnQ [7.52]
a~a~
where
I m
Q= aLaM . [7.53]
a~a~

Note that in expression Q the activities of products appear in the numerator while
the activities of reactants appear in the denominator. Each activity is raised to a power
equal to the coefficient of the constituent in the chemical equation.

ExampleZ7

From 1.000 mol H2 at 0.01000 bar, 25° C , and 0.500 mol O2 at 0.1000 bar, 25° C, reac-
tion produced 1.000 mol water vapor at 0.0500 bar and 25° C. What was the Gibbs
energy change?
For the given reaction

we have

From table 7.4
/)"Go = -228.62 kJ.
Substituting into equation (7.51) yields

1
/),.G = -228.62 kJ +(8.3145 x 10-3 kJ K- X298.15 K)ln15.81
= -228.62 kJ +6.84 kJ = -221.78 kJ.
The large negative value of /),.G indicates that there is a strong tendency for the reac-
tion to proceed under the given conditions. However, the thermodynamics does not
tell us anything about the rate of the process. This will be considered in the chapters
on kinetics.