PHYSICAL CHEMISTRY IN BRIEF

CHAP. 8: CHEMICAL EQUILIBRIUM [CONTENTS] 234

Note:For the negative values of ∆rG, the system composition shifts toward the products,

for the positive values it is vice versa, i.e. the amount of products decrease in the reaction

mixture. Theaffinityof reaction is sometimes defined using the relationA=−∆rG.

Example

Calculate the change in the Gibbs energy of the reaction

0 =CO(g)−C(s)−

1

2

O 2 (g)

at the beginning of the reaction when there is no carbon oxide present in the reacting mixture.

Solution

∆rG= ∆rGst+RTln

aCO

aCa^1 O/ 22

,

where

∆rGst=Gstm,CO−

1

2

Gstm,O 2 −Gstm,C.

The initial activity of carbon oxide (just like its fugacity, partial pressure and concentration) is

zero. Consequently, for any non-zero initial concentrations of the reactants,∆rG=−∞. (∆rGst

always has a finite value).

The dependence of∆rGon the activity of carbon oxide in the reaction mixture is shown in Figure

8.2.3.

8.2.4 Condition of chemical equilibrium

At equilibrium, the Gibbs energy of a closed system reaches its minimum at constant tempera-
ture and pressure [see3.4.6]. Hence, chemical equilibrium is attained at constant temperature
and pressure if (
∂G
∂ξ

)

T,p

= ∆rG= 0 [eq] (8.12)