Thermodynamics and Chemistry

CHAPTER 11 REACTIONS AND OTHER CHEMICAL PROCESSES

11.7 GIBBSENERGY ANDREACTIONEQUILIBRIUM 345



G



G



G

(a) (b) (c)

Figure 11.14 Gibbs energy versus advancement at constantTandpin systems of

pure phases.Gis a linear function ofwith slope equal toÅrG.

(a)ÅrGis negative;spontaneously increases.

(b)ÅrGis positive;spontaneously decreases.

(c)ÅrGis zero; the system is in reaction equilibrium at all values of.

bc

(^0)  eq
G
!
Figure 11.15 Gibbs energy as a function of advancement at constantTandpin a
closed system containing a mixture. The open circle is at the minimum value ofG.
(The reaction is the same as in Fig.11.6on page 317 .)
The condition of reaction equilibrium given byÅrGD 0 or

P

iiiD^0 is a general one
that is valid whether or not the reaction proceeds at constantT andp. Suppose a sponta-
neous reaction occurs in a closed system at constant temperature andvolume. The system
is at reaction equilibrium when

P

iiibecomes equal to zero. To relate this condition to
the change of a thermodynamic potential, we take the expression for the total differential of
the Helmholtz energy of an open system, with expansion work only, given by Eq.5.5.8on
page 141 :
dADSdTpdVC

X

i

idni (11.7.10)

When we make the substitution dniDid, we obtain an expression for the total differen-