CHAPTER 11 REACTIONS AND OTHER CHEMICAL PROCESSES
11.7 GIBBSENERGY ANDREACTIONEQUILIBRIUM 342
11.7 Gibbs Energy and Reaction Equilibrium
This section begins by examining the way in which the Gibbs energy changes as a chemical
process advances in a closed system at constantT andpwith expansion work only. A
universal criterion for reaction equilibrium is derived involving the molar reaction Gibbs
energy.
11.7.1 The molar reaction Gibbs energy
Applying the general definition of a molar differential reaction quantity (Eq.11.2.15) to the
Gibbs energy of a closed system withT,p, andas the independent variables, we obtain
the definition of themolar reaction Gibbs energyor molar Gibbs energy of reaction,ÅrG:
ÅrG
def
D
X
i
ii (11.7.1)
Equation11.2.16shows that this quantity is also given by the partial derivative
ÅrGD
@G
@
T;p
(11.7.2)
(closed system)
The total differential ofGis then
dGD SdTCVdpCÅrGd (11.7.3)
(closed system)
11.7.2 Spontaneity and reaction equilibrium
In Sec.5.8, we found that the spontaneous direction of a process taking place in a closed
system at constantTandp, with expansion work only, is the direction of decreasingG. In
the case of a chemical process occurring at constantT andp,ÅrGis the rate at whichG
changes with. Thus ifÅrGis positive,spontaneously decreases; ifÅrGis negative,
spontaneously increases. During a spontaneous process dandÅrGhave opposite signs.^14
Note how the equality of Eq.11.7.3agrees with the inequality dG <