CHAPTER 11 REACTIONS AND OTHER CHEMICAL PROCESSES
11.8 THETHERMODYNAMICEQUILIBRIUMCONSTANT 351
For instance, for the ammonia synthesis reaction N 2 (g) + 3 H 2 (g)!2 NH 3 (g) the reac-
tion quotient is given by
QrxnD
a^2 NH 3
aN 2 a^3 H 2
(11.8.7)
Qrxnis a dimensionless quantity. It is a function ofT,p, and the mixture composition, so
its value changes as the reaction advances.
The expression for the molar reaction Gibbs energy given by Eq.11.8.4can now be
written
ÅrGDÅrGCRTlnQrxn (11.8.8)
The value ofQrxnunder equilibrium conditions is thethermodynamic equilibrium con-
stant,K. The general definition ofKis
K
def
D
Y
i
.ai/eqi (11.8.9)
where the subscript eq indicates an equilibrium state. Note thatK, likeQrxn, is dimension-
less.
The IUPAC Green Book^19 givesK as an alternative symbol for the thermodynamic
equilibrium constant, the appended superscript denoting “standard.” An IUPAC Com-
mission on Thermodynamics^20 has furthermore recommended the name “standard
equilibrium constant,” apparently because its value depends on the choice of stan-
dard states. Using this alternative symbol and name could cause confusion, since the
quantity defined by Eq.11.8.9does not refer to reactants and products in their standard
states but rather to reactants and products in anequilibriumstate.
Substituting the equilibrium conditionsÅrGD 0 andQrxnDKin Eq.11.8.8gives an im-
portant relation between the standard molar reaction Gibbs energy and the thermodynamic
equilibrium constant:
ÅrGD RTlnK (11.8.10)
We can solve this equation forKto obtain the equivalent relation
KDexp