CHAPTER 11 REACTIONS AND OTHER CHEMICAL PROCESSES
11.8 THETHERMODYNAMICEQUILIBRIUMCONSTANT 355
as reactants or products, these values correctly giveÅrSusing Eq.11.8.22, orÅrGusing
Eq.11.8.24.
The relation of Eq.11.8.23does not apply to an ion, because we cannot write a for-
mation reaction for a single ion. Instead, the relation betweenÅfG,ÅfHandSmis
more complicated.
Consider first a hypothetical reaction in which hydrogen ions and one or more
elements form H 2 and a cation MzCwith charge numberzC:
zCHC(aq)Celements!.zC=2/H 2 (g)CMzC(aq)
For this reaction, using the convention thatÅfH,Sm, andÅfGare zero for the
aqueous HCion and the fact thatÅfHandÅfGare zero for the elements, we can
write the following expressions for standard molar reaction quantities:
ÅrHDÅfH.MzC/ (11.8.25)
ÅrSD.zC=2/Sm.H 2 /CSm.MzC/
X
elements
Si (11.8.26)
ÅrGDÅfG.MzC/ (11.8.27)
Then, fromÅrGDÅrH TÅrS, we find
ÅfG.MzC/DÅfH.MzC/