CHAPTER 12 EQUILIBRIUM CONDITIONS IN MULTICOMPONENT SYSTEMS
12.9 REACTIONEQUILIBRIA 408
Table 12.2 Expressions for activities (from Table9.5and Eqs.
10.1.14and10.3.16)
Species Activity
Pure gas a(g)D
f
p
Pure liquid or solid aD
Substanceiin a gas mix-
ture
ai(g)D
fi
p
Substanceiin a liquid or
solid mixture
aiD i (^) ixi
Solvent A of a solution aAD A (^) AxA
Nonelectrolyte solute B,
mole fraction basis
ax;BD x;B (^) x;BxB
Nonelectrolyte solute B,
concentration basis
ac;BD c;B (^) c;B
cB
c
Nonelectrolyte solute B,
molality basis
am;BD m;B (^) m;B
mB
m
Electrolyte solute B am;BD m;B (^)
mC
m
Cm
m
Ion in solution aCD C (^) C
mC
m
a D (^)
m
m
An approximate version of this relation, found by treatingVB^1 as independent of pressure,
is
kH,B.p 2 /kH,B.p 1 /exp
VB^1 .p 2 p 1 /
RT
(12.8.35)
Unlessjp 2 p 1 jis much greater than 1 bar, the effect of pressure onkH,Bis small; see
Prob. 12. 12 for an example.
12.9 Reaction Equilibria
The definition of the thermodynamic equilibrium constant of a reaction or other chemical
process is given by Eq.11.8.9:
KD
Y
i
.ai/eqi (12.9.1)
The activityaiof each reactant or product species is based on an appropriate standard state.
We can replace each activity on the right side of Eq.12.9.1by an expression in Table12.2.
For example, consider the following heterogeneous equilibrium that is important in the
formation of limestone caverns:
CaCO 3 .cr;calcite/CCO 2 .g/CH 2 O.sln/ïCa^2 C.aq/C2HCO 3 .aq/