Thermodynamics and Chemistry

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

aiDi (^) ixi
Solvent A of a solution aADA (^) AxA
Nonelectrolyte solute B,
mole fraction basis
ax;BDx;B (^) x;BxB
Nonelectrolyte solute B,
concentration basis
ac;BDc;B (^) c;B
cB
c
Nonelectrolyte solute B,
molality basis
am;BDm;B (^) m;B
mB
m
Electrolyte solute B am;BDm;B (^) 
mC
m
Cm
m

Ion in solution aCDC (^) C
mC
m
aD (^)
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/