Chemistry, Third edition

EQUILIBRIUM LAW AND EQUILIBRIUM CONSTANT

Equilibrium law and equilibrium

constant

Equilibrium law

For any chemical reaction at equilibrium we define an equilibrium constantKc(T)in

terms of concentration as follows. For the generalized reaction

aAbB... \===\cCdD...

Kc(T)

[C]c[D]d

... (15.3)
[A]a[B]b

The fact that all equilibrium reactions can be fitted to this general equation is some-

times called the equilibrium law. The coefficients in the chemical equation are a,b,c,

d,... and the square brackets represent the concentrations of the reactants and

productsat equilibrium(notthe starting concentrations). The TinKc(T)reminds us

that the equilibrium ‘constant’ depends upon temperature.

Applying the equilibrium law to the expression

H 2 (g)I 2 (g)\===\2HI(g)

gives

[HI(g)]^2

Kc(T)

[H 2 (g)][I 2 (g)] (15.4)

15.2

267

Example 15.1

Write down an expression for the equilibrium constant for the

reaction

2SO 2 (g)O 2 (g)\===\2SO 3 (g) (15.5)

and state its units.

Answer

We might represent this equation more clearly as:

2 SO 2 (g) 1 O 2 (g)\===\ 2 SO 3 (g)

in which the coefficients (2, 1 and 2, respectively) are displayed in bold.

Comparison with equation (15.3) shows that, for this reaction,

A = SO 2 B = O 2 C = SO 3 a= 2 b= 1 c= 2

The expression for the equilibrium constant therefore becomes

[SO 3 (g)]^2

Kc(T)

[SO 2 (g)]^2 [O 2 (g)]

The units of an equilibrium constant depend upon the number of concentration

terms in the expression. Here the units are

(mol dm^3 )^2 (mol dm^3 )^2

Kc(T)

(mol dm^3 )^2 (mol dm^3 )



(mol dm^3 )^2 (mol dm^3 )

i.e.Kc(T)possesses the units of mol^1 dm^3.