The Foundations of Chemistry

In general, numerical values for Kccan come only from experiments. Some equilib-

rium constant expressions and their numerical values at 25°C are

N 2 (g)O 2 (g) 34 2NO(g) Kc4.5 10 ^31

CH 4 (g)Cl 2 (g) 34 CH 3 Cl(g)HCl(g) Kc1.2 1018

N 2 (g)3H 2 (g) 34 2NH 3 (g) Kc3.6 108

We have used the subscript “eq” to emphasize that the concentrations in the equilibrium

constant expression are those at equilibrium. For the remainder of this text, we shall omit

these subscripts, remembering that calculations with Kcvalues always involve equilibrium

valuesof concentrations.

The thermodynamic definition of the equilibrium constant involves activities rather

than concentrations. The activityof a component of an ideal mixture is the ratio of its

concentration or partial pressure to a standard concentration (1 M) or pressure (1 atm).

For now, we can consider the activity of each species to be a dimensionless quantity whose

numerical value can be determined as follows.

1.For any pure liquid or pure solid, the activity is taken as 1.

2.For components of ideal solutions, the activity of each component is taken to be

the ratio of its molar concentration to a standard concentration of 1 M,so the units

cancel.

3.For gases in an ideal mixture, the activity of each component is taken to be the ratio

of its partial pressure to a standard pressure of 1 atm, so again the units cancel.

Because of the use of activities, the equilibrium constant has no units; the values we put into

Kcare numerically equal to molar concentrations, but are dimensionless,that is, they have no

units. In this text, calculations have usually included units along with numbers. Calcula-

tions involving equilibrium are frequently carried out without units; we will follow that

practice in this text.

The magnitude of Kcis a measure of the extent to which reaction occurs. For any

balanced chemical equation, the value of Kc

1.is constant at a given temperature;

2.changes if the temperature changes;

3.does not depend on the initial concentrations.

A value of Kcmuchgreater than 1 indicates that the “numerator concentrations” (prod-

ucts) would be much greater than the “denominator concentrations” (reactants); this means

that at equilibrium most of the reactants would be converted into products. For example,

the reaction CH 4 (g)Cl 2 (g) 34 CH 3 Cl(g)HCl(g) shown earlier goes nearly to comple-

tion; in Chapter 15, we called such a reaction “product-favored.” On the other hand, if

Kcis quite small, equilibrium is established when most of the reactants remain unreacted

and only small amounts of products are formed. The reaction N 2 (g)O 2 (g) 34 2NO(g)

shown earlier reaches equilibrium with only a tiny amount of NO present; in Chapter 15,

we called such a reaction “reactant-favored.”

[NH 3 ]eq^2



[N 2 ]eq[H 2 ]eq^3

[CH 3 Cl]eq[HCl]eq



[CH 4 ]eq[Cl 2 ]eq

[NO]eq^2



[N 2 ]eq[O 2 ]eq

These are three very important ideas.

712 CHAPTER 17: Chemical Equilibrium

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 16.6, Writing Equilibrium
Expressions.

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 16.5, The Meaning of the
Equilibrium Constant.