Chemistry, Third edition

EFFECTS OF CHANGING CONCENTRATION

rium constant is very large (so that the reaction goes to completion) or very small (so

that reaction hardly occurs), changes in concentration are found to have an insignifi-

cant effect upon equilibrium concentrations.

We could come to the same conclusions as in points 2 and 3 above by studying

the expression for the equilibrium constant, equation (15.7). As an example, con-

sider point 3. Imagine that we add fresh ester to a mixture already at equilibrium.

This causes [CH 3 COOC 2 H 5 (l)] to rise, and the ratio of concentrations,

[CH 3 COOC 2 H 5 (l)][H 2 O(l)]

[CH 3 COOH(l)][C 2 H 5 OH(l)]

temporarily exceeds Kc(T). The ratio eventually reduces to Kc(T)because some ester

decomposes into ethanoic acid and ethanol:

CH 3 COOC 2 H 5 (l)H 2 O(l) CH 3 COOH(l)C 2 H 5 OH(l)

The equilibrium mixture now contains fewer moles of ester than the sum of the

number of moles of added ester and the number of moles of ester at equilibrium

before the disturbance took place. Note that it may take several minutes for the

reaction to re-establish equilibrium, as shown in Fig. 15.4.

275

[CH 3 COOC 2 H 5 ][H 2 0]

[CH 3 COOH][C 2 H 5 0H]

Time/s

Equilibrium

restored

Extra

ester

added

4

6

Fig. 15.4Esterification mixture at 25 °C.

If an equilibrium is disturbed (here by

adding ester), the concentrations alter so

as to re-establish equilibrium.

Working out whether or not a reaction has reached equilibrium

The decomposition of ammonia gas was studied at 623 K:

2NH 3 (g)\===\N 2 (g)3H 2 (g)

It was found that Kc(T)0.45 mol^2 dm^6. In a separate experiment, some ammonia gas was

heated to 623 K and the composition of the mixture analysed after a set time. The concentra-

tions were: [N 2 (g)]0.04 mol dm^3 , H 2 (g)0.03 mol dm^3 , and NH 3 (g)0.02 mol dm^3.

Has the reaction reached equilibrium?

Exercise 15F

Chemistry under non-equilibrium conditions

In the laboratory, and in industry, we often deliberately disturb a chemical equilib-

rium. For example, the percentage of the number of moles of ester at equilibrium in

Experiment 2 in Table 15.3, is less than 50%. However, if the ester is physically

removedfrom the reaction mixture by distillation, the composition of the mixture

will adjust to restore the yield of ester. The overall yield of ester (obtained by sum-

ming the ester from several distillations) is then very much greater.