Chemistry, Third edition

15 · DYNAMIC CHEMICAL EQUILIBRIA

Effect of temperature on equilibrium constants

The effect of temperature upon Kc(T)is summarized as follows:

1.The equilibrium constant for an exothermic reaction decreases with increasing

temperature.

2.The equilibrium constant for an endothermic reaction increases with increasing

temperature.

For example, the formation of HI(g) from H 2 (g) and I 2 (g) is exothermic, and

Kc(T)for this reaction falls with increasing temperature (Fig. 15.6). This means that

the percentage of HI(g) in the equilibrium mixture falls as the temperature is

increased.

The variation of an equilibrium constant with temperature is often so dramatic

that adjusting the operating temperature of a reaction is the most effective way of

improving the yield of product. The following examples illustrate this.

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Non-equilibrium conditions

Kc(T)for the reaction of steam and heated solid iron,

2Fe(s)3H 2 O(g)\===\Fe 2 O 3 (s)3H 2 (g)

is very large at 600 K, yet it is quite easy to reverse this reaction by passing hydrogen gas

over strongly heated iron oxide using the apparatus shown in Fig. 15.5. How is this

possible?

Exercise 15G

Fig. 15.5Heating Fe 2 O 3 in a stream of H 2.

Effect of temperature upon equilibrium

Sketch a diagram, similar to Fig. 15.4, showing a plot of

[HI(g)]^2

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

against time when a HI/H 2 /I 2 mixture at equilibrium at 667 K (where Kc(T)61) is suddenly

warmed up to 764 K (when Kc(T)46), and then allowed to slowly cool back to 667 K.

Exercise 15H

KC

Temperature/K

200

0

800

400

400 800 1200

Fig. 15.6Variation with temperature of

the equilibrium constant for the reaction

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