Chemistry, Third edition

MEANING OF EQUILIBRIUM CONSTANTS

Values of equilibrium constants for reverse reactions

The equilibrium constant for the formation of hydrogen iodide,

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

is

[HI(g)]^2

Kc(T)f

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

where ‘f’ stands for ‘forward reaction’. The equilibrium constant for the reverse or

‘back’ reaction,

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

is

Kc(T)b

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

[HI(g)]^2

from which we see that

1

Kc(T)b

Kc(T)f

We conclude that if the equilibrium constant for the forward reaction is large, then the

equilibrium constant for the reverse process is small.

271

Equilibrium

constants for

reverse reactions

The equilibrium constant

for the formation of nitric

oxide (NO):

N 2 (g)O 2 (g)\===\2NO(g)

is 7  10 ^9 at 1100 K. In

an experiment, NO is

heated to 1100 K. Will the

NO decompose?

Exercise 15D

Equilibrium constants

(i)Which of the reactions shown in Table 15.1 go to

completion at the temperatures listed?

(ii)The equilibrium constant for the reactions of sodium and

magnesium ions with the complexing agent EDTA^4 at

room temperature are 50 mol^1 dm^3 and 5 

108 mol^1 dm^3 respectively. Which ion cannot be

satisfactorily analysed by titration with EDTA^4 , and why?

Exercise 15C

Equilibrium constants as indicators of whether or not a

reaction is allowed to occur

Chemical reactions with large equilibrium constants are said to be ‘thermodynamically

allowed’. However, equilibrium constants tell us nothing about whether or not the

reaction under consideration will occur at a fast enough speedto be detectable. Many

reactions possess large Kc(T)values, but are extremely slow in reaching equilibrium at

room temperature, for example:

●The formation of ammonia gas from hydrogen and nitrogen gases, occurs at a

negligible rate at room temperature.

●A mixture of hydrogen and oxygen gases at room temperature show no signs of

producing water, no matter how long it is kept.

Fortunately, the rates of reactions can (within certain limits) be controlled by

altering the experimental conditions, and reactions may often be accelerated using a

combination of high temperature, high reactant concentration and a catalyst.