CHAP. 8: CHEMICAL EQUILIBRIUM [CONTENTS] 255
8.5 Le Chatelier’s principle
Chemists are usually interested in what effect a change of temperature, pressure or the initial
composition of a mixture would have on its equilibrium composition. For the study of these
changes we use Le Chatelier’s principle which may be formulated as follows:
When a system in the state of equilibrium is subjected to a change, it strives
to minimize the effect of this change.
8.5.1 Effect of initial composition on the equilibrium extent of reaction
At a given temperature and pressure and at a constant amount of substance entering the
reaction, the equilibrium extent of reaction depends on the initial composition.
- The maximum extent of reaction at a given temperature and pressure and for a constant
total amount of initial substances can be attained when initial substances enter the
reaction in a stoichiometric ratio. - When one reactant is rarer (more expensive), its higher conversion is attained by adding
other reactants in excess.
Note:At a considerable dilution in reactions with ∆ν <0, however, the initial increase in
the equilibrium extent of reaction is followed by its decline. This is so because the excess
second component starts to act as an inert [see8.5.4].- Addition of any product to the initial mixture suppresses the reaction.
8.5.2 Effect of pressure
Reactions with ideal gaseous components
- If the reaction proceeds without any change in the total amount of substances (∆ν(g)=
0),pressure has no effecton the equilibrium composition of the mixture. - If the reaction proceeds with an increasing total amount of substances (∆ν(g)> 0), the
equilibrium extent of the reactiondecreaseswith increasing pressure. - If the reaction proceeds with a decreasing total amount of substances (∆ν(g) < 0), the
equilibrium extent of the reactionincreaseswith increasing pressure.