Everything Science Grade 12

CHAPTER 3. REACTION RATES 3.8

3.8 Le Chatelier’s principle ESCBC

A number of factors caninfluence the equilibriumof a reaction. These are:

1. concentration


2. temperature


3. pressure

Le Chatelier’s Principle helps to predict what effect a change in temperature, concen-

tration or pressure will have on the position of the equilibrium in a chemical reaction.

This is very important,particularly in industrial applications, where yields must be

accurately predicted andmaximised.

DEFINITION: Le Chatelier’s Principle

If a chemical system at equilibrium experiences achange in concentra-

tion, temperature or total pressure the equilibrium will shift in order to

minimise that change and a new equilibrium is established.

The effect of concentration on equi-

librium

ESCBD

If the concentration of asubstance is increased,the equilibrium will shift so that this

concentration decreases. So for example, if the concentration of a reactant was in-

creased, the equilibriumwould shift in the direction of the reaction that uses up the

reactants, so that the reactant concentration decreases and equilibriumis restored. In

the reaction between nitrogen and hydrogen toproduce ammonia:

N 2 (g) + 3H 2 (g)� 2 NH 3 (g)

• If the nitrogen or hydrogen concentration was increased, Le Chatelier’sprinciple
  predicts that equilibriumwill shift to favour the forward reaction so that the excess
  nitrogen and hydrogenare used up to produceammonia. Equilibriumshifts to
  the right.


• If the nitrogen or hydrogen concentration wasdecreased, the reverse reaction
  would be favoured so that some of the ammoniawould change back to nitrogen
  and hydrogen to restoreequilibrium.