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The effect of changing the conditions, including changing the concentration of one of the reactants or products, on an equilibrium system was extensively explored by the French chemist, Henri Le Chatelier.

He devised a general rule about what happens when the conditions of an equilibrium are changed, known now as Le Chatelier's Principle: 'If a dynamic equilibrium is disturbed by changing conditions, the position of equilibrium moves to counteract the change.'

Le Chatelier's Principle doesn't explain why the changes occur but it does provide a useful 'rule of thumb' to help predict the effect of change in many, but not all, cases. In some situations, application of Le Chatelier's Principle leads to an incorrect prediction so it must be used with great care.

Think about the equilibrium A  +  B  _∏  C  +  D

If the concentration of A in an equilibrium mixture is increased, then the system reacts in a way that counteracts that change, ie it reacts to use up some of the extra A. This is achieved by A and B reacting to form C and D.

What would happen to the equilibrium if the concentration of A was decreased?

The system would move in a way which counteracts that change, ie in a way that produces more of A. So C and D would react to form A and B.

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