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Chemical Equilibria and Le Chatelier's Principle

Many chemical reactions are reversible, which is to say they can progress in both the forwards and backwards directions, and are symbolised by a reversible arrow. In reversible reactions:

Forward and reverse reactions occur at the same rate

The concentrations of reactants and products remain constant

With equations that can achieve equilibrium, then if they are in a closed system, we would have both reactants and products together. This may sometimes be desirable, but most often we would want to change the equilibrium. For example, in an industrial process we would want to increase the amount of product from the reaction.

It is possible to change the position of the equilibrium:

If the proportion of the products in the equilibrium mixture is increased we can say the equilibrium has moved to the right.

If the proportion of the products in the equilibrium has decreased, we can say the equilibrium has moved to the left.

Le Chatelier's Principle

Le Chatelier's principle states that: If a system at equilibrium is disturbed, the equilibrium moves in the direction that tends to reduce the disturbance.

Le Chatelier's principle tells us that if any factor is changed which affects the equilibrium mixture, the equilibrium position will shift to oppose the change.

This principle cannot be used to predict quantities as it cannot tell us how far the position of the equilibrium will change, only that it will change and in which direction.

Effects of a change in concentration:

The principle tells us that if we increase the concentration of one of the reactants in a given reaction, the equilibrium will move in the direction that tends to reduce that reactant.

Consider:

A + B ⇌ C + D

If we add more of substance B to the reaction, we effectively increase its concentration in the reaction chamber. The only way the system can regain equilibrium is if some of B reacts with some of A to produce more C and D. This would move the equilibrium to the right.

The same would happen if we added more of substance A

If we removed either C or D from the reaction chamber, the equilibrium would again move to the right, with more C and D being produced, using up some of A and B in the process.

If we added more of substance D to the reaction chamber, then the equilibrium would move to the left, with more A and B being produced.