A-Level Chemistry AQA 7405

Chemical equilibria and Le Chatelier's principle

Many chemical reactions are reversible.

In a reversible reaction at equilibrium:

• forward and reverse reactions proceed at equal rates
• the concentrations of reactants and products remain constant.

Le Chatelier’s principle.

Le Chatelier's principle can be used to predict the effects of changes in temperature, pressure and concentration on the position of equilibrium in homogeneous reactions.

A catalyst does not affect the position of equilibrium.

Students should be able to:

• use Le Chatelier’s principle to predict qualitatively the effect of changes in temperature, pressure and concentration on the position of equilibrium
• explain why, for a reversible reaction used in an industrial process, a compromise temperature and pressure may be used.

Equilibrium constant K_c for homogeneous systems

The equilibrium constant K_c is deduced from the equation for a reversible reaction.

The concentration, in mol dm^–3, of a species X involved in the expression for K_c is represented by X

The value of the equilibrium constant is not affected either by changes in concentration or addition of a catalyst.

Students should be able to:

• construct an expression for K_c for a homogeneous system in equilibrium
• calculate a value for K_c from the equilibrium concentrations for a homogeneous system at constant temperature
• perform calculations involving K_c
• predict the qualitative effects of changes of temperature on the value of K_c