Topic 17: Equilibrium - 17.1 The equilibrium law

Nature of science: Employing quantitative reasoning-experimentally determined rate expressions for forward and backward reactions can be deduced directly from the stoichiometric equations and allow Le Chatelier's principle to be applied. (1.8, 1.9)


Essential idea: The position of equilibrium can be quantified by the equilibrium law. The equilibrium constant for a particular reaction only depends on the temperature

Le Châtelier's principle for changes in concentration can be explained by the equilibrium law

The position of equilibrium corresponds to a maximum value of entropy and a minimum in the value of the Gibbs free energy.

The Gibbs free energy change of a reaction and the equilibrium constant can both be used to measure the position of an equilibrium reaction and are related by the equation, ΔG = -RTlnK.

Applications and skills

Solution of homogeneous equilibrium problems using the expression for Kc.

Relationship between ΔG and the equilibrium constant.

Calculations using the equation ΔG = -RTlnK .


The expression ΔG = -RTlnK is given in the data booklet in section 1.

Students will not be expected to derive the expression ΔG = -RTlnK

The use of quadratic equations will not be assessed.