Learning Objectives

  • Understand what is meant by the position of equilibrium

The proportion of reactant and product in the equilibrium mixture is referred to as its equilibrium position.

  • Reactions where the mixture contains predominantly products are said to ‘lie to the right’:

For example: 2NO(g) ⇔ N2(g) + O2(g)

(At 700 K, the position of equilibrium lies a long way to the right, because there is a large amount of N2 and O2 and not very much NO at equilibrium.)

  • Reactions with predominantly reactants are said to ‘lie to the left’:

For example, water dissociates according to the equation:

H2O(l) ⇔ H+(aq) + OH(aq)

(At 298 K the number of water molecules present at equilibrium is over 250 million times greater than the total number of H+ and OH ions.)

  • Equilibrium is evenly balanced for some reactions as,

H2(g) + CO2(g) ⇔ H2O(g) + CO(g)

(at 1100 K, the total number of reactant particles is roughly equal to the total number of product particles at equilibrium.)

It is, however, often useful to be able to capture this information mathematically to compare the equilibrium mixtures of different reactions and the effect of different conditions.