1.6 Chemical equilibria and Le Chatelier’s principle (Equilibrium AS)
In contrast with kinetics, which is a study of how fast reactions occur, a study of equilibria indicates how far reactions will go. Le Chatelier’s principle can be used to predict the effects of changes in temperature, pressure and concentration on the yield of a reversible reaction. This has important consequences for many industrial processes, as many chemical reactions are reversible.
In a reversible reaction at dynamic equilibrium:
• forward and reverse reactions proceed at equal rates
• the concentrations of reactants and products remain constant
• the equilibrium constant Kc is deduced from the equation for a reversible reaction.
The position of the equilibrium and ‘yield’
The ‘yield‘ indicates the relative proportion of product compared to reactant in the reaction mixture at dynamic equilibrium.
If the equilibrium mixture mainly contains a high concentration of product (high yield), then this means the position of equilibrium is towards the RIGHT side e.g. ionisation of a strong acid in aqueous solution.
…………………………………………………HCl(aq) ⥂ Cl−(aq) + H+(aq)
Concentrations present at equilibrium: 0.05 mol dm−3 0.99 mol dm−3 0.99 mol dm−3
Conversely, if the equilibrium mixture mainly contains a high concentration of reactant (low yield), then this means the position of equilibrium is towards the LEFT side e.g. ionisation of a weak acid in aqueous solution.
…………………………………………………CH3COOH (aq) ⥄ CH3COO−(aq) + H+(aq)
Concentrations present at equilibrium: 1 mol dm−3 0.01 mol dm−3 0.01 mol dm−3
Le Chatelier’s principle
When a system at equilibrium is subjected to a change, the position of the equilibrium moves in the direction to oppose the change.
Applying Le Chatelier’s principle to predict the effect of change upon the position of the equilibrium (and thus yield) in homogeneous equilibria reactions:
Students should be able to:
Explain why, for a reversible reaction used in an industrial process, a compromise temperature and pressure may be used. The hydration of ethene to form ethanol and the reaction of carbon monoxide with hydrogen to form methanol are examples of how these principles have been applied. Students should know the importance of these alcohols as liquid fuels.
Assessment and practical opportunities
Students could carry out test-tube equilibrium shifts to show the effect of concentration and temperature (eg
[Co(H2O)62+] with concentrated HCl).
- Industrial examples of equilibria (Contact and Haber) - Le Chatelier’s Principle This was stated in AS and A2 Chemical Equilibria topics. In 1888, Henri Le Chatelier put forward his principle which dealt with the effects of constraints upon a system in equilibrium. If the conditions of a reversible reaction are changed and disturb the equilibrium, the composition of the mixture will tend to change to restore the equilibrium and to
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