MYP 10
Coursework

Equilibrium and reversible reactions


Le Chatelier's principle

"An equilibrium will respond to a change in conditions in such a way as to oppose the effect of that change"

Temperature change

In any equilibrium the forward and reverse reactions are opposite in terms of energy, one being exothermic and the other endothermic. This is a consequence of the law of conservation of energy.

A B (exothermic)

B A (endothermic)

If the temperature of an equilibrium is increased then the equilibrium will move in the direction of endothermic change and so some of the increased energy available from the temperature increase is absorbed.

Example: The Haber Process

In the Haber process (industrial manufacture of ammonia) the forward reaction is exothermic and the reverse reaction endothermic.

N2 + 3H2 2NH3

Increasing the temperature makes more energy available - the equilibrium responds by making more nitrogen and hydrogen and absorbing some of the extra energy. The equilibrium position shifts to the left hand side - there is now a greater proportion of N2 and H2 and less NH3.

Decreasing the temperature has the opposite effect - the equilibrium moves in the direction of exothermic change (forward) and produces a greater proportion of ammonia.

Pressure change

If the pressure of an equilibrium is increased the equilibrium moves in the direction of the fewest number of moles of gas so as to reduce the pressure on the system. This condition only applies to gaseous equilibria.

Example: The Haber Process

In the Haber process (industrial manufacture of ammonia) there are four moles of gas on the left hand side (the reactants) and two moles of gas on the right hand side (ammonia - the product). Moving to the left will increase the number of moles of gas and hence increase the pressure of the system. The forward reaction produces fewer moles and consequently reduces the pressure on the system.

N2 + 3H2 2NH3

Increasing the pressure therefore favours formation of ammonia.