Equilibrium - (CCEA)

• In a reversible reaction the products can react to turn back into the reactants.

• A dynamic equilibrium is formed in a reversible reaction when the rates of the forward and reverse reaction are equal and the amounts of the reactants and the products remains constant.

• Le Châtelier’s principle can be used to predict changes to the position of an equilibrium. This is useful when explaining the conditions used in the Haber process (making ammonia).

In many chemical reactions the reactants are converted into products and then the reaction stops.

For example:

2Mg + O₂ → 2MgO

In this reaction, once the magnesium oxide has formed, it cannot go back to reform magnesium and oxygen.

This is a non-reversible reaction.

Other chemical reactions are reversible reactions.

In these reactions, the products, once formed, can change back into the original reactants.

For example:

N₂ + 3H₂ ⇌ 2NH₃

In this reaction, once the ammonia (NH₃) has formed, it can ‘go back’ to reform nitrogen and hydrogen.

Reversible reactions are shown with reversible arrows (⇌).

The reversible arrow has two ‘half-arrows,’ one pointing in each direction.

Hydrated copper(II) sulfate is blue in appearance.

When it is heated, the water of crystallisation is removed, leaving anhydrous copper(II) sulfate which is white in appearance.

Equation: hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water

CuSO₄.5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l)

This reaction is reversible, as the hydrated copper(II) sulfate is reformed if water is added to anhydrous copper(II) sulfate.

When a reversible reaction happens in a closed system (closed container) , a dynamic equilibrium can be achieved.

In a dynamic equilibrium:

• the rates of the forward and reverse reactions are equal.

• the amounts of reactants and products remains constant.

For example, consider a reversible reaction where gas A reacts to form a different gas B:

An everyday illustration of dynamic equilibrium is a person trying to walk up an escalator that is moving downwards.

The person appears to be stationary (not moving), but this is just because the rate of their walking up is equal to the rate the escalator is moving downwards.

The position of an equilibrium can be described as on the ‘left’ or ‘right’.

This describes how much of the reactants (on the left) and the products (on the right) are present at equilibrium.

The diagram below shows an example of an equilibrium on the left and an equilibrium on the right.

The equilibrium position can be changed by altering the reaction conditions, such as by:

• changing the pressure.

• changing the concentration.

• changing the temperature.

The effect of changing a reaction condition on the position of equilibrium can be predicted using Le Châtelier’s principle.

This states that if a change is made to the conditions of a system at equilibrium, then the position of equilibrium moves to oppose that change in conditions.

If the concentration of a reactant or product is changed in a reaction at equilibrium, the position of the equilibrium moves to oppose the change.

For example:

CH₄(g) + H₂O(g) ⇌ CO(g) + 3H₂(g)

If the reaction is at equilibrium and the concentration of CH₄ is increased:

• The equilibrium moves to decrease the concentration of CH₄.
• The forward reaction is favoured which reduces the concentration of CH₄.
• The equilibrium position moves to the right.

For a reaction involving gases, the greater the number of gas particles present, the greater the pressure in the system.

If the pressure exerted on a reaction at equilibrium is changed, the position of the equilibrium moves to oppose the change in pressure.

For example:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

If the reaction is at equilibrium and the pressure is increased:

• The equilibrium moves to decrease the pressure.

• The equilibrium moves in the direction of the side of the reaction with fewer gas particles to reduce the pressure.

• The equilibrium position moves to the right, as there are two gas particles on the right hand side of the equation, and 4 gas particles on the left hand side of the equation.

In a reversible reaction, if the reaction is exothermic in one direction, it is endothermic in the other direction.

If the temperature of a reaction at equilibrium is changed, the position of the equilibrium moves to oppose the change in temperature.

For example:

2SO₂(g) + O₂(g) ⇌ 2SO₃(g) (forward reaction is exothermic)

If the reaction is at equilibrium and the temperature is increased:

• The equilibrium moves to decrease the temperature.

• The reverse reaction is endothermic, so this reaction is favoured to decrease the temperature.

• The equilibrium moves to the left.

Adding a catalyst

Adding a catalyst has no effect of the position of an equilibrium as it increases the rate of the forward and reverse reactions equally.

Ammonia is an important industrial product used to make fertilisers, explosives and dyes.

It is manufactured using the Haber process.

This involves a reversible reaction between nitrogen and hydrogen:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

The reaction can reach a dynamic equilibrium.

The following conditions are used in the Haber process:

The iron catalyst is used to increase the rate of the reaction.

The temperature and pressure conditions used are selected to give a compromise between the yield of ammonia, the rate of the reaction, and the cost of the process.

The reaction between nitrogen and hydrogen is exothermic in the forward direction.

Therefore a low temperature would cause the equilibrium to move to the right and increase the yield of ammonia.

However, a low temperature would cause a very slow rate of reaction.

The temperature used (450^oC) allows a compromise between yield and rate.

There are fewer gas particles on the right hand side of the reaction.

Therefore a high pressure would cause the equilibrium to move to the right and increase the yield of ammonia.

However, a high pressure is expensive to implement and would increase the risk of explosion.

The pressure used (200 atm) allows a compromise between yield and cost.