EDEXCEL GCSE - Reversible reactions and equilibria

Chemical reactions are reversible and may reach a dynamic equilibrium. The direction of reversible reactions can be altered by changing the reaction conditions. Ammonia is made by the Haber process.

Reversible reactions

In principle, all chemical reactions are reversible reactions. This means that the products can be changed back into the original reactants. This is not obvious when a reaction 'goes to completion', where very little or no reactants are left. Examples of reactions that go to completion are:

• complete combustion of a fuel
• many precipitation reactions
• effervescence reactions in which a gas escapes

It is more obvious in reactions that do not go to completion that the reaction is reversible. This is the case when the reaction mixture contains both reactants and products.

Examples of reversible reactions

Ammonium chloride

Ammonium chloride is a white solid. It breaks down when heated, forming ammonia and hydrogen chloride. When these two gases are cool enough, they react together to form ammonium chloride again. This reversible reaction can be modelled as:

Ammonium chloride ⇌ ammonia + hydrogen chloride

NH₄Cl(s) ⇌ NH₃(g) + HCl(g)

The symbol ⇌ has two half arrowheads, one pointing in each direction. It is used in equations that model reversible reactions:

• the forward reaction is the one that goes to the right
• the backward reaction is the one that goes to the left

Question

Write the balanced equation for the forward reaction in the breakdown of ammonium chloride.

NH₄Cl(s) → NH₃(g) + HCl(g)

v

Copper sulfate

Blue copper sulfate is described as hydrated. The copper ions in its crystal lattice structure are surrounded by water molecules. This water is driven off when blue hydrated copper sulfate is heated, leaving white anhydrous copper sulfate. This reaction is reversible:

Hydrated copper sulfate ⇌ anhydrous copper sulfate + water

CuSO₄.5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l)

A reversible reaction

1. Bunsen burner heats a bowl of hydrated copper sulfate

2. Water is driven off, leaving anhydrous copper sulfate

3. The burner is turned off and water is added using a pipette

4. The bowl now contains hydrated copper sulfate again

Dynamic equilibrium

When a reversible reaction happens in a closed container, it can achieve a dynamic equilibrium. At equilibrium:

• the forward and backward reactions are still happening
• the rates of the forward and backward reactions are the same
• the concentrations of the reactants and products remain constant (they do not change)

Question

Nitrogen reacts with hydrogen to produce ammonia: 

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Write the balanced equation for the backward reaction. 

Note that equilibrium can only be achieved if none of the reactants or products can escape. For example if calcium carbonate is heated in a sealed container, breakdown occurs and the following equilibrium is made:

CaCO₃(s) ⇌ CaO(s) + CO₂(g)

If this reaction is carried out in an open test tube the breakdown goes to completion:

CaCO₃(s) → CaO(s) + CO₂(g)

This occurs as the carbon dioxide gas escapes and the backwards reaction cannot occur.

The Haber process

Making ammonia

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 main stages in the Haber process

In the Haber process:

1. nitrogen (extracted from the air) and hydrogen (obtained from natural gas) are pumped through pipes
2. a compressor increases the gas pressure to 200 atmospheres
3. the pressurised gases are heated to 450°C and passed through a reaction chamber containing an iron catalyst to speed up the reaction
4. the reaction mixture is cooled so that ammonia liquefies and can be removed
5. unreacted nitrogen and hydrogen are recycled

The conditions are chosen to ensure the process of making ammonia is as profitable as possible.

Question

Explain why iron is used in the Haber process.

Question

State the reaction conditions used in the Haber process.

Changing the position of equilibrium - Higher

The equilibrium position of a reversible reaction is a measure of the concentrations of the reactants and products at equilibrium. Using the Haber process as an example:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

The equilibrium position is:

• to the left if the concentrations of N₂ and H₂ are greater than the concentration of NH₃
• to the right if the concentration of NH₃ is greater than the concentrations of N₂ and H₂

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

• changing the pressure
• changing the concentration
• changing the temperature

Changing the pressure

In a reaction involving gases, if the pressure is increased, the equilibrium position moves in the direction of the fewest molecules of gas.

There are fewer molecules on the right hand side of the equation for the Haber process:

If the pressure is increased, the equilibrium position moves to the right.

Question

Calcium carbonate decomposes when it is heated: CaCO₃(s) ⇌ CaO(s) + CO₂(g)

Predict the effect of increasing the pressure.

Changing the concentration

In a reaction involving solutions, if the concentration of a soluteis increased, the equilibrium position moves in the direction away from this solute. For example, bismuth chloride reacts with water in a reversible reaction:

BiCl₃(aq) + H₂O(l) ⇌ BiOCl(s) + 2HCl(aq)

The concentration of hydrochloric acid can be increased by adding more hydrochloric acid. When this happens, the equilibrium position moves to the left, away from HCl(aq) in the equation.

Question

Iron(III) ions react with thiocyanate ions, SCN^-, in a reversible reaction:

Fe³⁺(aq) + SCN^-(aq) ⇌ FeSCN²⁺(aq)

Predict the effect of adding more iron(III) ions.

Changing the temperature

In a reversible reaction, if the reaction is exothermic in one direction, it is endothermic in the other direction. If the temperature is increased, the equilibrium position moves in the direction of the endothermic process. For example, sulfur dioxide reacts with oxygen in a reversible reaction:

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

If the forward reaction is exothermic, the backward reaction must be endothermic. Therefore, if the temperature is increased, the equilibrium position moves to the left.

Question

Hydrogen can be manufactured by reacting carbon with steam:

C(s) + H₂O(g) ⇌ H₂(g) + CO(g) (forward reaction is endothermic)

Predict the effect of increasing the temperature, at constant pressure.

The equilibrium position will move to the right, in the direction of the endothermic reaction.

Summary of condition changes to the equilibrium position

Change	Equilibrium:
Pressure increased	Position moves towards the fewest molecules of gas
Concentration of a reactant increased	Position moves away from that reactant
Temperature increased	Position moves in the direction of the endothermic reaction