6.3Chemical Reactions

Reversible Reactions and Equilibrium — Cambridge O Level Chemistry 5070 Revision Notes & Topical Questions

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Lesson summary

Explain reversible reactions and dynamic equilibrium, predict shifts using Le Chatelier's principle and describe the stages of the Haber process.

Part 1 of 9Reversible reactions

Complete reactions
- reactants are used up and the reaction stops.
Reversible reactions
- products can react to re-form the reactants.
Forward reaction
- forms the products.
- Reverse reaction
  - forms the reactants.
- Symbol
  - written with two half arrows, ⇌.
- Example
  - N₂ + 3H₂ ⇌ 2NH₃ in the Haber process.

Water of crystallisation
- water built into a salt crystal in a fixed ratio, affecting shape and colour.
Copper(II) sulfate
- crystallises as CuSO4.5H2O.
- Dot
  - written between the salt and the water.
Anhydrous salt
- has lost its water of crystallisation, usually on heating.
Cobalt(II) chloride
- CoCl2.6H2O (pink) ⇌ CoCl₂ + 6H₂O (blue).
- Copper(II) sulfate
  - CuSO4.5H2O (blue) ⇌ CuSO₄ + 5H₂O (white).
- Water tests
  - adding water turns blue anhydrous cobalt chloride pink and white anhydrous copper sulfate blue.

Definition
- equilibrium is reached when the forward and reverse reaction rates are equal.
Dynamic
- both reactions continue constantly at the same rate.
- Constant concentrations
  - reactants and products stay constant if conditions do not change.
- Closed system
  - nothing can escape from the vessel.
Haber example
- at the start the forward rate is highest because hydrogen and nitrogen concentrations are highest.
Forward rate
- falls as reactants are used.
- Reverse rate
  - rises as ammonia builds up.
- Equilibrium
  - both rates become equal.

Meaning
- the relative concentrations of reactants and products at equilibrium.
Shift to the left
- more reactant.
- Shift to the right
  - more product.
Le Chatelier's principle
- when a condition at equilibrium is changed, the system moves to oppose the change.
Use
- predict changes of temperature, pressure and concentration.
- Pressure
  - increasing pressure moves equilibrium to the side with fewer gas molecules.

Change	Equilibrium shifts towards	Reason
Increase temperature	The endothermic direction	System absorbs the extra heat
Decrease temperature	The exothermic direction	System releases heat
Increase pressure	The side with fewer gas molecules	System lowers pressure
Decrease pressure	The side with more gas molecules	System raises pressure
Increase concentration of a substance	Away from that substance	System uses it up
Add a catalyst	No shift	Both rates increase equally

ICl + Cl₂ ⇌ ICl₃
- dark brown ICl, yellow ICl₃.
Heating
- the mixture turns dark brown, so the equilibrium moved left, meaning the backward reaction is endothermic.
- More ICl or Cl₂
  - shifts right, giving more yellow product.
- Less ICl or Cl₂
  - shifts left, giving more brown reactant.
2NO₂ ⇌ N₂O₄
- brown to colourless.
Gas molecules
- two on the left, one on the right.
- Higher pressure
  - shifts right, so the mixture becomes paler.
Catalysts
- they speed up both reactions equally, so equilibrium is reached sooner but the position is unchanged.

Hydrogen from methane in natural gas and nitrogen from air are fed into a compressor.
The gases are compressed to about 200 atm (20 000 kPa).
They pass over layers of iron catalyst at 450 °C, where some form ammonia: N₂ + 3H₂ ⇌ 2NH₃.
The mixture is cooled so ammonia liquefies and is removed to pressurised storage.
Unreacted nitrogen and hydrogen are recycled.

Pressure
- higher pressure gives a higher yield of ammonia at any temperature.
Temperature
- lower temperature gives a higher yield.
Real conditions
- chosen by economic, chemical and practical considerations.

The reaction A ⇌ B is exothermic in the forward direction. State the effect of raising the temperature on the yield of B and explain. [2]2 marks
1. The yield of B decreases.
2. The equilibrium shifts in the endothermic (reverse) direction to absorb the added heat.
For 2NO₂ ⇌ N₂O₄, predict and explain the effect of increasing pressure. [3]3 marks
1. Two gas molecules on the left, one on the right.
2. Equilibrium shifts to the right, the side with fewer gas molecules.
3. The mixture becomes paler as more colourless N₂O₄ forms.
Describe what is meant by dynamic equilibrium. [2]2 marks
1. Forward and reverse reactions continue at the same rate.
2. The concentrations of reactants and products remain constant in a closed system.
State the colour change when water is added to anhydrous copper(II) sulfate and write the equation. [2]2 marks
1. White to blue.
2. CuSO₄ + 5H₂O ⇌ CuSO4.5H2O.

Term	Meaning
Reversible reaction	A reaction that can go in both the forward and reverse directions.
Dynamic equilibrium	A state where forward and reverse reaction rates are equal and concentrations are constant.
Le Chatelier's principle	A system at equilibrium opposes any change made to its conditions.
Anhydrous	Containing no water of crystallisation.
Hydrated	Containing water of crystallisation.
Position of equilibrium	The relative amounts of reactants and products at equilibrium.

Saying the reaction stops at equilibrium — both reactions continue at equal rates.
Saying a catalyst changes the yield — it only speeds the approach to equilibrium.
Applying pressure effects when the number of gas molecules is equal on both sides.
Writing a single arrow for a reversible reaction.

Last updated: 19 October 2026