Equilibrium – IGCSE Chemistry Definition

Equilibrium is the state reached when a reversible reaction proceeds in both directions at equal rates, so the concentrations of reactants and products remain constant. The 0620 syllabus (Supplement) requires you to define dynamic equilibrium and apply Le Chatelier’s principle to predict how changes in conditions shift the position of equilibrium.

The 0620 definition

A chemical equilibrium is a dynamic state in which the rate of the forward reaction equals the rate of the reverse reaction. The concentrations of reactants and products remain constant (but not necessarily equal).

Key word: dynamic — both reactions are still happening, just at equal rates.

Conditions for equilibrium

1. The reaction must be reversible
2. The system must be closed (no substances added or removed)
3. Both forward and reverse reactions must be occurring at equal rates

What equilibrium is NOT

• Equilibrium does not mean equal amounts of reactants and products. The position of equilibrium can favour one side.
• Equilibrium does not mean the reactions have stopped. Both reactions continue — the system is dynamic.
• Equilibrium is not permanent — changing conditions shifts it.

Shifting the position of equilibrium

The position of equilibrium can be shifted by changing temperature, pressure (for gases), or concentration. A catalyst does NOT shift the equilibrium position — it only helps equilibrium be reached faster.

Change	Effect on equilibrium
Increase temperature	Shifts in the endothermic direction
Decrease temperature	Shifts in the exothermic direction
Increase pressure	Shifts toward the side with fewer gas molecules
Decrease pressure	Shifts toward the side with more gas molecules
Increase concentration of reactant	Shifts toward products
Add a catalyst	No shift — equilibrium reached faster but position unchanged

Worked exam question

The Haber process: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (forward reaction is exothermic). (a) What happens to the position of equilibrium if the temperature is increased? Explain. (2) (b) What effect does a catalyst have on the position of equilibrium? (1)

Mark scheme

(a) The equilibrium shifts to the left / toward the reactants / in the reverse/endothermic direction [1]; because increasing temperature favours the endothermic reaction to absorb the extra heat [1]

(b) No effect on the position of equilibrium [1] — a catalyst only increases the rate at which equilibrium is reached

Common exam mistakes

• Saying equilibrium means “the reaction has stopped”. It has not stopped — both forward and reverse reactions continue at equal rates. Always use the word “dynamic.”
• Claiming a catalyst shifts the equilibrium. Catalysts speed up both the forward and reverse reactions equally — the position does not change.
• Confusing “constant concentrations” with “equal concentrations”. At equilibrium, concentrations are constant but rarely equal.