This guide covers IGCSE Chemistry reversible reactions and equilibrium. You will revise reversible reactions (⇌; can go both ways), dynamic equilibrium (rates equal; concentrations constant; in closed system), Le Chatelier (effect of concentration, temperature, pressure on position of equilibrium), and Haber process (N2 + 3H2 ⇌ 2NH3) as example.

Key concepts to revise

• Reversible – Reaction can go forward and backward (e.g. N2 + 3H2 ⇌ 2NH3); use ⇌.
• Dynamic equilibrium – In closed system; forward and backward rates equal; concentrations of all species constant (but not necessarily equal).
• Le Chatelier – If you change concentration, temperature or pressure, equilibrium shifts to reduce that change. Increase concentration of reactant → shift to products. Increase temperature → shift to endothermic direction. Increase pressure (gases) → shift to side with fewer moles of gas.
• Haber process – N2 + 3H2 ⇌ 2NH3; high pressure (fewer moles on right); moderate temperature (compromise: rate vs yield); Fe catalyst; used to make ammonia (fertilizers).

Exam tips and command words

• State – What is meant by dynamic equilibrium (rates equal; concentrations constant; closed system).
• Describe – The effect of increasing concentration of a reactant (shift to products); increasing temperature for exothermic forward reaction (shift to reactants).
• Explain – Why high pressure is used in Haber process (4 mol → 2 mol; shift to right; more NH3).
• Predict – The direction of shift when concentration, temperature or pressure is changed (Le Chatelier).

Common mistakes

• Saying at equilibrium “reaction stops” (reaction continues both ways; rates are equal).
• Saying “equilibrium shifts to the left” without stating why (e.g. to reduce the increase in temperature if forward is exothermic).
• Not stating that equilibrium is only in a closed system (no matter in or out).

Revision checklist

• Define dynamic equilibrium (rates equal; concentrations constant; closed system).
• State the effect of increasing concentration of a reactant on position of equilibrium (shift to products).
• Explain the use of high pressure in Haber process (shift to right; more NH3).
• Predict the effect of increasing temperature on an exothermic forward reaction (shift to reactants).

Next steps

Book a free trial with an IGCSE Chemistry tutor to practise equilibrium and Le Chatelier questions, or explore Tutopiya’s learning portal for more revision resources.