Reversible Reactions and Equilibrium

IGCSE Edexcel Chemistry
3.17–3.18 Reversible Reactions
Key Concepts: Some reactions are reversible and shown with the symbol ⇌. In a closed system, the forward and reverse reactions occur at equal rates at equilibrium.

Section A: Reversible Reactions and Equilibrium

1. Define chemical equilibrium. [2]
2. Describe the reversible reaction of hydrated copper(II) sulfate. [3]
3. Describe the reversible reaction involving ammonium chloride and heat. [3]
4. Explain what is meant by dynamic equilibrium in a closed system. [2]
Total marks: 10

Mark Scheme

1. Forward and reverse reactions occur at equal rates and concentrations remain constant [2]
2. Hydrated copper(II) sulfate (blue) ⇌ anhydrous copper(II) sulfate (white) + water; heating drives dehydration [3]
3. Solid NH₄Cl ⇌ NH₃ + HCl gases on heating; gases recombine on cooling [3]
4. Rates equal in closed system, but reactions continue; concentrations constant [2]