GCSE Chemistry: Reversible Reactions and Equilibrium

Reversible reactions introduce the strangest idea in GCSE Chemistry: a reaction that runs both ways at once, settling into a balance that responds to being disturbed. This quiz covers the concept from the double arrow to full Le Chatelier predictions.

The foundations establish what reversibility means: the ⇌ symbol, the classic laboratory demonstration of hydrated copper(II) sulfate turning white when heated and blue again with water, and the energy symmetry rule — if the forward direction is exothermic, the reverse absorbs exactly the same amount of energy.

Dynamic equilibrium gets careful treatment, because the word "dynamic" is what students misunderstand: both reactions continue, at equal rates, in a closed system, with concentrations constant but not equal — a distinction the multi-select questions probe directly.

The Higher tier core is Le Chatelier's principle applied across all three variables. Temperature is tested on the ammonia synthesis (raising temperature favours the endothermic reverse direction). Pressure is tested twice, in both directions of reasoning: increasing yield of methanol by increasing pressure (fewer gas moles on the product side), and the phosphorus pentachloride decomposition, where increased pressure pushes the equilibrium back towards the single-molecule side. Concentration effects appear alongside pressure in a combined yield-optimisation question — the format industrial-context exam questions use.

The catalyst question closes the loop on a persistent misconception: a catalyst brings equilibrium faster but leaves its position untouched.

Every reaction is given with full state symbols and thermal information, so each prediction can be reasoned rather than recalled — precisely the transferable skill the exam tests with unfamiliar equilibria.

• Interpret the reversible reaction symbol and classic reversible changes
• Apply the energy symmetry of forward and reverse reactions
• Define dynamic equilibrium precisely: equal rates, constant concentrations, closed system
• Predict equilibrium shifts from temperature, pressure and concentration changes using Le Chatelier's principle (Higher)
• State the effect of a catalyst on equilibrium position and the time to reach it

Topic scope follows the Department for Education's GCSE combined science subject content, rate and extent of chemical change strand: reversible reactions, dynamic equilibrium and the effect of changing conditions on equilibrium position.