A-Level Chemistry: Equilibria

Equilibrium questions separate students who can recite Le Chatelier from those who know exactly what changes the position of equilibrium, what changes the constant, and what changes neither. This quiz is built around those distinctions.

The conceptual questions define dynamic equilibrium by its real criterion — equal forward and reverse rates in a closed system — and test the catalyst fact that trips up so many candidates: a catalyst speeds the approach to equilibrium but shifts nothing. The most important discrimination in the topic gets its own question: only temperature changes the numerical value of K; concentration and pressure changes merely move the position while K stays fixed.

Le Chatelier reasoning is exercised on the two canonical systems. The ammonia synthesis question (corrected during verification so that all three genuine yield-raising actions — higher pressure, product removal, lower temperature — are credited) tests the full logic of industrial compromise, and the PCl₅ dissociation question applies the fewer-moles-of-gas rule under pressure increase.

The calculations are concrete and verified: a Kc computation from equilibrium concentrations (1.60, with the squared product term handled correctly), a partial-pressure calculation from mole fractions, and the reciprocal relationship between forward and reverse Kc (4.0 becomes 0.25). Kp is covered through its expression for the sulfur dioxide oxidation and its properties — defined at fixed temperature, unmoved by total pressure changes.

Explanations consistently name the principle and show the algebra, making the set a fast, complete revision of the equilibrium strand across both years of the course.

• Define dynamic equilibrium by equal forward and reverse rates
• Distinguish what changes K (temperature only) from what changes only the position
• Apply Le Chatelier's principle to pressure, temperature and product-removal changes
• Calculate Kc from equilibrium concentrations and use reciprocal relationships
• Work with Kp, mole fractions and partial pressures

Aligned to the equilibria core of the DfE A level science subject content: dynamic equilibrium, Le Chatelier's principle, Kc and Kp calculations, and industrial compromise conditions.