Part 3: Changing conditions and equilibrium yield

Explain separately what happens to composition, Kp and the time taken to reach equilibrium.

Only temperature changes Kp for a fixed equation

Raising temperature favours the endothermic direction. If the forward reaction is exothermic, heating shifts equilibrium towards reactants and reduces Kp. If the forward reaction is endothermic, heating increases Kp. A catalyst or a pressure change at fixed temperature does not alter Kp.

For ammonia formation, the forward reaction is exothermic, so cooling increases its equilibrium constant. Cooling can simultaneously slow both reaction directions. The equilibrium amount and the speed of approaching it answer different questions.

Compression changes composition without changing Kp

At fixed temperature, reducing volume increases all gas partial pressures immediately. When the gaseous coefficients differ between sides, the mixture then reacts towards the side with fewer moles of gas. The new composition restores the same Kp value.

For N₂O₄ ⇌ 2NO₂, double all pressures instantaneously. The pressure quotient p(NO₂)²/p(N₂O₄) becomes twice its previous value, greater than Kp. The reverse reaction consumes NO₂ until the quotient again equals Kp. Thus compression favours N₂O₄.

For H₂ + I₂ ⇌ 2HI, equal total coefficients mean the pressure factors cancel. Uniform compression of an ideal mixture at constant temperature causes no equilibrium shift. Kp stays unchanged in both cases.

How pressure was changed matters

Adding inert gas at fixed volume and temperature increases total pressure but leaves each reacting gas’s partial pressure unchanged: each still has the same n, T and V. There is therefore no shift for an ideal gas mixture.

Adding inert gas while maintaining constant total pressure requires the container to expand. Reacting partial pressures decrease, which favours the side with more gas moles if the coefficients differ. Do not apply “higher total pressure favours fewer moles” without identifying the physical change.

Adding a reacting gas at fixed volume changes its own partial pressure and disturbs the reaction quotient. Removing a product can promote further net product formation. Kp remains fixed at the same temperature.

A catalyst changes the journey, not the final ratio

A catalyst accelerates attainment of equilibrium by providing an alternative pathway for both directions. The equilibrium composition and Kp at that temperature stay the same. It can still improve production rate and allow operation closer to the favourable lower-temperature equilibrium for an exothermic synthesis.

Industrial conditions balance equilibrium yield, rate, compression energy, equipment strength and operating cost. High pressure may improve ammonia yield but is not free. Recycling unreacted gases can improve overall feed utilisation even if the single-pass conversion is incomplete. No particular plant pressure or temperature is universal.

Structure an equilibrium explanation

Name the change, state the endothermic/exothermic direction or gas-mole comparison, then give the equilibrium shift and the effect on Kp separately. If the question asks about rate, explain that separately using kinetics.

The June 2023 Paper 1 Q04 report highlighted confusion between pressure effects on composition and on Kp. Its pressure-expression notation also mattered. These lessons use explicit p(X) notation and show the algebra so the distinction remains visible.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.

Sources