Chemical equilibria & Kc
A-level Chemistry (7405) · exam-style practice, examiner-report intelligence and the tools that drill it.
The topic on one screen
- A dynamic equilibrium: forward and reverse reactions continue at equal rates in a closed system, so concentrations stay constant.
- Le Chatelier: the position shifts to oppose any change in concentration, pressure or temperature.
- Increase pressure → shifts to the side with fewer gas moles. Increase temperature → shifts in the endothermic direction.
- A catalyst does NOT move the position or change Kc — it only speeds up how fast equilibrium is reached.
- Kc = [products] / [reactants], each raised to its balancing number. Only temperature changes Kc.
- In Kc calculations, convert every amount in moles to a concentration by dividing by the total volume FIRST.
Where students actually lose marks
The most common lost mark in Kc calculations is failing to divide the amount, in moles, of each species by the total volume before putting it into the expression.
June 2024 Paper 2 examiner report (Q02.2)
Even on a straightforward Kc expression, significant numbers failed to use the volume when substituting values. Concentrations, not moles, go into Kc.
June 2022 Paper 2 examiner report (Q02.3)
On an unfamiliar Le Chatelier context, the best students still applied the same rule cleanly. State the change, state the direction the system opposes it, then the effect.
June 2024 Paper 2 examiner report (Q02.3)
Try it — exam-style
Write the expression for Kc for the reaction N2(g) + 3H2(g) ⇌ 2NH3(g).
At equilibrium a 2.0 dm3 flask contains 0.40 mol H2, 0.40 mol I2 and 1.60 mol HI for H2(g) + I2(g) ⇌ 2HI(g). Calculate Kc.
The reaction N2(g) + 3H2(g) ⇌ 2NH3(g) is exothermic. State and explain the effect of increasing temperature on the yield of ammonia.
State the effect of adding a catalyst on the position of an equilibrium and on the value of Kc.
Questions are written in the style of past AQA papers (source shown on each) — never copied from them.
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