OCR GCSE Chemistry A (Gateway)
OCR GCSE Chemistry A C5.3 Equilibria Exam Questions
Free OCR GCSE Chemistry A exam questions for C5.3 Equilibria, with a mark scheme for every pack. Covers reversible reactions, dynamic equilibrium, energy changes in reversible reactions, and Le Chatelier's principle for concentration, temperature and pressure.
C5.3 Equilibria exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
StemUp mark scheme flashcards
GCSE Chemistry Triple Science Flashcards for 2027 Onwards
Every answer is written in mark scheme wording, so you learn exactly what examiners give marks for. Then test yourself with the questions on this page.
From £7.99View flashcards
About OCR GCSE Chemistry A C5.3 Equilibria
C5.3 covers reactions that can go in both directions and how their position of equilibrium can be controlled. On the OCR Gateway Chemistry A course it completes Topic C5 Monitoring and controlling chemical reactions, assessed in Paper 2 (Foundation) or Paper 4 (Higher). You learn what dynamic equilibrium means and, at Higher tier, use Le Chatelier's principle to predict the effect of changing conditions. It builds on energetics in C3.2 and rates in C5.2, and is applied to industrial processes such as the Haber process in C6.1. Examiners frequently test definitions, predictions with full explanations, and the compromise between yield and rate.
What you need to know
- Reversible reactions In a reversible reaction the products can react to re-form the reactants, shown by the symbol ⇌. For example, hydrated copper sulfate (blue) loses water on heating to form anhydrous copper sulfate (white), and adding water reverses this.
- Energy in reversible reactions If the forward reaction is exothermic, the reverse reaction is endothermic, and the same amount of energy is transferred in each direction.
- Dynamic equilibrium In a closed system a reversible reaction reaches dynamic equilibrium, where the forward and reverse reactions happen at the same rate. The concentrations of reactants and products then stay constant, but both reactions continue.
- Le Chatelier's principle (Higher) If a change is made to the conditions of a system at equilibrium, the position of equilibrium shifts to oppose the change.
- Changing conditions (Higher) Increasing the concentration of a reactant shifts equilibrium to the right. Increasing temperature favours the endothermic direction. Increasing pressure shifts equilibrium to the side with fewer moles of gas.
- Catalysts and equilibrium A catalyst speeds up the forward and reverse reactions equally, so equilibrium is reached faster but its position does not change.
Common mistakes in C5.3 Equilibria
- Saying that at equilibrium the reactions have stopped. Both reactions continue at equal rates.
- Stating that the concentrations of reactants and products are equal at equilibrium. They are constant, not necessarily equal.
- Writing that a catalyst increases the yield at equilibrium. It does not change the position of equilibrium.
- Counting all molecules, including solids or liquids, when predicting pressure effects. Count only moles of gas on each side.
- Explaining a shift without saying which direction is endothermic or exothermic. Always link temperature changes to the energy change of the forward reaction.
Try three C5.3 Equilibria exam-style questions
1. N2(g) + 3H2(g) ⇌ 2NH3(g). Predict and explain the effect of increasing pressure on the yield of ammonia. (3 marks)
Show the mark scheme answer
Yield of ammonia increases; equilibrium shifts to the right; because there are fewer moles of gas on the right (2) than the left (4), so the system opposes the increase in pressure.
2. The forward reaction in the previous question is exothermic. Explain the effect of increasing temperature on the yield of ammonia. (2 marks)
Show the mark scheme answer
Yield decreases; the endothermic (reverse) reaction is favoured to oppose the increase in temperature, so equilibrium shifts to the left.
3. State two features of a dynamic equilibrium. (2 marks)
Show the mark scheme answer
Forward and reverse reactions happen at the same rate; concentrations of reactants and products remain constant; it only occurs in a closed system. Any two.
C5.3 Equilibria FAQs
What does OCR GCSE Chemistry A C5.3 Equilibria cover?
It covers reversible reactions, dynamic equilibrium, energy changes in reversible reactions and, for Higher, using Le Chatelier's principle to predict the effect of changing conditions.
Which paper is C5.3 examined in?
Topic C5 is assessed in Paper 2 (Foundation) or Paper 4 (Higher).
How do I answer Le Chatelier questions in C5.3?
State the direction of the shift, give the effect on yield, then explain using the rule: equilibrium moves to oppose the change, naming the fewer moles of gas or the endothermic direction as appropriate.
