Edexcel International A Level Chemistry
Edexcel International A Level Chemistry Equilibria Exam Questions
Free Edexcel International A Level Chemistry exam questions for Equilibria, with a mark scheme for every pack. Covers dynamic equilibrium, closed systems, Le Chatelier's principle, effects of concentration, pressure, temperature and catalysts, and industrial compromises.
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.
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About Edexcel International A Level Chemistry Equilibria
Equilibria in Unit 2 of Edexcel International A Level Chemistry introduces reversible reactions and the idea of dynamic equilibrium. You use Le Chatelier's principle to predict how changing concentration, pressure and temperature shifts the position of equilibrium, and why a catalyst does not. These ideas are applied to industrial processes, where conditions are a compromise between yield, rate and cost. The topic is closely linked with Kinetics and is taken further in Unit 4 Chemical equilibria, where equilibrium constants are calculated. Examiners frequently ask you to define dynamic equilibrium and to give full, reasoned predictions of the effect of a change.
What you need to know
- Dynamic equilibrium In a closed system the forward and reverse reactions continue at equal rates, so concentrations of reactants and products stay constant.
- Le Chatelier's principle If a system at equilibrium is subject to a change, the position of equilibrium shifts to oppose the change.
- Concentration and pressure Increasing the concentration of a reactant shifts the position to the right. Increasing pressure shifts the position towards the side with fewer gas molecules; it has no effect if both sides have the same number.
- Temperature Increasing temperature shifts the position in the endothermic direction. Decreasing temperature favours the exothermic direction.
- Catalysts A catalyst speeds up the forward and reverse reactions equally, so equilibrium is reached faster but its position does not change.
- Industrial compromise In the Haber process (exothermic, fewer moles of gas on the right) a lower temperature would give a higher yield but too slow a rate, and very high pressure is expensive and hazardous, so compromise conditions are chosen.
Common mistakes in Equilibria
- Saying the reactions stop at equilibrium. Both reactions continue at the same rate.
- Saying the concentrations of reactants and products are equal at equilibrium. They are constant, not necessarily equal.
- Claiming a catalyst increases the yield. It only increases the rate at which equilibrium is reached.
- Giving the direction of shift without a reason. Full marks need the reason, for example 'fewer moles of gas on the right'.
- Mixing up which direction temperature favours. Always check the sign of delta H for the forward reaction.
Try three Equilibria exam-style questions
1. State two features of a dynamic equilibrium. (2 marks)
Show the mark scheme answer
Rate of forward reaction equals rate of reverse reaction; concentrations of reactants and products remain constant (in a closed system).
2. For N2(g) + 3H2(g) and 2NH3(g), delta H = -92 kJ mol-1, explain the effect on the equilibrium yield of ammonia of increasing the pressure and of increasing the temperature. (4 marks)
Show the mark scheme answer
Increasing pressure shifts position to the right; because there are fewer gas molecules on the right (2 compared with 4), so yield increases; increasing temperature shifts position to the left; in the endothermic direction, so yield decreases.
3. Explain why an iron catalyst is used in the Haber process even though it does not change the yield. (2 marks)
Show the mark scheme answer
It increases the rate of both forward and reverse reactions equally; so equilibrium is reached more quickly, allowing an economic rate at a lower temperature.
Equilibria FAQs
What does Edexcel International A Level Chemistry Equilibria cover?
It covers reversible reactions, dynamic equilibrium, Le Chatelier's principle for changes in concentration, pressure and temperature, the role of catalysts and compromise conditions in industry.
Which unit is Equilibria examined in?
This introductory equilibria topic is part of Unit 2; equilibrium constants Kc and Kp are covered in Chemical equilibria in Unit 4.
How do I write a full-mark Le Chatelier answer?
State the change, the direction the position shifts, the reason linked to the equation (moles of gas or sign of delta H), and the effect on the yield.
