OxfordAQA International A Level Chemistry
OxfordAQA International A Level Chemistry 3.1.7 Chemical Equilibria, Le Chatelier's Principle and Kc Exam Questions
Free OxfordAQA International A Level Chemistry revision for 3.1.7 Chemical Equilibria, Le Chatelier's Principle and Kc: what you need to know, common mistakes and exam-style questions with answers. Covers dynamic equilibrium, Le Chatelier's principle, effects of temperature, pressure, concentration and catalysts, industrial compromise conditions and the equilibrium constant Kc.
Practise 3.1.7 Chemical Equilibria, Le Chatelier's Principle and Kc with past papers
Once you have learnt this topic, find the 3.1.7 Chemical Equilibria, Le Chatelier's Principle and Kc questions in real OxfordAQA International A Level Chemistry past papers and mark them with the mark scheme. Doing lots of real questions is the quickest way to learn how examiners ask about each idea.
StemUp mark scheme flashcards
A-Level Chemistry 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 £10.99View flashcards
About OxfordAQA International A Level Chemistry 3.1.7 Chemical Equilibria, Le Chatelier's Principle and Kc
This Unit 2 physical chemistry topic covers reversible reactions. You need to describe dynamic equilibrium, use Le Chatelier's principle to predict the effect of changing temperature, pressure and concentration on the position of equilibrium, and explain why a catalyst does not change the position. You also need to explain the compromise conditions used in industry. The second part covers the equilibrium constant Kc for homogeneous systems: writing expressions, calculating values and units, and explaining how temperature changes Kc. It leads to Kp in 3.1.12 and to acids and bases in 3.1.10. Examiners commonly set Kc calculations from initial and equilibrium amounts.
What you need to know
- Dynamic equilibrium In a closed system the forward and reverse reactions occur at the same rate, so the concentrations of reactants and products stay constant.
- Le Chatelier's principle If a change is made to a system at equilibrium, the position of equilibrium shifts to oppose the change. Increasing pressure shifts it towards the side with fewer gas molecules; increasing temperature shifts it in the endothermic direction.
- Catalysts A catalyst increases the rates of the forward and reverse reactions equally, so equilibrium is reached faster but the position does not change.
- Industrial conditions Industry uses compromise conditions, for example in making ammonia or methanol, balancing yield, rate, cost of high pressure and energy, and safety.
- Kc For aA + bB ⇌ cC + dD, Kc = [C]^c[D]^d / [A]^a[B]^b using equilibrium concentrations. Work out units by cancelling the concentration units.
- What changes Kc Only temperature changes Kc. If the forward reaction is exothermic, raising the temperature decreases Kc. Changing concentration or pressure moves the position but Kc stays the same.
Common mistakes in 3.1.7 Chemical Equilibria, Le Chatelier's Principle and Kc
- Saying the reactions stop at equilibrium. Both reactions continue at equal rates.
- Saying a catalyst increases yield. It does not change the position of equilibrium, only how quickly it is reached.
- Using initial amounts in the Kc expression. Use equilibrium concentrations, dividing moles by the volume.
- Saying Kc changes when pressure changes. Kc only changes with temperature.
Try three 3.1.7 Chemical Equilibria, Le Chatelier's Principle and Kc exam-style questions
1. For H2 + I2 ⇌ 2HI, equilibrium concentrations are [H2] = 0.020, [I2] = 0.020 and [HI] = 0.14 mol dm-3. Calculate Kc. (2 marks)
Show the mark scheme answer
Kc = [HI]^2 / [H2][I2] = 0.14^2 / (0.020 x 0.020); = 0.0196 / 0.0004 = 49 (no units).
2. For N2 + 3H2 ⇌ 2NH3, ΔH = -92 kJ mol-1, explain the effect of increasing the temperature on the yield of ammonia. (2 marks)
Show the mark scheme answer
Forward reaction is exothermic so equilibrium shifts in the endothermic (reverse) direction to oppose the increase; yield of ammonia decreases.
3. Explain why a compromise pressure is used in industry for a reaction where high pressure increases yield. (2 marks)
Show the mark scheme answer
High pressure is expensive to generate and equipment is costly / dangerous; so a lower pressure gives an acceptable yield at lower cost.
3.1.7 Chemical Equilibria, Le Chatelier's Principle and Kc FAQs
What does OxfordAQA International A Level Chemistry 3.1.7 Chemical Equilibria, Le Chatelier's Principle and Kc cover?
It covers dynamic equilibrium, Le Chatelier's principle, effects of temperature, pressure, concentration and catalysts, industrial compromise conditions and the equilibrium constant Kc.
Which unit is 3.1.7 Chemical Equilibria, Le Chatelier's Principle and Kc examined in?
It is examined in Unit 2, the second International AS paper, which covers organic and physical chemistry. The Unit 5 practical and synoptic paper at A2 can also assess it.
How do I work out the units of Kc?
Write mol dm-3 for each concentration term in the expression and cancel. For example, Kc = [C]^2 / [A][B] has no units, while Kc = [C] / [A][B] has units of mol-1 dm3.
