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AQA A-Level Chemistry: Reversible Reactions and Le Chatelier's Principle — mark scheme explained

Machine-verifiedchecked against the AQA A-Level Chemistry specificationlast verified 3 July 2026

The short answer

In A-Level Chemistry, understanding reversible reactions and the concept of equilibrium is fundamental. This section delves into the principles governing these reactions and how changes in conditions can affect the position of equilibrium. We will explore Le Chatelier’s principle and its application to predict the effects of temperature, pressure, and concentration on equilibrium.

The question

Consider the reaction: N 2 (g) + 3H 2 (g) ⇌ 2NH 3 (g). Predict how increasing the temperature will affect the position of equilibrium.

[Paraphrased for study — not reproduced from any exam paper.]

3 marks

Mark scheme, decoded

How the examiner actually awards the marks on this topic.

Gradora's own decode of the marking approach — not the exam board's published mark scheme.

How marks are awarded

For prediction questions, marks are typically awarded for correctly identifying the type of reaction (exothermic or endothermic) and applying Le Chatelier’s principle to predict the effect of changes in conditions. For explanation questions, marks are given for clear and concise descriptions that demonstrate a deep understanding of equilibrium shifts and industrial applications.

What the command words demand

Predict: Use Le Chatelier’s principle to determine how a change in conditions will affect the position of equilibrium.
Explain: Provide a detailed description of the concept, including relevant principles and relationships.
Determine: Find or derive a specific value or quantity based on given information.
Discuss: Examine and interpret data or graphs to draw conclusions about the behavior of the system.

Model answer

A full-mark response to the question above, worked through step by step.

Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, apply Le Chatelier’s principle, and check your work.

Identify the type of reaction (exothermic or endothermic).1 mark
The Haber process is exothermic (ΔH < 0).
Apply Le Chatelier’s principle to predict the effect of increasing temperature.1 mark
For an exothermic reaction, increasing the temperature shifts the equilibrium to the left, favoring the reactants.
State the final answer.1 mark
Increasing the temperature will shift the equilibrium to the left, favoring N 2 (g) and H 2 (g).

Final answer: The equilibrium will shift to the left, favoring N 2 (g) and H 2 (g).

Work through every step correctly and you earn all 3 marks.

Another worked example

For the reaction: CO(g) + 2H 2 (g) ⇌ CH 3 OH(g), predict how increasing the pressure will affect the position of equilibrium.

4 marks

Determine the change in the number of moles of gas (Δn).1 mark
Reactants: 1 + 2 = 3 moles of gasProducts: 1 mole of gasΔn = 1 - 3 = -2 (decrease in moles of gas)
Apply Le Chatelier’s principle to predict the effect of increasing pressure.2 marks
For a reaction where Δn < 0, increasing the pressure shifts the equilibrium to the right, favoring the products.
State the final answer.1 mark
Increasing the pressure will shift the equilibrium to the right, favoring CH 3 OH(g).

Final answer: The equilibrium will shift to the right, favoring CH 3 OH(g).

Work through every step correctly and you earn all 4 marks.

Common mistakes

Misunderstanding the direction of equilibrium shifts for exothermic reactions when temperature increases.
Why it happens: Students often confuse the effect of temperature on exothermic and endothermic reactions, leading to incorrect predictions.
Fix: Remember that increasing the temperature for an exothermic reaction (ΔH < 0) shifts the equilibrium to the left, favoring the reactants.
Forgetting that catalysts do not affect the position of equilibrium.
Why it happens: Students sometimes think that a catalyst can change the final concentrations of reactants and products, which is incorrect.
Fix: Always remember that a catalyst only speeds up both the forward and reverse reactions equally, allowing the system to reach equilibrium faster but not changing the position of equilibrium.
Misinterpreting the effect of pressure on reactions involving gases.
Why it happens: Students may struggle with determining the change in the number of moles of gas (Δn) and its impact on equilibrium.
Fix: Practice calculating Δn for different reactions and applying Le Chatelier’s principle to predict the effect of changes in pressure. For Δn 0, increasing pressure shifts the equilibrium to the left.
Incorrectly predicting the effect of concentration changes on equilibrium.
Why it happens: Students may not fully understand how changes in the concentration of a reactant or product affect the position of equilibrium.
Fix: Practice applying Le Chatelier’s principle to predict the direction of equilibrium shifts when concentrations change. Increasing the concentration of a reactant shifts the equilibrium to the right, and decreasing it shifts the equilibrium to the left.
Failing to explain why compromise conditions are used in industrial processes.
Why it happens: Students may not fully grasp the balance between yield and rate of reaction in industrial processes, leading to incomplete or incorrect explanations.
Fix: Practice explaining how compromise conditions (temperature and pressure) optimize both yield and cost-effectiveness in industrial processes. For example, in the Haber process, a high temperature increases the rate but favors the reverse reaction, while a low temperature favors the forward reaction but decreases the rate.
Confusing the effects of changes in conditions on equilibrium with other chemical concepts.
Why it happens: Students may mix up Le Chatelier’s principle with other principles, leading to incorrect predictions and explanations.
Fix: Review and practice applying Le Chatelier’s principle specifically to reversible reactions. Focus on how changes in temperature, pressure, and concentration affect the position of equilibrium.

Where the marks go

The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.

Question type	What you’re asked to do	Marks
Le Chatelier Prediction	Predict how raising temperature shifts the equilibrium position for this reversible reaction.	3
Le Chatelier Prediction	Predict how increasing pressure shifts the equilibrium position using the change in gas moles.	4
Le Chatelier Prediction	Predict the direction the equilibrium position shifts when a reactant concentration is decreased.	3
Catalyst Equilibrium Effect	Predict how adding a catalyst affects the position of equilibrium in a reversible reaction.	2
Explain Compromise Conditions	Explain why a moderate temperature balances reaction rate against equilibrium yield in ammonia production.	4
Total across these question types		16

Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.