A-Level · Chemistry · AQA · Mark scheme decoded

AQA A-Level Chemistry: Free-Radical Substitution of Methane with Chlorine — mark scheme explained

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

The short answer

The reaction between methane (CH 4 ) and chlorine (Cl 2 ) is a classic example of a free-radical substitution mechanism in organic chemistry. This reaction is important for understanding the behavior of alkanes and halogens, as well as the principles of radical reactions.

The question

Write the equation for the initiation step in the free-radical substitution of methane with chlorine.

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

1 mark

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 writing equations, marks are typically awarded for correct reactants, products, and radical symbols. For explanations, marks are given for clear and concise descriptions that demonstrate a deep understanding of the mechanism. Predicting products requires accurate identification of all possible outcomes.

What the command words demand

Write: Provide the chemical equation for a specific step in the mechanism.
Explain: Describe the process and behavior of radicals in the free-radical substitution mechanism.
Identify: Recognize and name the different steps (initiation, propagation, termination) in the reaction.
Predict: Determine the products of the reaction based on the given reactants and conditions.

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, write out equations, and check your work.

Identify the reactant and the product.0 marks
Reactant: Cl 2Product: 2 Cl•
Write the equation for the initiation step.1 mark
Cl 2 → 2 Cl•

Final answer: Cl 2 → 2 Cl•

Work through every step correctly and you earn all 1 mark.

Another worked example

Write the equations for the two propagation steps in the free-radical substitution of methane with chlorine.

2 marks

Identify the reactants and products for the first propagation step.0 marks
Reactants: Cl•, CH 4Products: CH 3 •, HCl
Write the equation for the first propagation step.1 mark
Cl• + CH 4 → CH 3 • + HCl
Identify the reactants and products for the second propagation step.0 marks
Reactants: CH 3 •, Cl 2Products: CH 3 Cl, Cl•
Write the equation for the second propagation step.1 mark
CH 3 • + Cl 2 → CH 3 Cl + Cl•

Final answer: 1. Cl• + CH 4 → CH 3 • + HCl 2. CH 3 • + Cl 2 → CH 3 Cl + Cl•

Work through every step correctly and you earn all 2 marks.

Common mistakes

Forgetting to include the radical symbol (•) in equations.
Why it happens: Students sometimes overlook the importance of indicating radicals with a dot (•), leading to incorrect representations of the reaction steps.
Fix: Always use the radical symbol (•) when writing equations for initiation, propagation, and termination steps.
Misinterpreting the role of radicals in the mechanism.
Why it happens: Students may not fully understand that radicals are highly reactive species that initiate and propagate the reaction.
Fix: Practice explaining the behavior of radicals, emphasizing their reactivity and how they participate in each step of the mechanism.
Forgetting to balance equations for propagation steps.
Why it happens: Students may overlook the need to ensure that the number of atoms on both sides of the equation is equal, leading to incorrect answers.
Fix: Always check that your equations are balanced, with the same number of each type of atom on both sides of the reaction arrow.
Confusing initiation and propagation steps.
Why it happens: Students may mix up the different steps in the mechanism, leading to incorrect answers when writing out the equations.
Fix: Practice identifying and writing out each step of the mechanism separately. Initiation involves breaking a Cl-Cl bond, while propagation involves the formation and reaction of radicals.
Failing to identify all possible termination steps.
Why it happens: Students may only consider one or two termination steps, missing others that are equally valid.
Fix: Practice writing out all possible termination steps, including the combination of two chlorine radicals, a methyl radical and a chlorine radical, and two methyl radicals.
Not understanding the energy requirements for initiation.
Why it happens: Students may not fully grasp why energy is needed to break the Cl-Cl bond during initiation.
Fix: Review the concept of bond strength and the energy required to break bonds. Emphasize that breaking a strong Cl-Cl bond requires significant energy, which can be provided by heat or light (UV radiation).

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
Write Equation	Write the initiation step equation for chlorine forming radicals under UV light.	1
Propagation Step Equations	Write balanced equations for both propagation steps in methane's free-radical chlorination.	2
Radical Termination Equations	Write three termination step equations combining radicals in the chlorination of methane.	3
Explain Reaction Mechanism	Explain why energy input is needed to break the Cl-Cl bond during initiation.	2
Total across these question types		8

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