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 2 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.]

Mark scheme, decoded

What each mark is really for — in plain English — and the wording trap that loses it.

S1
Step 1: Identify the reactant and the product.
S2
Reactant: Cl 2
S3
Product: 2 Cl•
S4
Step 2: Write the equation for the initiation step.
S5
Cl 2 → 2 Cl•

Model answer

Worked through, with each step tagged to the mark it earns.

S1
Step 1: Identify the reactant and the product.
S2
Reactant: Cl 2
S3
Product: 2 Cl•
S4
Step 2: Write the equation for the initiation step.
S5
Cl 2 → 2 Cl•
Final answer: Cl 2 → 2 Cl•

Common mistakes

Forgetting to include the radical symbol (•) in equations. — Always use the radical symbol (•) when writing equations for initiation, propagation, and termination steps.
Misinterpreting the role of radicals in the mechanism. — 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. — 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. — 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. — 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. — 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

Full worked solution (all marking points)1 mark