A-Level · Chemistry · AQA · Mark scheme decoded

AQA A-Level Chemistry: Redox Reactions and Oxidation States — mark scheme explained

Machine-verifiedchecked against the AQA A-Level Chemistry specificationlast verified 2 July 2026

The short answer

Redox reactions are a fundamental concept in chemistry, involving the transfer of electrons from one species to another. Understanding redox reactions is crucial for grasping various chemical processes, including corrosion, combustion, and electrochemical cells.

The question

Determine the oxidation state of sulfur in H 2 SO 4 . [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 known oxidation states.
S2
Hydrogen (H) has an oxidation state of +1, and oxygen (O) has an oxidation state of -2.
S3
Step 2: Set up the equation for the sum of oxidation states.
S4
2(+1) + x + 4(-2) = 0
S5
Step 3: Simplify the equation.
S6
2 + x - 8 = 0
S7
x - 6 = 0
S8
Step 4: Solve for x (the oxidation state of sulfur).
S9
x = +6

Model answer

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

S1
Step 1: Identify the known oxidation states.
S2
Hydrogen (H) has an oxidation state of +1, and oxygen (O) has an oxidation state of -2.
S3
Step 2: Set up the equation for the sum of oxidation states.
S4
2(+1) + x + 4(-2) = 0
S5
Step 3: Simplify the equation.
S6
2 + x - 8 = 0
S7
x - 6 = 0
S8
Step 4: Solve for x (the oxidation state of sulfur).
S9
x = +6
Final answer: +6

Common mistakes

Misinterpreting the rules for assigning oxidation states. — Review and memorize the rules for assigning oxidation states. Practice with a variety of compounds to reinforce understanding.
Forgetting to balance charges in half-equations. — Always ensure that the charge is balanced on both sides of a half-equation. Add or remove electrons as necessary to achieve balance.
Incorrectly combining half-equations. — Double-check your work when combining half-equations. Ensure that all charges and atoms are balanced, and cancel out any species that appear on both sides.
Confusing oxidation and reduction processes. — Remember that oxidation involves electron loss, while reduction involves electron gain. Practice writing half-equations for various redox reactions to reinforce this understanding.
Failing to balance the number of electrons in half-equations. — Always check and balance the number of electrons transferred in both half-equations. Multiply one or both half-equations by a suitable integer if necessary.
Incorrectly identifying oxidising and reducing agents. — Practice identifying the species that gains electrons (oxidising agent) and the species that loses electrons (reducing agent). Look for changes in oxidation states to help with this identification.

Where the marks go

Full worked solution (all marking points)3 marks