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 3 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.]

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 calculation questions, marks are typically awarded for correct substitution of values into formulas, accurate arithmetic, and providing the final answer with appropriate units. For conceptual questions, marks are given for clear and concise explanations that demonstrate a deep understanding of the topic.

What the command words demand

Determine: Find or derive a specific value or quantity based on given information.
Write: Provide the required equation, formula, or statement.
Balance: Ensure that the number of atoms and charges are equal on both sides of an equation.
Identify: Recognize and name the specified component or process.

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, perform calculations, and check your work.

Identify the known oxidation states.0 marks
Hydrogen (H) has an oxidation state of +1, and oxygen (O) has an oxidation state of -2.
Set up the equation for the sum of oxidation states.1 mark
2(+1) + x + 4(-2) = 0
Simplify the equation.1 mark
2 + x - 8 = 0x - 6 = 0
Solve for x (the oxidation state of sulfur).1 mark
x = +6

Final answer: +6

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

Another worked example

Write the half-equations for the redox reaction between Fe 2+ and MnO 4 - in acidic solution.

6 marks

Identify the oxidation and reduction processes.0 marks
Fe 2+ → Fe 3+ (oxidation)MnO 4 - → Mn 2+ (reduction)
Write the oxidation half-equation.1 mark
Fe 2+ → Fe 3+ + e -
Write the reduction half-equation, balancing oxygen and hydrogen atoms with H 2 O and H + .2 marks
MnO 4 - + 8H + + 5e - → Mn 2+ + 4H 2 O
Balance the number of electrons in both half-equations.1 mark
Fe 2+ → Fe 3+ + e -5(Fe 2+ → Fe 3+ + e - )MnO 4 - + 8H + + 5e - → Mn 2+ + 4H 2 O
Combine the balanced half-equations.2 marks
5Fe 2+ + MnO 4 - + 8H + → 5Fe 3+ + Mn 2+ + 4H 2 O

Final answer: 5Fe 2+ + MnO 4 - + 8H + → 5Fe 3+ + Mn 2+ + 4H 2 O

Work through every step correctly and you earn all 6 marks.

Common mistakes

Misinterpreting the rules for assigning oxidation states.
Why it happens: Students may confuse the rules for different elements, leading to incorrect oxidation state assignments.
Fix: 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.
Why it happens: Students may overlook the need to balance charges, leading to incorrect half-equations.
Fix: 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.
Why it happens: Students may make arithmetic errors or fail to cancel out species that appear on both sides of the equation.
Fix: 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.
Why it happens: Students may mix up the concepts of oxidation (electron loss) and reduction (electron gain), leading to incorrect half-equations.
Fix: 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.
Why it happens: Students may not realize that the number of electrons must be equal in both half-equations before combining them.
Fix: 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.
Why it happens: Students may struggle to identify which species is the oxidising agent and which is the reducing agent in a given reaction.
Fix: 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

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
Oxidation State Calculation	Work out the oxidation state of sulfur in a compound using known values.	3
Redox Half-Equations	Write and combine balanced half-equations for the Fe2+ and MnO4- reaction in acid.	6
Oxidation State Calculation	Work out the oxidation number of nitrogen within the ammonium ion using known values and the overall charge.	3
Write Half-Equations	Write balanced oxidation and reduction half-equations for the reaction between copper and silver ions.	5
Total across these question types		17

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