AQA A Level Chemistry
AQA A Level Chemistry 3.1.7 Oxidation, Reduction and Redox Equations Exam Questions
Free AQA A Level Chemistry exam questions for 3.1.7 Oxidation, Reduction and Redox Equations, with a mark scheme for every pack. Covers oxidation states, oxidation and reduction as electron transfer, oxidising and reducing agents, half equations and combining them into redox equations.
3.1.7 Oxidation, Reduction and Redox Equations exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About AQA A Level Chemistry 3.1.7 Oxidation, Reduction and Redox Equations
Redox is a short but heavily used AS topic in AQA A Level physical chemistry. You need to define oxidation and reduction in terms of electrons, work out oxidation states of elements in compounds and ions, identify oxidising and reducing agents, and write and combine half equations, including those in acidic solution balanced with H+ and H2O. These skills are essential for Group 7 chemistry, electrode potentials, transition metal redox titrations and many organic oxidation reactions. Examiners often ask for an unfamiliar half equation, an overall equation from two half equations, or the change in oxidation state of a named element.
What you need to know
- Oxidation and reduction Oxidation is loss of electrons and an increase in oxidation state; reduction is gain of electrons and a decrease in oxidation state. Both always happen together in a redox reaction.
- Oxidation state rules Uncombined elements are 0. The sum of oxidation states equals the overall charge. Oxygen is usually -2 (but -1 in peroxides), hydrogen is usually +1 (but -1 in metal hydrides), and fluorine is always -1.
- Agents An oxidising agent is an electron acceptor and is itself reduced. A reducing agent is an electron donor and is itself oxidised.
- Half equations Balance the element being oxidised or reduced, add H2O to balance oxygen, add H+ to balance hydrogen, then add electrons to balance charge. For example MnO4- + 8H+ + 5e- → Mn2+ + 4H2O.
- Combining half equations Multiply each half equation so the numbers of electrons are equal, add them, and cancel electrons and any species on both sides. The final equation must balance for atoms and charge.
Common mistakes in 3.1.7 Oxidation, Reduction and Redox Equations
- Writing oxidation states as 2+ instead of +2. The sign comes before the number for oxidation states; 2+ is used for ionic charges.
- Assuming oxygen is always -2 and hydrogen always +1. Check for peroxides such as H2O2 and hydrides such as NaH.
- Saying the oxidising agent is oxidised. The oxidising agent gains electrons, so it is reduced.
- Leaving electrons in the overall equation. Electrons must cancel completely when half equations are combined.
- Balancing atoms but not charge in a half equation. Always check the total charge on each side.
Try three 3.1.7 Oxidation, Reduction and Redox Equations exam-style questions
1. Give the oxidation state of manganese in MnO4- and of sulfur in S2O3 2-. (2 marks)
Show the mark scheme answer
Mn in MnO4- is +7; S in S2O3 2- is +2 (average oxidation state).
2. Write a half equation for the reduction of MnO4- to Mn2+ in acidic solution, and use it with Fe2+ → Fe3+ + e- to write the overall ionic equation. (2 marks)
Show the mark scheme answer
MnO4- + 8H+ + 5e- → Mn2+ + 4H2O; MnO4- + 8H+ + 5Fe2+ → Mn2+ + 4H2O + 5Fe3+.
3. In the reaction 2Fe3+ + 2I- → 2Fe2+ + I2, identify the oxidising agent and explain your answer in terms of electrons and oxidation states. (2 marks)
Show the mark scheme answer
Fe3+ is the oxidising agent; it gains an electron and its oxidation state decreases from +3 to +2 (it is reduced while it oxidises I- to I2).
3.1.7 Oxidation, Reduction and Redox Equations FAQs
What does AQA A Level Chemistry 3.1.7 Redox cover?
It covers oxidation states, oxidation and reduction as electron transfer, oxidising and reducing agents, and writing and combining half equations to form full redox equations.
Which paper is 3.1.7 Redox examined in?
It is examined in Paper 1, and Paper 3 can also include it. Redox skills are used throughout the inorganic content of Paper 1.
How do I balance half equations in acidic solution?
Balance the key element first, then oxygen with H2O, hydrogen with H+, and finally charge with electrons. Check the total charge on both sides before combining.
