OxfordAQA International A Level Chemistry
OxfordAQA International A Level Chemistry 3.1.5 Oxidation, Reduction and Redox Equations Exam Questions
Free OxfordAQA International A Level Chemistry revision for 3.1.5 Oxidation, Reduction and Redox Equations: what you need to know, common mistakes and exam-style questions with answers. Covers oxidation and reduction as electron transfer, oxidation states, oxidising and reducing agents, half-equations and combining them into overall redox equations.
Practise 3.1.5 Oxidation, Reduction and Redox Equations with past papers
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About OxfordAQA International A Level Chemistry 3.1.5 Oxidation, Reduction and Redox Equations
Oxidation, reduction and redox equations is a Unit 1 topic that gives you the tools used across inorganic 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, write half-equations and combine them into balanced overall equations. These skills are needed for Group 7 reactions, electrode potentials, transition metal chemistry and redox titrations in later units. Examiners often ask you to deduce oxidation states, write half-equations in acid conditions, and identify which species has been oxidised or reduced in an unfamiliar reaction.
What you need to know
- Definitions Oxidation is loss of electrons and reduction is gain of electrons. An oxidising agent accepts electrons and is reduced; a reducing agent donates electrons and is oxidised.
- Oxidation state rules Uncombined elements are 0. The sum of oxidation states equals the charge on the species. Oxygen is usually -2 (except in peroxides and with fluorine), hydrogen is +1 (except in metal hydrides, -1), and fluorine is always -1.
- Using oxidation states An increase in oxidation state is oxidation and a decrease is reduction. Roman numerals in names show oxidation states, for example iron(III).
- Half-equations Balance the main atom, add H2O to balance oxygen, add H+ to balance hydrogen, then add electrons to balance charge. For example MnO4- + 8H+ + 5e- → Mn2+ + 4H2O.
- Overall equations Multiply the half-equations so the electrons cancel, add them and cancel species that appear on both sides.
Common mistakes in 3.1.5 Oxidation, Reduction and Redox Equations
- Saying an oxidising agent is oxidised. It causes oxidation and is itself reduced.
- Forgetting that the sum of oxidation states in an ion equals its charge.
- Balancing oxygen with O2 in half-equations. Use water molecules and H+ ions in acidic conditions.
- Leaving electrons in the overall equation. They must cancel completely.
Try three 3.1.5 Oxidation, Reduction and Redox Equations exam-style questions
1. Give the oxidation state of chromium in Cr2O7 2-. (1 mark)
Show the mark scheme answer
2x + 7(-2) = -2, so x = +6.
2. Write a half-equation for the reduction of Cr2O7 2- to Cr3+ in acidic solution. (2 marks)
Show the mark scheme answer
Cr2O7 2- + 14H+ + 6e- → 2Cr3+ + 7H2O; correct species with balanced charge.
3. Combine the half-equations Fe2+ → Fe3+ + e- and MnO4- + 8H+ + 5e- → Mn2+ + 4H2O into an overall equation. (2 marks)
Show the mark scheme answer
Multiply the iron half-equation by 5; MnO4- + 8H+ + 5Fe2+ → Mn2+ + 4H2O + 5Fe3+.
3.1.5 Oxidation, Reduction and Redox Equations FAQs
What does OxfordAQA International A Level Chemistry 3.1.5 Oxidation, Reduction and Redox Equations cover?
It covers oxidation and reduction as electron transfer, oxidation states, oxidising and reducing agents, half-equations and combining them into overall redox equations.
Which unit is 3.1.5 Oxidation, Reduction and Redox Equations examined in?
It is examined in Unit 1, the first International AS paper, which covers inorganic and physical chemistry. The Unit 5 practical and synoptic paper at A2 can also assess it.
How do I check a redox equation is balanced?
Check that atoms of each element balance and that the total charge on each side is the same. Also check that electrons lost in oxidation equal electrons gained in reduction.
