CCEA A Level Chemistry
CCEA A Level Chemistry 1.7 Redox Exam Questions
Free CCEA A Level Chemistry revision for 1.7 Redox: what you need to know, common mistakes and exam-style questions with answers. Covers oxidation states including peroxides and metal hydrides, oxidation and reduction, oxidising and reducing agents, disproportionation, half-equations and Roman numeral names.
Practise 1.7 Redox with past papers
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About CCEA A Level Chemistry 1.7 Redox
Section 1.7 of Unit AS 1 gives you the tools for every redox reaction in the course. You need to assign oxidation states, including in peroxides (oxygen is -1) and metal hydrides (hydrogen is -1), define oxidation and reduction in terms of electrons and oxidation state, identify oxidising and reducing agents and recognise disproportionation. You must write half-equations and combine them to form balanced redox equations, and use Roman numerals in names such as chlorate(I) and chlorate(V). These skills are used immediately in the halogens (1.8) and again in A2 volumetric analysis, transition metals and electrode potentials.
What you need to know
- Oxidation state rules Elements are 0; oxygen is usually -2 (but -1 in peroxides such as H2O2); hydrogen is usually +1 (but -1 in metal hydrides such as NaH); the sum in a compound is 0 and in an ion equals the charge.
- Definitions Oxidation is loss of electrons or an increase in oxidation state; reduction is gain of electrons or a decrease in oxidation state. An oxidising agent accepts electrons and is reduced; a reducing agent donates electrons and is oxidised.
- Disproportionation The same element is simultaneously oxidised and reduced, for example Cl2 + 2NaOH gives NaCl + NaClO + H2O, where chlorine goes from 0 to -1 and +1.
- Half-equations Balance atoms other than O and H, add H2O for oxygen, H+ for hydrogen, then electrons for charge. Example: MnO4- + 8H+ + 5e- gives Mn2+ + 4H2O.
- Combining half-equations Multiply so electrons cancel, then add and simplify. Check that atoms and charges balance.
- Naming Roman numerals give the oxidation state: chlorate(I) ClO-, chlorate(V) ClO3-, iron(II), manganate(VII) MnO4-.
Common mistakes in 1.7 Redox
- Using -2 for oxygen in hydrogen peroxide. In peroxides oxygen is -1.
- Saying the oxidising agent is oxidised. The oxidising agent is reduced.
- Leaving electrons in a combined redox equation. Electrons must cancel completely.
- Balancing half-equations with O2 instead of H2O. In aqueous acidic conditions use H2O and H+.
- Confusing oxidation state with charge on a covalent atom. Oxidation state is a bookkeeping number, written with the sign first, for example +6.
Try three 1.7 Redox exam-style questions
1. Give the oxidation state of chromium in Cr2O7 2-. (1 mark)
Show the mark scheme answer
2Cr + 7(-2) = -2 so 2Cr = +12; Cr = +6.
2. Write the overall equation for the reaction of acidified manganate(VII) with iron(II) ions. (2 marks)
Show the mark scheme answer
MnO4- + 8H+ + 5e- gives Mn2+ + 4H2O and Fe2+ gives Fe3+ + e- (x5); MnO4- + 8H+ + 5Fe2+ gives Mn2+ + 4H2O + 5Fe3+.
3. Explain why the reaction of chlorine with cold sodium hydroxide is a disproportionation. (2 marks)
Show the mark scheme answer
Chlorine is both oxidised and reduced; oxidation state changes from 0 to -1 in Cl- and 0 to +1 in ClO-.
1.7 Redox FAQs
What does CCEA A Level Chemistry 1.7 Redox cover?
It covers oxidation states, definitions of oxidation and reduction, oxidising and reducing agents, disproportionation, half-equations and naming with Roman numerals.
Which unit is 1.7 Redox examined in?
It is examined in Unit AS 1: Basic Concepts in Physical and Inorganic Chemistry, a 1 hour 30 minute paper with a multiple-choice section and structured questions. Related practical skills can also be assessed in Unit AS 3.
How do I get better at writing half-equations?
Follow the same order every time: balance the key element, add water for oxygen, add H+ for hydrogen, then add electrons to balance charge, and check both atoms and charges at the end.
