Edexcel A Level Chemistry Topic 3: Redox I Exam Questions

About Edexcel A Level Chemistry Topic 3: Redox I

Topic 3 of Edexcel A Level Chemistry, Redox I, introduces oxidation and reduction in terms of electron transfer and oxidation numbers. You need to assign oxidation numbers in elements, compounds and ions, use them to name compounds, and identify oxidising and reducing agents. The topic shows how metals generally form positive ions by losing electrons and non-metals form negative ions by gaining electrons, and introduces disproportionation. A key skill is writing ionic half-equations and combining them into full redox equations. Redox I is assessed in Papers 1 and 2 and provides the foundation for Group 7 chemistry in Topic 4 and electrode potentials in Topic 14 (Redox II).

What you need to know

• Oxidation and reduction Oxidation is loss of electrons or an increase in oxidation number; reduction is gain of electrons or a decrease in oxidation number.
• Assigning oxidation numbers Elements are 0, oxygen is usually -2 (but -1 in peroxides), hydrogen is +1 (but -1 in metal hydrides), and the sum equals the charge on the species.
• Naming compounds Roman numerals show oxidation numbers, for example iron(III) chloride, sulfate(VI) and chlorate(I).
• Oxidising and reducing agents An oxidising agent gains electrons and is reduced; a reducing agent loses electrons and is oxidised.
• Disproportionation The same element is simultaneously oxidised and reduced, for example chlorine in water: Cl2 + H2O → HCl + HClO, where Cl goes from 0 to -1 and +1.
• Half-equations Balance atoms other than O and H, add H2O to balance O, add H+ to balance H, then add electrons to balance charge. Combine half-equations so electrons cancel.

Common mistakes in Topic 3: Redox I

• Describing the oxidising agent as the species oxidised; it is the species reduced.
• Forgetting that oxygen in hydrogen peroxide has an oxidation number of -1.
• Leaving electrons in the overall equation after combining half-equations.
• Writing oxidation numbers as 2+ instead of +2; the sign comes first for oxidation numbers.
• Balancing charge with H+ before balancing oxygen with water, which leads to wrong equations.

Try three Topic 3: Redox I exam-style questions