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Oxidation–Reduction · Part 1 of 3
12 exam-style questions with model answers, plus 16 quick multi-choice questions — every question on this part of the standard, grouped by the 4 pages of notes they come from.
Write a full answer before you reveal the model one. That comparison is where the learning happens.
A student writes: "When zinc is added to copper(II) sulfate solution, the zinc is oxidised." Explain why this statement on its own would not reach Achieved, and rewrite it so that it would.
State what a Merit answer must add to an Achieved answer for a redox reaction, and illustrate it using the reaction between zinc metal and copper(II) sulfate solution.
Explain what makes an Excellence answer different from a Merit answer in this standard, and demonstrate the difference using the reaction between zinc metal and copper(II) sulfate solution.
Chlorine water is added to a colourless solution of potassium iodide, and the solution turns brown. Identify which species is oxidised and which is reduced, and give a reason for each.
For the reaction between chlorine water and potassium iodide solution, write the two half equations and explain how each one accounts for the observation.
Chlorine water turns potassium iodide solution brown, but adding iodine solution to potassium chloride solution produces no change. Analyse these two results in terms of electron transfer, and explain what they show about chlorine and iodine as oxidants.
Determine the oxidation number of sulfur in each of: SO2, SO32− and H2S.
Sulfur dioxide is bubbled through acidified potassium dichromate solution. Use oxidation numbers to explain which species is oxidised and which is reduced, and state what colour change you would expect.
Hydrogen peroxide reacts with acidified potassium permanganate solution, and separately with acidified potassium iodide solution. Use oxidation numbers to analyse the role played by hydrogen peroxide in each reaction, and justify why the same substance can behave in two different ways.
State whether the reaction 2Na(s) + Cl2(g) → 2NaCl(s) is a redox reaction, and give a reason using oxidation numbers.
Explain, using oxidation numbers, how many electrons are transferred when one Cr2O72− ion is reduced to Cr3+ ions, and how this determines the half equation.
When dilute nitric acid reacts with copper metal, a colourless gas is produced and the solution turns blue. When concentrated nitric acid is used, a brown gas is produced instead. Analyse both reactions using oxidation numbers, and evaluate what the different observations reveal about the reduction of nitrogen.