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30 exam-style questions with model answers, plus 40 quick multi-choice questions — every question on the site for this standard, grouped by the 10 pages of notes they come from.
Write a full answer before you reveal the model one — that comparison is where the marks come from. Every block links back to the notes that teach it.
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.
Write balanced half equations for (a) aluminium metal being oxidised to Al3+, and (b) bromine being reduced to bromide ions.
A student writes the half equation Cl2(aq) + e− → Cl−(aq) for the reduction of chlorine. Explain the two errors and give the correct half equation.
Explain why a half equation must have electrons on one side only, and justify the claim that checking the charge is a more reliable test of a half equation than checking the atoms.
Balance the half equation for the reduction of hydrogen peroxide to water in acidic solution, showing your steps.
Balance the half equation for the reduction of the iodate ion, IO3−, to iodine, I2, in acidic solution, and explain how the electron count can be confirmed from the oxidation numbers.
Explain why acidified potassium permanganate is used rather than a neutral solution, and evaluate what would be observed if a student forgot to add the acid.
Combine the half equations Al(s) → Al3+(aq) + 3e− and Cu2+(aq) + 2e− → Cu(s) into a balanced overall equation.
Acidified permanganate reacts with hydrogen peroxide. Write the overall equation, and explain how the observations follow from it.
A student combines the permanganate and iron(II) half equations and obtains MnO4− + 8H+ + Fe2+ → Mn2+ + 4H2O + Fe3+ + 4e−. Analyse the error, give the correct equation, and justify why the presence of electrons in a final equation is always fatal to the answer.
State the colour change you would observe when acidified potassium permanganate is added to a solution containing iron(II) ions, and name the species responsible for each colour.
Hydrogen sulfide gas is bubbled through acidified potassium dichromate solution. Describe the observations and explain them using half equations.
Two colourless solutions are mixed and no visible change occurs; a third colourless solution is then added and the mixture turns brown. Evaluate what can and cannot be concluded from these observations, and explain what further evidence would be needed.
A piece of copper wire is placed in silver nitrate solution. Describe two observations and write the overall ionic equation.
Chlorine water is added separately to potassium bromide and to potassium chloride solutions. Explain what would be observed in each case and what this shows about chlorine.
A student concludes from a single experiment — adding chlorine water to potassium iodide and seeing a brown colour — that chlorine is the strongest oxidant of the halogens. Evaluate this conclusion and describe the minimum set of experiments needed to justify a full order.
List the six components of a complete answer for one redox reaction in this standard.
A student writes: "The solution went from blue to colourless and a brown solid formed. Cu2+ + 2e− → Cu." Explain what is missing for Merit and rewrite the answer so that it would reach Merit.
Explain why, in this standard, an answer containing more correct chemical facts can score lower than an answer containing fewer, and justify what an Excellence answer does differently.