A student adds a few drops of iron(III) solution to a mixture of potassium iodide and potassium peroxodisulfate solutions. The reaction happens in two steps.
Step 1: 2Fe3++2I−→2Fe2++I2\mathrm{2Fe^{3+} + 2I^{-} \rightarrow 2Fe^{2+} + I_{2}}2Fe3++2I−→2Fe2++I2
Step 2: 2Fe2++S2O82−→2Fe3++2SO42−\mathrm{2Fe^{2+} + S_{2}O_{8}^{2-} \rightarrow 2Fe^{3+} + 2SO_{4}^{2-}}2Fe2++S2O82−→2Fe3++2SO42−
Which species acts as the catalyst for the overall reaction?
Fe2+\mathrm{Fe^{2+}}Fe2+
Fe3+\mathrm{Fe^{3+}}Fe3+
I2\mathrm{I_{2}}I2
SO42−\mathrm{SO_{4}^{2-}}SO42−
56 exam-style questions on OCR GCSE Chemistry 5.2 Controlling reactions, covering 5.2.1 Measuring and interpreting rate of reaction, 5.2.2 Factors affecting rate and collision theory, and 5.2.3 Catalysts, activation energy and enzymes. Each one has a worked solution and a mark scheme showing where the marks go.