1. 1
   Some roadside breath tests use acidified orange dichromate(VI) ions, which oxidise ethanol in the breath and turn green:
   2Cr₂O₇²⁻(aq) + 3C₂H₅OH(aq) + 16H⁺(aq) → 4Cr³⁺(aq) + 3CH₃COOH(aq) + 11H₂O(l)

   (a)

   What is the oxidation state of chromium in the dichromate(VI) ion, Cr₂O₇²⁻?

   [1 mark]

   • A+3
   • B+7
   • C+12
   • D+6

   (b)

   Which species is the oxidising agent in this reaction?

   [1 mark]

   • AC₂H₅OH
   • BH⁺
   • CCr₂O₇²⁻
   • DCr³⁺

   (c)

   Deduce the half-equation for the reduction of dichromate(VI) ions to chromium(III) ions in acidic solution.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student placed clean strips of four metals, W, X, Y and Z, into separate solutions of the nitrates of the other three metals. W reacted with the nitrates of X, Y and Z. X reacted only with the nitrate of Z, and the solution slowly turned blue. Y reacted with the nitrates of X and Z. Z did not react with any of the solutions.

   (a)

   What is the order of ease of oxidation of the four metals, from most easily oxidised to least?

   [1 mark]

   • AW > Y > X > Z
   • BW > X > Y > Z
   • CZ > X > Y > W
   • DY > W > X > Z

   (b)

   A voltaic cell is made from a Y electrode in a solution of Y nitrate and an X electrode in a solution of X nitrate, joined by a wire and a salt bridge. Which statement is correct?

   [1 mark]

   • AElectrons flow from X to Y in the external circuit, and X is the anode.
   • BElectrons flow from Y to X in the external circuit, and Y is the negative electrode.
   • CElectrons flow through the salt bridge from Y to X.
   • DY is the cathode because Y atoms are reduced.

   (c)

   X is copper, Y is magnesium and Z is silver. Predict which of the four metals will release hydrogen gas from dilute hydrochloric acid, and write an ionic equation for one of these reactions.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A lead–acid car battery is a secondary cell containing electrodes of lead and lead(IV) oxide in sulfuric acid. When the battery is discharging, the reactions at the electrodes are:
   Negative electrode: Pb(s) + SO₄²⁻(aq) → PbSO₄(s) + 2e⁻
   Positive electrode: PbO₂(s) + 4H⁺(aq) + SO₄²⁻(aq) + 2e⁻ → PbSO₄(s) + 2H₂O(l)

   (a)

   Deduce the oxidation state of lead in Pb, PbO₂ and PbSO₄, and hence identify which electrode is the anode during discharge.

   [3 marks]

   (b)

   Deduce the overall equation for the discharge reaction and the half-equations at each electrode while the battery is being recharged, and explain how the concentration of sulfuric acid changes during discharge.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student investigates the redox chemistry of some compounds containing three carbon atoms: propan-1-ol, propan-2-ol, propanal, propanone, propanoic acid and propyne. Boiling points: propanal 49 °C, propan-1-ol 97 °C, propanoic acid 141 °C. Oxidising agent available: acidified potassium dichromate(VI). Reducing agents available: lithium aluminium hydride (LiAlH₄), sodium borohydride (NaBH₄), and hydrogen gas with a nickel catalyst.

   (a)

   Propan-1-ol and propan-2-ol are each warmed with acidified potassium dichromate(VI). Deduce equations, using [O] for the oxidising agent, for the oxidation reactions, and explain how the boiling point data allow either propanal or propanoic acid to be obtained from propan-1-ol. State the colour change you would observe.

   [6 marks]

   (b)

   Deduce equations, using [H] for the reducing agent, to show how propanoic acid can be reduced to propan-1-ol and how propanone can be reduced to propan-2-ol, identifying a suitable reducing agent for each. Then deduce the products when propyne reacts with hydrogen, and explain how the degree of unsaturation changes.

   [6 marks]

   Total for question 4: 12 marks

End of questions