Standard electrode potentials and the hydrogen electrodeEdexcel International A Level Chemistry: Subtopic test

1. 1
   Acidified potassium dichromate(VI) is used to oxidise iron(II) ions to iron(III) ions in a laboratory analysis. The ionic equation for the reaction is: Cr₂O₇²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O

   (a)

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

   [1 mark]

   • A+3
   • B+6
   • C+7
   • D+12

   (b)

   Which species is oxidised in this reaction?

   [1 mark]

   • ACr₂O₇²⁻
   • BH⁺
   • CFe²⁺
   • DH₂O

   (c)

   Explain, in terms of electrons and oxidation number, why the dichromate(VI) ion acts as an oxidising agent in this reaction.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student sets up a standard hydrogen electrode (SHE) to use as the reference when measuring electrode potentials.

   (a)

   Which set of conditions describes the standard hydrogen electrode?

   [1 mark]

   • A273 K, hydrogen gas at 100 kPa, 1.00 mol dm⁻³ H⁺(aq), platinum electrode
   • B298 K, hydrogen gas at 100 kPa, 0.10 mol dm⁻³ H⁺(aq), platinum electrode
   • C298 K, hydrogen gas at 100 kPa, 1.00 mol dm⁻³ H⁺(aq), zinc electrode
   • D298 K, hydrogen gas at 100 kPa, 1.00 mol dm⁻³ H⁺(aq), platinum electrode coated with platinum black

   (b)

   Why is a reference electrode such as the SHE needed to measure electrode potentials?

   [1 mark]

   • AOnly a potential difference between two half-cells can be measured, not the potential of a single half-cell
   • BThe SHE gives the largest possible voltage, which makes readings easier to see
   • CThe SHE supplies the ions needed by the other half-cell
   • DThe SHE stops the other half-cell from reaching equilibrium

   (c)

   Explain why platinum is used as the electrode in the standard hydrogen electrode.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student measures the standard electrode potential of the Fe³⁺(aq)|Fe²⁺(aq) half-cell and, separately, of the Zn²⁺(aq)|Zn(s) half-cell, in each case by connecting the half-cell to a standard hydrogen electrode.

   (a)

   Describe how the student would set up the Fe³⁺(aq)|Fe²⁺(aq) half-cell and complete the circuit so that its standard electrode potential can be measured.

   [3 marks]

   (b)

   The Zn²⁺(aq)|Zn(s) half-cell gives a reading of −0.76 V under standard conditions. Explain what the negative sign shows about zinc compared with hydrogen. Predict how the reading would change if the Zn²⁺(aq) concentration were reduced to 0.10 mol dm⁻³, and give a reason.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A teacher plans for her class to measure standard electrode potentials for three half-cells in turn, each connected to a standard hydrogen electrode: Cu²⁺(aq)|Cu(s), Cl₂(g)|Cl⁻(aq) and Fe³⁺(aq)|Fe²⁺(aq).

   (a)

   Describe how each of the three half-cells is constructed and connected so that its standard electrode potential can be measured.

   [6 marks]

   (b)

   Evaluate the use of the standard hydrogen electrode as the reference electrode for measuring standard electrode potentials.

   [6 marks]

   Total for question 4: 12 marks

End of questions