Redox EquilibriaEdexcel International A Level Chemistry: Topic test

1. 1
   A student measures the standard electrode potential of the Ni²⁺(aq)|Ni(s) half-cell by connecting it to a standard hydrogen electrode. The voltmeter reads 0.25 V and the nickel electrode is the negative terminal.

   (a)

   What is the standard electrode potential, $E^\circ$, of the Ni²⁺(aq)|Ni(s) half-cell?

   [1 mark]

   • A+0.25 V
   • B0.00 V
   • C+0.50 V
   • D−0.25 V

   (b)

   Which set of conditions is used for the standard hydrogen electrode?

   [1 mark]

   • AHydrogen gas at 100 kPa, 1.00 mol dm⁻³ H⁺(aq), platinum electrode and 298 K
   • BHydrogen gas at 100 kPa, 1.00 mol dm⁻³ H⁺(aq), platinum electrode and 273 K
   • CHydrogen gas at 100 kPa, 0.100 mol dm⁻³ H⁺(aq), platinum electrode and 298 K
   • DHydrogen gas at 100 kPa, 1.00 mol dm⁻³ H⁺(aq), copper electrode and 298 K

   (c)

   Explain why a reference electrode such as the standard hydrogen electrode is needed, and state the role of the platinum in it.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A cell is made from a zinc half-cell and a lead half-cell under standard conditions. Standard electrode potentials: Zn²⁺(aq)|Zn(s) −0.76 V; Pb²⁺(aq)|Pb(s) −0.13 V.

   (a)

   What is the standard emf, $E^\circ_{cell}$, of the cell?

   [1 mark]

   • A+0.89 V
   • B+0.63 V
   • C−0.63 V
   • D+1.26 V

   (b)

   Which is the correct conventional cell diagram?

   [1 mark]

   • APb(s) | Pb²⁺(aq) ‖ Zn²⁺(aq) | Zn(s)
   • BZn(s) | Zn²⁺(aq) | Pb²⁺(aq) | Pb(s)
   • CZn(s) | Zn²⁺(aq) ‖ Pb²⁺(aq) | Pb(s)
   • DZn²⁺(aq) | Zn(s) ‖ Pb(s) | Pb²⁺(aq)

   (c)

   Write the half-equation for the reaction at the positive electrode of the cell and state what happens to the mass of the lead electrode as the cell supplies current.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Standard electrode potentials at 298 K: VO₂⁺(aq),H⁺(aq)|VO²⁺(aq) +1.00 V; VO²⁺(aq),H⁺(aq)|V³⁺(aq) +0.34 V; V³⁺(aq)|V²⁺(aq) −0.26 V; Zn²⁺(aq)|Zn(s) −0.76 V; Fe³⁺(aq)|Fe²⁺(aq) +0.77 V. A yellow solution of vanadium(V), as VO₂⁺, in acid is reduced by two different reducing agents.

   (a)

   Zinc is added to the acidified yellow solution. Deduce the lowest oxidation state of vanadium reached, using the data.

   [3 marks]

   (b)

   Excess iron(II) ions are added instead to an acidified solution of vanadium(V). Use $E^\circ_{cell}$ values to deduce the lowest oxidation state of vanadium reached.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Household bleach contains sodium chlorate(I), NaClO. In an analysis, 10.0 cm³ of bleach is diluted to 250 cm³ in a volumetric flask. A 25.0 cm³ portion is added to excess potassium iodide and dilute sulfuric acid: ClO⁻ + 2I⁻ + 2H⁺ → Cl⁻ + I₂ + H₂O. The iodine formed is titrated with 0.100 mol dm⁻³ sodium thiosulfate: I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻. The mean titre is 18.40 cm³. Each burette reading has an uncertainty of ±0.05 cm³. Standard electrode potentials in acid conditions: ClO⁻,H⁺|Cl⁻ +1.49 V; I₂|I⁻ +0.54 V; S₄O₆²⁻|S₂O₃²⁻ +0.09 V. Mr of NaClO = 74.5.

   (a)

   Calculate the concentration of sodium chlorate(I) in the bleach in mol dm⁻³ and in g dm⁻³.

   [6 marks]

   (b)

   Use the electrode potentials to show that both reactions in the analysis are feasible. Calculate the percentage uncertainty in the titre and explain why starch is added only near the end point.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   A fuel cell with an acidic electrolyte uses ethanol as the fuel. The overall equation is C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l). The two electrodes are separated by a proton-conducting membrane.

   (a)

   At which electrode is ethanol oxidised, and what is the sign of that electrode?

   [1 mark]

   • AThe cathode, which is the positive electrode
   • BThe cathode, which is the negative electrode
   • CThe anode, which is the negative electrode
   • DThe anode, which is the positive electrode

   (b)

   How many moles of electrons are transferred for each mole of ethanol oxidised?

   [1 mark]

   • A3
   • B4
   • C6
   • D12

   (c)

   Write the half-equation for the reaction at the electrode where ethanol is oxidised.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   A cell is set up with two half-cells, each with a platinum electrode. One contains Sn⁴⁺(aq) and Sn²⁺(aq), and the other contains Fe³⁺(aq) and Fe²⁺(aq). Standard electrode potentials: Sn⁴⁺(aq)|Sn²⁺(aq) +0.15 V; Fe³⁺(aq)|Fe²⁺(aq) +0.77 V. All ions are initially at 1.00 mol dm⁻³ and the temperature is 298 K.

   (a)

   What is the standard emf, $E^\circ_{cell}$, of this cell?

   [1 mark]

   • A+0.62 V
   • B+0.92 V
   • C−0.62 V
   • D+0.15 V

   (b)

   The concentration of Fe³⁺(aq) is reduced to 0.100 mol dm⁻³, with all other concentrations unchanged. What happens to the emf of the cell?

   [1 mark]

   • AIt stays at +0.62 V, because E° is a constant.
   • BIt falls to below +0.62 V
   • CIt rises to above +0.62 V.
   • DIt becomes negative.

   (c)

   Write the conventional cell diagram for this cell, with the negative electrode on the left.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   Hydrogen peroxide, H₂O₂, can disproportionate. Standard electrode potentials: O₂(g),H⁺(aq)|H₂O₂(aq) +0.68 V; H₂O₂(aq),H⁺(aq)|H₂O(l) +1.77 V. Aqueous hydrogen peroxide can nevertheless be stored in a clean brown bottle for months.

   (a)

   Write the equation for the disproportionation of hydrogen peroxide and use the electrode potentials to show that it is feasible.

   [3 marks]

   (b)

   Explain, in terms of $E^\circ_{cell}$, entropy and equilibrium, and the rate, why the stored solution does not decompose rapidly.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   An iron tablet of mass 1.20 g contains iron(II) ions as its only source of iron. The tablet is dissolved in dilute sulfuric acid and made up to 250 cm³ in a volumetric flask. A 25.0 cm³ portion is titrated with 0.00200 mol dm⁻³ potassium manganate(VII) solution and the mean titre is 21.50 cm³. The ionic equation is MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺. Ar of Fe = 55.8. Standard electrode potentials: MnO₄⁻,H⁺|Mn²⁺ +1.51 V; Cl₂|Cl⁻ +1.36 V; Fe³⁺|Fe²⁺ +0.77 V.

   (a)

   Calculate the percentage by mass of iron in the tablet. State how the end point is recognised.

   [6 marks]

   (b)

   A student suggests acidifying the iron solution with dilute hydrochloric acid instead of dilute sulfuric acid. Use the electrode potentials to explain why this would give an inaccurate result, and why dilute sulfuric acid is suitable.

   [6 marks]

   Total for question 8: 12 marks

End of questions