Electrode potentials and cellsAQA A-Level Chemistry: Subtopic test

1. 1
   A cell is set up with a zinc half-cell and a copper half-cell joined by a salt bridge. Standard electrode potentials: Zn²⁺(aq) + 2e⁻ ⇌ Zn(s), E° = −0.76 V; Cu²⁺(aq) + 2e⁻ ⇌ Cu(s), E° = +0.34 V.

   (a)

   What is the standard EMF of the cell?

   [1 mark]

   • A+1.10 V
   • B+0.42 V
   • C−1.10 V
   • D+0.34 V

   (b)

   Which is the conventional representation of this cell?

   [1 mark]

   • ACu(s) | Cu²⁺(aq) || Zn²⁺(aq) | Zn(s)
   • BZn²⁺(aq) | Zn(s) || Cu(s) | Cu²⁺(aq)
   • CZn(s) | Zn²⁺(aq) | Cu²⁺(aq) | Cu(s)
   • DZn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s)

   (c)

   Write the equation for the overall cell reaction and state the direction of electron flow in the external circuit.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student measures the standard electrode potential of the Ag⁺(aq) | Ag(s) half-cell by connecting it to a standard hydrogen electrode using a salt bridge and a voltmeter.

   (a)

   Which set of conditions applies to the measurement of a standard electrode potential?

   [1 mark]

   • A273 K, 100 kPa and 1.00 mol dm⁻³ concentration of ions
   • B298 K, 100 kPa and 0.10 mol dm⁻³ concentration of ions
   • C298 K, 100 kPa and 1.00 mol dm⁻³ concentration of ions
   • D298 K, 1000 kPa and 1.00 mol dm⁻³ concentration of ions

   (b)

   Which description of the standard hydrogen electrode is correct?

   [1 mark]

   • AA zinc electrode in 1.00 mol dm⁻³ Zn²⁺(aq) at 298 K, assigned E° = 0.00 V
   • BA platinum electrode in 1.00 mol dm⁻³ H⁺(aq), with hydrogen gas at 100 kPa passed over it at 298 K, assigned E° = 0.00 V
   • CA platinum electrode in 1.00 mol dm⁻³ H⁺(aq), with hydrogen gas at 100 kPa passed over it at 298 K, assigned E° = +1.00 V
   • DA carbon electrode in 1.00 mol dm⁻³ HCl(aq) with chlorine gas at 100 kPa passed over it at 298 K, assigned E° = 0.00 V

   (c)

   Explain why a high-resistance voltmeter is used and what the purpose of the salt bridge is.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A chemist investigates the reactions of aqueous iron(II) and iron(III) ions with halogens and halide ions at standard conditions. Standard electrode potentials: Fe³⁺(aq) + e⁻ ⇌ Fe²⁺(aq), E° = +0.77 V; I₂(aq) + 2e⁻ ⇌ 2I⁻(aq), E° = +0.54 V; Br₂(aq) + 2e⁻ ⇌ 2Br⁻(aq), E° = +1.07 V.

   (a)

   Use the electrode potentials to deduce whether iron(III) ions will oxidise iodide ions. Calculate the EMF and write an equation for any reaction.

   [3 marks]

   (b)

   Deduce, using the electrode potentials, whether bromine will oxidise iron(II) ions to iron(III) ions. Give the EMF and the equation for the reaction, and compare the oxidising power of bromine and iron(III) ions.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Electrochemical cells are used to measure electrode potentials. Standard electrode potentials: Zn²⁺(aq) + 2e⁻ ⇌ Zn(s), E° = −0.76 V; Cu²⁺(aq) + 2e⁻ ⇌ Cu(s), E° = +0.34 V; Ag⁺(aq) + e⁻ ⇌ Ag(s), E° = +0.80 V; Fe³⁺(aq) + e⁻ ⇌ Fe²⁺(aq), E° = +0.77 V.

   (a)

   Describe how the EMF of a cell made from a zinc half-cell and a copper half-cell can be measured in the laboratory, and state the conditions needed for the value to be the standard EMF.

   [6 marks]

   (b)

   A cell is made from a silver half-cell and an Fe³⁺(aq)/Fe²⁺(aq) half-cell. Write the half-equations for the reactions at each electrode, and the overall equation, calculate the EMF, and write the conventional representation of the cell. Explain why a platinum electrode is used in the Fe³⁺/Fe²⁺ half-cell.

   [6 marks]

   Total for question 4: 12 marks

End of questions