Standard electrode potentialsEdexcel A-Level Chemistry: Subtopic test

1. 1
   A teacher demonstrates three redox changes. In the first, sodium burns in chlorine: 2Na + Cl₂ → 2NaCl. In the second, orange acidified potassium dichromate(VI) turns green as it is reduced. In the third, purple acidified potassium manganate(VII) is decolourised when it is added to iron(II) sulfate solution.

   (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)

   In the half-equation MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O, what happens to the manganese?

   [1 mark]

   • AIt is oxidised from +2 to +7
   • BIt is oxidised from +7 to +2
   • CIt is reduced from +2 to +7
   • DIt is reduced from +7 to +2

   (c)

   Use oxidation numbers and electron transfer to identify the species oxidised and the species reduced when sodium burns in chlorine.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   To compare the tendency of different species to gain electrons, chemists measure electrode potentials against a single reference. The reference used is the standard hydrogen electrode, which is given an electrode potential of exactly 0.00 V.

   (a)

   Which set of conditions applies to the standard hydrogen electrode?

   [1 mark]

   • AHydrogen gas at 100 kPa, 1.00 mol dm⁻³ H⁺(aq), 298 K
   • BHydrogen gas at 100 kPa, 1.00 mol dm⁻³ H⁺(aq), 273 K
   • CHydrogen gas at 10 kPa, 1.00 mol dm⁻³ H⁺(aq), 298 K
   • DHydrogen gas at 100 kPa, 0.10 mol dm⁻³ H⁺(aq), 298 K

   (b)

   Which material is used for the electrode in the standard hydrogen electrode?

   [1 mark]

   • AZinc
   • BCopper
   • CPlatinum
   • DMagnesium

   (c)

   Explain why a reference electrode is needed to measure the standard electrode potential of a half-cell.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In Core Practical 10, a student investigates electrochemical cells by measuring standard electrode potentials. Each half-cell is connected to a standard hydrogen electrode so that a voltmeter can give the electrode potential. The student has zinc metal, zinc sulfate solution, iron(II) sulfate and iron(III) sulfate solutions, a platinum electrode, filter paper and potassium nitrate solution.

   (a)

   Describe how the student sets up the zinc half-cell and connects it to the standard hydrogen electrode so that the standard electrode potential of Zn²⁺/Zn can be measured.

   [3 marks]

   (b)

   Describe how the student would measure the standard electrode potential of the Fe³⁺/Fe²⁺ half-cell, and state the conditions needed.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student studies the cell made from a copper half-cell and a standard hydrogen electrode, and the redox reaction in which acidified manganate(VII) oxidises iron(II) ions. The standard electrode potential of Cu²⁺(aq)/Cu(s), measured against the standard hydrogen electrode, is +0.34 V.

   (a)

   Describe how the standard electrode potential of Cu²⁺/Cu would be measured, and explain what the value of +0.34 V shows about the direction of electron flow in the external circuit.

   [6 marks]

   (b)

   Explain, using oxidation numbers and electron transfer, what happens to manganese and iron in this reaction. Include the half-equations and the ratio in which the ions react.

   [6 marks]

   Total for question 4: 12 marks

End of questions