Redox IIEdexcel A-Level Chemistry: Topic test

20 questions, 54 marks

Edexcel A-Level Chemistry

Redox II topic test

Total 54 marks

Name

Class

Date

1
Magnesium ribbon continues to burn when lowered into a gas jar of carbon dioxide, forming a white solid and black specks of carbon: 2Mg(s) + CO₂(g) → 2MgO(s) + C(s).
(a)
What is the change in the oxidation number of carbon in this reaction?
[1 mark]
A+4 to +2
B0 to +4
C+4 to 0
D+2 to 0
(b)
Which statement about the reaction is correct?
[1 mark]
AMagnesium is the reducing agent because it loses electrons
BCarbon dioxide is the reducing agent because it is reduced
CMagnesium is the oxidising agent because its oxidation number increases
DMagnesium is reduced because its oxidation number increases from 0 to +2
(c)
Write the half-equation for the oxidation of magnesium and explain, in terms of electron transfer, why carbon dioxide is the oxidising agent.
[2 marks]
Total for question 1: 4 marks
2
A student sets up a cell from a nickel half-cell and a lead half-cell. Each metal strip dips into a 1.00 mol dm⁻³ solution of its nitrate at 298 K, and the two solutions are joined by a salt bridge. Standard electrode potentials: Ni²⁺(aq) + 2e⁻ ⇌ Ni(s), E° = −0.25 V; Pb²⁺(aq) + 2e⁻ ⇌ Pb(s), E° = −0.13 V.
(a)
What is the standard emf, E°cell, of this cell?
[1 mark]
A+0.38 V
B+0.12 V
C−0.12 V
D+0.25 V
(b)
Which is the correct conventional cell diagram for this cell?
[1 mark]
APb(s)|Pb²⁺(aq)||Ni²⁺(aq)|Ni(s)
BNi²⁺(aq)|Ni(s)||Pb(s)|Pb²⁺(aq)
CNi(s)|Pb²⁺(aq)||Ni²⁺(aq)|Pb(s)
DNi(s)|Ni²⁺(aq)||Pb²⁺(aq)|Pb(s)
(c)
The student repeats the experiment using 2.00 mol dm⁻³ lead(II) nitrate and 1.00 mol dm⁻³ nickel(II) nitrate. Predict how the measured emf changes and explain why.
[2 marks]
Total for question 2: 4 marks
3
A student determines the percentage of copper in a sample of brass. A 1.250 g sample is dissolved in acid and made up to 250 cm³ with distilled water. A 25.0 cm³ portion is added to an excess of potassium iodide solution, which reacts: 2Cu²⁺ + 4I⁻ → 2CuI + I₂. The iodine formed is titrated with 0.0500 mol dm⁻³ sodium thiosulfate solution, using starch indicator near the end point: I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻. The mean titre is 22.40 cm³.
(a)
Calculate the amount, in mol, of Cu²⁺ in the 25.0 cm³ portion.
[3 marks]
(b)
Calculate the percentage by mass of copper in the brass (A_r Cu = 63.5), and explain why the starch indicator is added only near the end point.
[4 marks]
Total for question 3: 7 marks
4
A company designs a rechargeable nickel–cadmium cell for a portable power pack at a remote weather station. The cell has an alkaline electrolyte. Standard electrode potentials: NiO(OH)(s) + H₂O(l) + e⁻ ⇌ Ni(OH)₂(s) + OH⁻(aq), E° = +0.49 V; Cd(OH)₂(s) + 2e⁻ ⇌ Cd(s) + 2OH⁻(aq), E° = −0.81 V.
(a)
Calculate E°cell for the cell, write the equation for the reaction at each electrode and the overall equation as the cell discharges, and explain how the cell can be recharged.
[6 marks]
(b)
The company is also considering a hydrogen–oxygen fuel cell with an alkaline electrolyte for the power pack. Write the equation for the reaction at each electrode of the fuel cell and evaluate the use of the fuel cell compared with the nickel–cadmium cell.
[6 marks]
Total for question 4: 12 marks
5
A chemist builds a standard hydrogen electrode (SHE) to use as a reference when measuring the standard electrode potentials of other half-cells.
(a)
Which set of conditions applies to the standard hydrogen electrode?
[1 mark]
AHydrogen gas at 100 kPa, H⁺(aq) at 1.00 mol dm⁻³, 298 K
BHydrogen gas at 100 kPa, H⁺(aq) at 0.100 mol dm⁻³, 298 K
CHydrogen gas at 100 kPa, H⁺(aq) at 1.00 mol dm⁻³, 373 K
DHydrogen gas at 10 kPa, H⁺(aq) at 1.00 mol dm⁻³, 298 K
(b)
Which electrode is used in the standard hydrogen electrode, and why?
[1 mark]
AA zinc rod, because zinc is more reactive than hydrogen
BA carbon rod, because it is cheap and reacts with the hydrogen
CPlatinum coated with finely divided platinum, because it is an inert conductor that provides a surface for the H⁺/H₂ equilibrium
DA copper rod, because it is a good conductor
(c)
Explain why a reference electrode is needed to measure standard electrode potentials.
[2 marks]
Total for question 5: 4 marks
6
Hydrogen peroxide solution is sold in dark bottles. Standard electrode potentials: H₂O₂(aq) + 2H⁺(aq) + 2e⁻ ⇌ 2H₂O(l), E° = +1.77 V; O₂(g) + 2H⁺(aq) + 2e⁻ ⇌ H₂O₂(aq), E° = +0.68 V. The decomposition 2H₂O₂(aq) → 2H₂O(l) + O₂(g) is very slow at room temperature.
(a)
Why is the decomposition of hydrogen peroxide described as a disproportionation?
[1 mark]
AHydrogen is oxidised and oxygen is reduced
BOxygen in H₂O₂ is reduced from −1 to 0 and hydrogen is unchanged
COxygen in H₂O₂ is oxidised from −2 to −1 in the products
DOxygen in H₂O₂ (−1) is both oxidised to 0 in O₂ and reduced to −2 in H₂O
(b)
What is E°cell for the decomposition of hydrogen peroxide?
[1 mark]
A+2.45 V
B+1.09 V
C−1.09 V
D+1.77 V
(c)
The value of E°cell shows that the decomposition is feasible, but hydrogen peroxide can be stored for months. Explain why.
[2 marks]
Total for question 6: 4 marks
7
A student builds a cell from an aluminium half-cell and a copper half-cell. Standard electrode potentials: Al³⁺(aq) + 3e⁻ ⇌ Al(s), E° = −1.66 V; Cu²⁺(aq) + 2e⁻ ⇌ Cu(s), E° = +0.34 V.
(a)
Calculate E°cell, write the conventional cell diagram and write the overall cell reaction.
[3 marks]
(b)
The student uses 0.100 mol dm⁻³ copper(II) sulfate solution, with 1.00 mol dm⁻³ aluminium ions at 298 K, and a high-resistance voltmeter. The reading is slightly lower than +2.00 V. Explain why the reading is lower, and why a high-resistance voltmeter is used.
[4 marks]
Total for question 7: 7 marks
8
A student investigates tin(II) ions as a reducing agent. Standard electrode potentials: Sn⁴⁺(aq) + 2e⁻ ⇌ Sn²⁺(aq), E° = +0.15 V; I₂(aq) + 2e⁻ ⇌ 2I⁻(aq), E° = +0.54 V; Fe³⁺(aq) + e⁻ ⇌ Fe²⁺(aq), E° = +0.77 V; Zn²⁺(aq) + 2e⁻ ⇌ Zn(s), E° = −0.76 V.
(a)
Use the data to predict whether tin(II) ions will reduce (i) Fe³⁺(aq), (ii) I₂(aq) and (iii) Zn²⁺(aq) under standard conditions. Show your working, and state what the sign of E°cell tells you about ΔS total and K.
[6 marks]
(b)
Describe how the student could measure E°cell for a cell made from the Sn⁴⁺/Sn²⁺ half-cell and the Fe³⁺/Fe²⁺ half-cell. Include the details of each half-cell, the connections between them, the cell diagram and the reading expected.
[6 marks]
Total for question 8: 12 marks

End of questions

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).