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Secondary 4 Pure Chemistry Redox Electrochemistry Quiz
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Questions
Secondary 4 Pure Chemistry Quiz - Redox Electrochemistry
Name: _________________________ Class: _________________________ Date: _________________________ Score: ______ / 40
Duration: 45 minutes Total Marks: 40
Instructions:
- Answer ALL questions in the spaces provided.
- Show all working for calculation questions.
- State symbols are required in all chemical equations unless otherwise stated.
- You may use a calculator.
- The number of marks is given in brackets [ ] at the end of each question or part question.
Section A: Multiple Choice (10 marks)
Choose the best answer for each question. Circle your answer.
1. Which statement correctly describes oxidation in terms of electron transfer? A) Gain of electrons B) Loss of electrons C) Gain of protons D) Loss of protons [1 mark]
2. In the electrolysis of molten sodium chloride using inert electrodes, which product forms at the anode? A) Sodium metal B) Chlorine gas C) Hydrogen gas D) Oxygen gas [1 mark]
3. A simple cell is constructed using zinc and copper electrodes in their respective sulfate solutions. Which electrode is the negative terminal? A) Copper electrode B) Zinc electrode C) Both electrodes equally D) Neither electrode [1 mark]
4. Which substance is an oxidising agent? A) Hydrogen gas B) Carbon monoxide C) Potassium dichromate(VI) D) Zinc metal [1 mark]
5. During the electrolysis of aqueous copper(II) sulfate using copper electrodes, what happens to the mass of the anode? A) It increases B) It decreases C) It remains unchanged D) It first increases then decreases [1 mark]
Section B: Structured Questions (10 marks)
Answer all questions in the spaces provided.
6. Which reaction is NOT a redox reaction? A) 2Mg + O₂ → 2MgO B) HCl + NaOH → NaCl + H₂O C) Zn + CuSO₄ → ZnSO₄ + Cu D) 2Fe + 3Cl₂ → 2FeCl₃ [1 mark]
7. In the electrolysis of dilute sulfuric acid using platinum electrodes, the gas collected at the cathode is: A) Oxygen B) Sulfur dioxide C) Hydrogen D) Sulfur trioxide [1 mark]
8. Which metal can be used to electroplate iron to prevent rusting AND provide sacrificial protection if the coating is scratched? A) Copper B) Silver C) Zinc D) Tin [1 mark]
9. The oxidation state of manganese in KMnO₄ is: A) +2 B) +4 C) +6 D) +7 [1 mark]
10. A hydrogen fuel cell produces electrical energy. What is the overall reaction? A) H₂ + O₂ → H₂O₂ B) 2H₂ + O₂ → 2H₂O C) H₂ + Cl₂ → 2HCl D) 2H₂O → 2H₂ + O₂ [1 mark]
Section C: Structured Questions (10 marks)
Answer all questions in the spaces provided.
11. A student investigates the electrolysis of molten lead(II) bromide, PbBr₂, using graphite electrodes.
(a) Write the half-equation for the reaction occurring at the cathode. Include state symbols. [2 marks]
(b) State and explain the observation made at the anode. [2 marks]
(c) Explain why lead(II) bromide must be molten for electrolysis to occur. [2 marks]
12. The diagram below represents a simple chemical cell constructed using magnesium and copper electrodes immersed in solutions of their respective salts.
[V]
|
----|----
| |
[Mg] [Cu]
| |
MgSO₄ CuSO₄
(aq) (aq)
(a) Identify the anode in this cell and explain your choice. [2 marks]
(b) Write the half-equation for the reaction occurring at the copper electrode. [1 mark]
(c) Explain the function of the salt bridge in this cell. [2 marks]
13. Copper can be purified by electrolysis.
(a) State the material used for the anode and the cathode in this process. [2 marks]
Anode: ___________________________________________________________________________
Cathode: _________________________________________________________________________
(b) Write the half-equation for the reaction occurring at the cathode. [1 mark]
(c) Explain what happens to the impurities in the impure copper anode during electrolysis. [2 marks]
14. A student tests an unknown solution by adding acidified potassium dichromate(VI). The solution turns from orange to green.
(a) What does this colour change indicate about the unknown solution? [1 mark]
(b) Name the type of reaction that has occurred. [1 mark]
15. The table below shows the results of experiments where different metals were added to solutions of metal ions. A tick (✓) indicates a reaction occurred; a cross (✗) indicates no reaction.
| Metal | Mg²⁺(aq) | Zn²⁺(aq) | Fe²⁺(aq) | Cu²⁺(aq) |
|---|---|---|---|---|
| Mg | ✗ | ✓ | ✓ | ✓ |
| Zn | ✗ | ✗ | ✓ | ✓ |
| Fe | ✗ | ✗ | ✗ | ✓ |
| Cu | ✗ | ✗ | ✗ | ✗ |
(a) Using the data, arrange the four metals in order of increasing reactivity. [1 mark]
(b) Write the ionic equation for the reaction between zinc and copper(II) sulfate solution. Include state symbols. [2 marks]
(c) Explain, in terms of electron transfer, why no reaction occurs when copper is added to iron(II) sulfate solution. [2 marks]
Section D: Data-Based and Extended Response Questions (10 marks)
Answer all questions in the spaces provided.
16. A student sets up the apparatus shown below to investigate the electrolysis of aqueous sodium chloride using inert electrodes.
+ --------- -
| |
[Carbon] [Carbon]
| |
| NaCl(aq) |
|___________|
(a) Name the gas produced at the anode and explain why this gas, rather than oxygen, is discharged. [2 marks]
(b) The solution around the cathode becomes alkaline after electrolysis. Explain this observation using relevant half-equations. [3 marks]
17. Describe, in terms of electron transfer, what is meant by the term "oxidising agent". Give one example of a common oxidising agent used in the laboratory and write the half-equation for its reduction. [3 marks]
18. Explain why aluminium metal is not extracted by electrolysis of aqueous aluminium chloride but is instead extracted by electrolysis of molten aluminium oxide dissolved in cryolite. [2 marks]
19. A student wants to electroplate an iron spoon with silver using a simple electrolytic cell.
(a) State the material that should be used for the anode. [1 mark]
(b) Suggest a suitable electrolyte for this process. [1 mark]
(c) Write the half-equation for the reaction occurring at the iron spoon. [1 mark]
20. Compare the energy changes and the redox processes occurring in a simple chemical cell and an electrolytic cell. State one similarity and one difference. [2 marks]
END OF QUIZ
Check your answers carefully before submitting.
Answers
Secondary 4 Pure Chemistry Quiz - Redox Electrochemistry
ANSWER KEY AND MARKING SCHEME
Total Marks: 40
Section A: Multiple Choice (10 marks)
| Question | Answer | Mark |
|---|---|---|
| 1 | B | 1 |
| 2 | B | 1 |
| 3 | B | 1 |
| 4 | C | 1 |
| 5 | B | 1 |
Marking notes:
- Q1: Oxidation is loss of electrons (OIL RIG mnemonic).
- Q2: In molten NaCl, Cl⁻ ions are discharged at the anode: 2Cl⁻ → Cl₂ + 2e⁻.
- Q3: Zinc is more reactive than copper; the more reactive metal acts as the negative terminal (anode in a cell).
- Q4: Potassium dichromate(VI) is a common oxidising agent; it accepts electrons and is reduced.
- Q5: With copper electrodes in CuSO₄(aq), the anode dissolves: Cu(s) → Cu²⁺(aq) + 2e⁻, so its mass decreases.
Section B: Multiple Choice (10 marks)
| Question | Answer | Mark |
|---|---|---|
| 6 | B | 1 |
| 7 | C | 1 |
| 8 | C | 1 |
| 9 | D | 1 |
| 10 | B | 1 |
Marking notes:
- Q6: Neutralisation (HCl + NaOH) is not a redox reaction; no change in oxidation states occurs.
- Q7: In dilute H₂SO₄, H⁺ ions are discharged at the cathode: 2H⁺ + 2e⁻ → H₂.
- Q8: Zinc provides both barrier protection and sacrificial protection because it is more reactive than iron.
- Q9: KMnO₄: K = +1, O₄ = -8, so Mn = +7 (1 + x + 4(-2) = 0, x = +7).
- Q10: The hydrogen fuel cell overall reaction is 2H₂ + O₂ → 2H₂O.
Section C: Structured Questions (10 marks)
Question 11 (6 marks)
(a) Half-equation at the cathode: Pb²⁺(l) + 2e⁻ → Pb(l) [2 marks]
- Award 1 mark for correct species (Pb²⁺ + 2e⁻ → Pb).
- Award 1 mark for correct state symbols (l).
(b) Observation at the anode: Brown fumes / reddish-brown gas evolved [1 mark] Explanation: Bromide ions are oxidised to bromine gas: 2Br⁻(l) → Br₂(g) + 2e⁻ [1 mark]
- Accept: "Brown gas with pungent smell" or "Red-brown vapour".
- Must link observation to the discharge of bromide ions.
(c) Lead(II) bromide must be molten because: In the solid state, the ions are held in fixed positions in the ionic lattice and cannot move / are not mobile. When molten, the ions are free to move and can carry electric current / conduct electricity. [2 marks]
- Award 1 mark for stating ions cannot move in solid.
- Award 1 mark for stating ions are mobile in molten state.
Question 12 (5 marks)
(a) Anode: Magnesium electrode [1 mark] Explanation: Magnesium is more reactive than copper / Magnesium has a greater tendency to lose electrons / Magnesium is oxidised more readily than copper. [1 mark]
- In a simple cell, the more reactive metal acts as the anode (negative terminal) where oxidation occurs.
(b) Half-equation at copper electrode: Cu²⁺(aq) + 2e⁻ → Cu(s) [1 mark]
- Reduction occurs at the cathode (copper electrode).
- State symbols must be correct.
(c) Function of salt bridge: The salt bridge completes the circuit by allowing ions to flow between the two half-cells. Anions move towards the anode and cations move towards the cathode, maintaining electrical neutrality in each half-cell. [2 marks]
- Award 1 mark for "completes the circuit / allows ion flow".
- Award 1 mark for "maintains electrical neutrality" or description of ion movement.
Question 13 (5 marks)
(a) Anode: Impure copper [1 mark] Cathode: Pure copper [1 mark]
(b) Half-equation at cathode: Cu²⁺(aq) + 2e⁻ → Cu(s) [1 mark]
- Copper(II) ions from the electrolyte are reduced and deposited as pure copper on the cathode.
(c) Impurities in the impure copper anode: More reactive metals (e.g., Zn, Fe) dissolve into the solution as ions but are not discharged at the cathode. Less reactive metals (e.g., Ag, Au) do not dissolve and fall to the bottom of the cell as anode sludge / anode mud. [2 marks]
- Award 1 mark for describing the fate of more reactive impurities.
- Award 1 mark for describing the fate of less reactive impurities (anode sludge).
Question 14 (2 marks)
(a) The colour change from orange to green indicates: The unknown solution contains a reducing agent / The unknown solution can reduce potassium dichromate(VI). [1 mark]
- Accept: "The solution is a reducing agent" or "The dichromate(VI) ions are reduced to chromium(III) ions."
(b) Type of reaction: Redox reaction / Oxidation-reduction reaction [1 mark]
- The dichromate(VI) is reduced (orange → green) while the unknown substance is oxidised.
Question 15 (5 marks)
(a) Order of increasing reactivity: Copper < Iron < Zinc < Magnesium [1 mark]
- Must be in correct order from least to most reactive.
- Accept: Cu, Fe, Zn, Mg.
(b) Ionic equation: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s) [2 marks]
- Award 1 mark for correct formulae.
- Award 1 mark for correct state symbols.
- Spectator ions (SO₄²⁻) must be omitted.
(c) Explanation: Copper is less reactive than iron / Copper is below iron in the reactivity series. Copper atoms cannot lose electrons to reduce iron(II) ions / Copper has a lower tendency to form ions than iron. For a reaction to occur, the added metal must be more reactive than the metal ion in solution so that electron transfer can take place. [2 marks]
- Award 1 mark for stating copper is less reactive than iron.
- Award 1 mark for explaining in terms of electron transfer (Cu cannot reduce Fe²⁺ / Cu cannot donate electrons to Fe²⁺).
Section D: Data-Based and Extended Response Questions (10 marks)
Question 16 (5 marks)
(a) Gas produced at the anode: Chlorine gas [1 mark] Explanation: In concentrated aqueous sodium chloride, chloride ions are discharged in preference to hydroxide ions due to the concentration effect / Chloride ions are present in higher concentration than hydroxide ions, so they are selectively discharged despite oxygen being higher in the discharge series. [1 mark]
- Accept reference to "selective discharge" or "concentration effect".
- The relevant half-equation: 2Cl⁻(aq) → Cl₂(g) + 2e⁻.
(b) Explanation of alkaline solution at cathode: At the cathode, hydrogen ions (from water) are discharged in preference to sodium ions: 2H⁺(aq) + 2e⁻ → H₂(g) OR 2H₂O(l) + 2e⁻ → H₂(g) + 2OH⁻(aq). This leaves behind an excess of hydroxide ions (OH⁻) in the solution around the cathode, making it alkaline. [3 marks]
- Award 1 mark for identifying that H⁺ ions (or water) are discharged at the cathode.
- Award 1 mark for correct half-equation.
- Award 1 mark for explaining that OH⁻ ions accumulate, causing alkalinity.
- Accept: "Sodium ions remain in solution with hydroxide ions, forming sodium hydroxide which is alkaline."
Question 17 (3 marks)
An oxidising agent is a substance that accepts electrons / causes another substance to be oxidised, and is itself reduced. [1 mark] Example: Potassium dichromate(VI) / acidified potassium manganate(VII) / chlorine. [1 mark] Half-equation for reduction: Cr₂O₇²⁻(aq) + 14H⁺(aq) + 6e⁻ → 2Cr³⁺(aq) + 7H₂O(l) (accept other valid examples with correct half-equation). [1 mark]
- Award marks for correct definition, valid example, and correct corresponding half-equation.
Question 18 (2 marks)
In aqueous aluminium chloride, water is present. At the cathode, hydrogen ions (from water) are discharged in preference to aluminium ions because aluminium is more reactive than hydrogen. Therefore, hydrogen gas is produced instead of aluminium metal. [1 mark] In the electrolysis of molten aluminium oxide, there is no water present. The only ions are Al³⁺ and O²⁻, so aluminium ions are discharged at the cathode to form aluminium metal. [1 mark]
- Accept reference to the reactivity series and selective discharge.
Question 19 (3 marks)
(a) Anode material: Silver metal / Pure silver [1 mark]
(b) Suitable electrolyte: Silver nitrate solution / any soluble silver salt solution (e.g., silver ethanoate, silver sulfate) [1 mark]
(c) Half-equation at iron spoon (cathode): Ag⁺(aq) + e⁻ → Ag(s) [1 mark]
- The iron spoon is the cathode where reduction occurs.
Question 20 (2 marks)
Similarity: Both cells involve redox reactions / involve electron transfer / involve oxidation at the anode and reduction at the cathode. [1 mark] Difference: A simple chemical cell converts chemical energy into electrical energy (spontaneous reaction), while an electrolytic cell converts electrical energy into chemical energy (non-spontaneous reaction). [1 mark]
- Accept other valid similarities and differences related to energy conversion and spontaneity.
END OF ANSWER KEY