Secondary 3 Chemistry Redox Electrochemistry Quiz

Secondary 3 Chemistry Quiz - Redox Electrochemistry

ANSWER KEY AND MARKING SCHEME

Total Marks: 40

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Section A: Oxidation and Reduction (Questions 1–5)

1. Define oxidation in terms of electron transfer. [1]

• Answer: Oxidation is the loss of electrons from a substance / an atom or ion.
• Marking: Award 1 mark for "loss of electrons". Accept "removal of electrons" or "donation of electrons".

2. Define reduction in terms of oxidation state change. [1]

• Answer: Reduction is the decrease in oxidation state of an element.
• Marking: Award 1 mark for "decrease in oxidation state" or "oxidation state becomes more negative/less positive".

3. Determine the oxidation state of chromium in the dichromate ion, Cr₂O₇²⁻. [2]

• Answer: +6
• Working: Let oxidation state of Cr = x. 2x + 7(−2) = −2 → 2x − 14 = −2 → 2x = 12 → x = +6.
• Marking: Award 1 mark for correct working/setup, 1 mark for correct answer (+6). Accept answer only if correct.

4. In the reaction: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s), identify the oxidising agent. Explain your reasoning. [2]

• Answer: The oxidising agent is Cu²⁺ (or copper(II) ions / CuSO₄).
• Explanation: Cu²⁺ ions gain electrons (are reduced) to form Cu, causing Zn to be oxidised. The oxidising agent is the species that is itself reduced.
• Marking: Award 1 mark for identifying Cu²⁺/CuSO₄ as the oxidising agent, 1 mark for explaining that Cu²⁺ is reduced / gains electrons.

5. State the colour change observed when aqueous potassium iodide is added to aqueous iron(III) chloride. Name the substance reduced in this reaction. [2]

• Answer: Colour change: Colourless/pale yellow solution turns brown (or reddish-brown). Substance reduced: Iron(III) ions / Fe³⁺.
• Marking: Award 1 mark for correct colour change (brown/reddish-brown), 1 mark for identifying Fe³⁺ as the substance reduced. Accept "iodine formed" as part of colour change description.

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Section B: Electrolysis (Questions 6–12)

6. Define the term electrolysis. [2]

• Answer: Electrolysis is the decomposition of an ionic compound (electrolyte) by passing an electric current through it when molten or in aqueous solution.
• Marking: Award 1 mark for "decomposition by electric current", 1 mark for "molten or aqueous ionic compound/electrolyte". Accept "breaking down using electricity".

7. Explain why ionic compounds must be molten or dissolved in water to undergo electrolysis. [2]

• Answer: In the solid state, ions are held in fixed positions in the lattice and cannot move. When molten or dissolved, the ions become mobile / free to move, allowing them to carry electric current and migrate to the electrodes.
• Marking: Award 1 mark for stating ions are immobile in solid state, 1 mark for stating ions become mobile when molten/dissolved.

8. During the electrolysis of molten lead(II) bromide using inert electrodes: (a) Name the product formed at the cathode. [1]

• Answer: Lead / Pb.
• Marking: Award 1 mark for lead (accept Pb).

(b) Write the half-equation for the reaction occurring at the anode. [2]

• Answer: 2Br⁻ → Br₂ + 2e⁻
• Marking: Award 1 mark for correct reactants and products (Br⁻ → Br₂), 1 mark for correct balancing and electrons.

(c) State one observation made at the anode. [1]

• Answer: Brown fumes / reddish-brown gas evolved / bubbles of brown gas.
• Marking: Award 1 mark for any correct observation (brown gas/fumes, or pungent smell).

9. A student electrolyses dilute aqueous sodium chloride using carbon electrodes. (a) Name the gas produced at the anode. [1]

• Answer: Oxygen / O₂.
• Marking: Award 1 mark for oxygen.

(b) Explain why sodium metal is not formed at the cathode. [2]

• Answer: In aqueous solution, both Na⁺ and H⁺ ions are attracted to the cathode. H⁺ ions are discharged preferentially because hydrogen is lower in the reactivity series than sodium (or H⁺ is a weaker reducing agent / more easily reduced). Hydrogen gas is produced instead of sodium.
• Marking: Award 1 mark for identifying H⁺ is discharged instead of Na⁺, 1 mark for explaining selective discharge (H⁺ more easily reduced / lower in reactivity series).

10. During the electrolysis of aqueous copper(II) sulfate using copper electrodes: (a) State what happens to the mass of the anode. Explain your answer. [2]

• Answer: The mass of the anode decreases. Copper atoms at the anode lose electrons and dissolve into the solution as Cu²⁺ ions: Cu(s) → Cu²⁺(aq) + 2e⁻.
• Marking: Award 1 mark for stating mass decreases, 1 mark for explanation (copper dissolves/oxidises).

(b) Write the half-equation for the reaction at the cathode. [1]

• Answer: Cu²⁺(aq) + 2e⁻ → Cu(s)
• Marking: Award 1 mark for correct half-equation.

11. Predict the products at both electrodes during the electrolysis of concentrated aqueous sodium chloride using inert electrodes. Explain the selective discharge at each electrode. [3]

• Answer:
  • Cathode: Hydrogen gas (H₂). Both Na⁺ and H⁺ are attracted. H⁺ is discharged because it is lower in the reactivity series / more easily reduced. 2H⁺ + 2e⁻ → H₂.
  • Anode: Chlorine gas (Cl₂). Both Cl⁻ and OH⁻ are attracted. In concentrated solution, Cl⁻ is discharged preferentially despite OH⁻ being more easily oxidised, because the concentration of Cl⁻ is much higher. 2Cl⁻ → Cl₂ + 2e⁻.
• Marking: Award 1 mark for correct cathode product and explanation, 1 mark for correct anode product, 1 mark for explanation of selective discharge at anode (concentration effect).

12. State one industrial application of electrolysis and name the substance produced. [1]

• Answer: Any one of: Extraction of aluminium (produces aluminium metal) / Electroplating (produces coated metal objects) / Purification of copper (produces pure copper) / Manufacture of chlorine and sodium hydroxide from brine (produces chlorine, hydrogen, and sodium hydroxide).
• Marking: Award 1 mark for a correct application with the substance produced.

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Section C: Simple Cells and Fuel Cells (Questions 13–16)

13. In a simple chemical cell consisting of zinc and copper electrodes dipped in an electrolyte: (a) Which electrode is the negative terminal? [1]

• Answer: Zinc / Zn.
• Marking: Award 1 mark for zinc.

(b) Explain your answer in terms of electron flow. [2]

• Answer: Zinc is more reactive than copper. Zinc atoms lose electrons more readily and dissolve as Zn²⁺ ions. The electrons accumulate on the zinc electrode, making it the negative terminal. Electrons flow from the zinc electrode through the external circuit to the copper electrode.
• Marking: Award 1 mark for stating zinc is more reactive/loses electrons more readily, 1 mark for explaining electron accumulation on zinc making it negative.

14. State two differences between a simple chemical cell and an electrolytic cell. [2]

• Answer: Any two of:
  1. A simple cell converts chemical energy to electrical energy; an electrolytic cell converts electrical energy to chemical energy.
  2. In a simple cell, the reaction is spontaneous; in an electrolytic cell, the reaction is non-spontaneous (requires external power source).
  3. In a simple cell, the anode is negative and cathode is positive; in an electrolytic cell, the anode is positive and cathode is negative.
• Marking: Award 1 mark for each correct difference (max 2).

15. A hydrogen-oxygen fuel cell produces electricity through a chemical reaction. (a) Write the overall equation for the reaction occurring in a hydrogen fuel cell. [1]

• Answer: 2H₂ + O₂ → 2H₂O
• Marking: Award 1 mark for correct balanced equation.

(b) State one environmental advantage of using hydrogen fuel cells over fossil fuels. [1]

• Answer: The only product is water (no greenhouse gas emissions / no air pollutants such as CO₂, SO₂, NOₓ) / Hydrogen can be produced from renewable sources.
• Marking: Award 1 mark for any valid environmental advantage.

16. Explain why the voltage of a simple cell decreases over time when it is used continuously. [1]

• Answer: The reactants are consumed / the concentration of ions in the electrolyte decreases / the more reactive metal electrode dissolves away, reducing the rate of the redox reaction.
• Marking: Award 1 mark for any valid reason related to reactant depletion or concentration decrease.

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Section D: Integrated and Applied Questions (Questions 17–20)

17. A student adds a piece of zinc metal to aqueous copper(II) sulfate solution. (a) Write the ionic equation for this reaction. [1]

• Answer: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)
• Marking: Award 1 mark for correct ionic equation with state symbols.

(b) Explain, in terms of electron transfer, why this is a redox reaction. [2]

• Answer: Zinc atoms lose electrons (are oxidised) to form Zn²⁺ ions: Zn → Zn²⁺ + 2e⁻. Copper(II) ions gain electrons (are reduced) to form copper atoms: Cu²⁺ + 2e⁻ → Cu. Since both oxidation and reduction occur, this is a redox reaction.
• Marking: Award 1 mark for identifying Zn is oxidised (loses electrons), 1 mark for identifying Cu²⁺ is reduced (gains electrons).

18. During the extraction of aluminium by electrolysis, aluminium oxide is dissolved in molten cryolite. (a) State the purpose of adding cryolite. [1]

• Answer: To lower the melting point of aluminium oxide / to reduce the operating temperature / to save energy costs.
• Marking: Award 1 mark for lowering melting point or reducing temperature.

(b) Write the half-equation for the formation of aluminium at the cathode. [1]

• Answer: Al³⁺ + 3e⁻ → Al
• Marking: Award 1 mark for correct half-equation.

19. A student sets up an electrolytic cell to electroplate an iron spoon with silver. State the substance used as: (a) The anode: [1]

• Answer: Silver / Ag / a piece of pure silver metal.
• Marking: Award 1 mark for silver.

(b) The electrolyte: [1]

• Answer: Aqueous silver nitrate / silver nitrate solution / any soluble silver salt solution (e.g., silver cyanide solution).
• Marking: Award 1 mark for a soluble silver salt solution.

20. Explain why reactive metals such as potassium and sodium are extracted by electrolysis rather than by reduction with carbon. [1]

• Answer: Potassium and sodium are more reactive than carbon / are above carbon in the reactivity series. Carbon cannot reduce their oxides; electrolysis is required to reduce their ions.
• Marking: Award 1 mark for stating they are more reactive than carbon / above carbon in reactivity series.

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END OF ANSWER KEY