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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: 50 minutes
Instructions
- Answer ALL questions.
- Write your answers in the spaces provided.
- Show all working clearly where calculations are required.
- The number of marks for each question or part-question is shown in brackets [ ].
- You may use a calculator where appropriate.
- This quiz contains 20 questions across three sections: Section A (Multiple Choice), Section B (Short Answer), and Section C (Structured Response).
Section A: Multiple Choice Questions (Questions 1–5) [10 marks]
For each question, choose the most suitable answer (A, B, C, or D). Write your answer in the space provided.
1. Which of the following statements best describes oxidation in terms of electron transfer?
A. Gain of electrons
B. Loss of electrons
C. Gain of oxygen only
D. Loss of hydrogen only
Answer: ________ [1]
2. In the reaction: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s), which substance is the reducing agent?
A. Zn(s)
B. CuSO₄(aq)
C. ZnSO₄(aq)
D. Cu(s)
Answer: ________ [1]
3. During electrolysis of molten sodium chloride using inert electrodes, what is formed at the cathode?
A. Chlorine gas
B. Sodium metal
C. Hydrogen gas
D. Oxygen gas
Answer: ________ [1]
4. Which of the following metals is most likely to be extracted by electrolysis rather than reduction with carbon?
A. Iron
B. Copper
C. Aluminium
D. Zinc
Answer: ________ [1]
5. In a simple electrochemical cell made of magnesium and copper electrodes immersed in dilute sulfuric acid, which observation is correct?
A. The magnesium electrode increases in mass.
B. The copper electrode dissolves.
C. Bubbles of hydrogen gas are produced at the copper electrode.
D. Electrons flow from the copper electrode to the magnesium electrode through the external circuit.
Answer: ________ [1]
Section B: Short Answer Questions (Questions 6–15) [15 marks]
Answer each question in the space provided. Show working where necessary.
6. State the oxidation number of sulfur in each of the following:
(a) H₂S ________ [1]
(b) SO₃ ________ [1]
(c) Na₂SO₄ ________ [1]
7. Define the term oxidising agent in terms of electron transfer. [2]
8. A strip of iron metal is placed in a solution of silver nitrate.
(a) Write the ionic equation for the reaction that occurs. [2]
(b) Identify the substance that is reduced. Explain your answer in terms of oxidation number change. [2]
9. State two observations you would expect when an aqueous solution of copper(II) sulfate is electrolysed using carbon electrodes. [2]
Observation 1: _________________________________________________________________
Observation 2: _________________________________________________________________
10. Explain why aluminium is extracted from its ore by electrolysis rather than by reduction with carbon. [2]
11. A student sets up a simple cell using a zinc rod and an iron rod connected by a wire, both placed in dilute sulfuric acid.
(a) Which metal acts as the negative electrode? Explain your answer. [2]
(b) Write the half-equation for the reaction occurring at the negative electrode. [1]
12. State one industrial application of electrolysis and briefly explain the process involved. [2]
Application: __________________________________________________________________
Explanation: __________________________________________________________________
13. In the reaction between chlorine gas and potassium bromide solution:
Cl₂(g) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)
(a) Identify the species that is oxidised. [1]
(b) Explain, in terms of electron transfer, why this is a redox reaction. [2]
14. Describe the rusting of iron. In your answer, state the conditions required and write the overall word equation for rust formation. [3]
Conditions: ___________________________________________________________________
Word equation: _______________________________________________________________
15. A student electrolyses concentrated sodium chloride solution (brine) using inert electrodes.
(a) Name the product formed at the anode. [1]
(b) Name the product formed at the cathode. [1]
(c) Explain why chlorine gas is produced at the anode instead of oxygen gas. [2]
Section C: Structured Response Questions (Questions 16–20) [15 marks]
Answer all questions. Show all working clearly. Use complete sentences where explanations are required.
16. The reactivity series of metals (from most reactive to least reactive) is:
Potassium > Sodium > Calcium > Magnesium > Aluminium > Zinc > Iron > Copper > Silver
(a) Explain why metals above carbon in the reactivity series cannot be extracted by reduction with carbon. [2]
(b) A student adds a piece of zinc metal to a solution containing a mixture of copper(II) nitrate and silver nitrate.
(i) Describe the observations the student would make. [2]
(ii) Write the ionic equation for the reaction between zinc and silver ions. [1]
(iii) Explain which metal ion, Cu²⁺ or Ag⁺, is more easily reduced. Refer to the reactivity series in your answer. [2]
17. The diagram below (described in words) shows the electrolysis of dilute sulfuric acid using inert platinum electrodes.
At electrode X (connected to the positive terminal of the power supply): a gas is produced that relights a glowing splint.
At electrode Y (connected to the negative terminal of the power supply): a gas is produced that produces a "pop" sound with a lighted splint.
(a) Identify electrode X as the anode or cathode. [1]
(b) Name the gas produced at electrode X. [1]
(c) Name the gas produced at electrode Y. [1]
(d) Write the half-equation for the reaction at electrode Y. [1]
(e) Explain why the concentration of sulfuric acid increases during the electrolysis. [2]
18. Electroplating is used to coat a spoon with silver.
(a) State what the spoon should be connected to in the circuit (positive or negative terminal). [1]
(b) Name the material used for the other electrode. [1]
(c) Name a suitable electrolyte for this electroplating process. [1]
(d) Write the half-equation for the reaction occurring at the spoon. [1]
(e) Explain why the concentration of the electrolyte remains approximately constant during electroplating. [2]
19. Consider the following redox reaction:
Fe₂O₃(s) + 3CO(g) → 2Fe(s) + 3CO₂(g)
(a) Determine the oxidation number of iron in Fe₂O₃. [1]
(b) Determine the oxidation number of carbon in CO and in CO₂. [2]
In CO: _______________________________________________________________________
In CO₂: _____________________________________________________________________
(c) Identify the oxidising agent and the reducing agent in this reaction. Explain your answer in terms of oxidation number changes. [3]
Oxidising agent: ______________________________________________________________
Explanation: ________________________________________________________________
Reducing agent: ______________________________________________________________
Explanation: ________________________________________________________________
20. A student investigates the reactivity of four metals (P, Q, R, and S) by placing each metal in solutions of the other metals' sulfate solutions. The results are shown in the table below. A tick (✓) means a reaction occurred; a cross (✗) means no reaction occurred.
| Metal placed ↓ / Solution → | PSO₄ | QSO₄ | RSO₄ | SO₄ |
|---|---|---|---|---|
| P | — | ✗ | ✗ | ✗ |
| Q | ✓ | — | ✗ | ✗ |
| R | ✓ | ✓ | — | ✗ |
| S | ✓ | ✓ | ✓ | — |
(a) Arrange the four metals in order of decreasing reactivity. [1]
(b) Explain how you determined this order using evidence from the table. [3]
(c) If metal S is copper, predict what would happen when copper is placed in dilute hydrochloric acid. Explain your answer with reference to the reactivity series. [2]
End of Quiz
Answers
Secondary 4 Pure Chemistry Quiz - Redox Electrochemistry
Answer Key
Section A: Multiple Choice Questions
1. B – Loss of electrons [1]
Explanation: Oxidation is defined as the loss of electrons (OIL RIG: Oxidation Is Loss, Reduction Is Gain). While oxidation can also involve gain of oxygen or loss of hydrogen, the fundamental definition in terms of electron transfer is loss of electrons.
2. A – Zn(s) [1]
Explanation: Zinc loses electrons (is oxidised) and causes Cu²⁺ ions to gain electrons (be reduced). The substance that is oxidised is the reducing agent. Zn goes from oxidation number 0 to +2, confirming it is oxidised.
3. B – Sodium metal [1]
Explanation: In molten NaCl, the ions present are Na⁺ and Cl⁻. At the cathode (negative electrode), Na⁺ ions are reduced to sodium metal: Na⁺ + e⁻ → Na. Chlorine gas forms at the anode. Hydrogen and oxygen are not relevant here since there is no water present (molten, not aqueous).
4. C – Aluminium [1]
Explanation: Aluminium is above carbon in the reactivity series, meaning it is more reactive than carbon and cannot be reduced by carbon. Electrolysis is required to extract metals above carbon (K, Na, Ca, Mg, Al). Iron, copper, and zinc are below carbon and can be extracted by reduction with carbon.
5. C – Bubbles of hydrogen gas are produced at the copper electrode. [1]
Explanation: Magnesium is more reactive than copper, so magnesium acts as the negative electrode (anode) and dissolves. At the copper electrode (positive/cathode), H⁺ ions from the acid are reduced to H₂ gas. Electrons flow from magnesium to copper through the external circuit. The copper electrode does not dissolve, and the magnesium electrode decreases in mass.
Section B: Short Answer Questions
6. [3 marks]
(a) H₂S: –2 [1]
Working: Let oxidation number of S = x. Then: 2(+1) + x = 0 → x = –2
(b) SO₃: +6 [1]
Working: Let oxidation number of S = x. Then: x + 3(–2) = 0 → x = +6
(c) Na₂SO₄: +6 [1]
Working: Let oxidation number of S = x. Then: 2(+1) + x + 4(–2) = 0 → 2 + x – 8 = 0 → x = +6
Marking note: Students must show working for full credit. Accept correct answers without working for 1 mark each, but encourage showing method.
7. [2 marks]
An oxidising agent is a substance that accepts electrons (gains electrons / is reduced) and causes another substance to be oxidised. [2]
Marking note: Award 1 mark for "accepts/gains electrons" and 1 mark for stating that it causes another substance to be oxidised (or that it is itself reduced). Answers that only say "causes oxidation" without mentioning electron transfer receive 1 mark only.
8. [4 marks]
(a) Ionic equation: Fe(s) + 2Ag⁺(aq) → Fe²⁺(aq) + 2Ag(s) [2]
Marking note: Award 2 marks for correct balanced equation with state symbols. Award 1 mark for correct reactants and products but unbalanced or missing state symbols.
(b) Silver ions (Ag⁺) are reduced. [1] The oxidation number of silver decreases from +1 in Ag⁺ to 0 in Ag, meaning it gains electrons. [1]
Marking note: Students must identify Ag⁺ (not Ag metal) as the species reduced. The explanation must reference oxidation number change or electron gain.
9. [2 marks]
Observation 1: A brown/pink solid (copper) is deposited on the cathode. [1]
Observation 2: Bubbles of gas (oxygen) are produced at the anode. [1]
Alternative acceptable answers: The blue colour of the solution fades (becomes paler) [1]; the cathode increases in mass [1].
Marking note: Award 1 mark per valid observation. "Gas produced" alone is insufficient — the electrode must be specified or the gas named.
10. [2 marks]
Aluminium is above carbon in the reactivity series, meaning aluminium is more reactive than carbon. [1] Therefore, carbon cannot reduce aluminium oxide to aluminium. Electrolysis is required because it uses electrical energy to force the reduction of Al³⁺ ions to aluminium metal. [1]
Marking note: The key point is the position of aluminium relative to carbon in the reactivity series. Answers that only say "aluminium is very reactive" without reference to carbon or the reactivity series receive 1 mark.
11. [3 marks]
(a) Zinc acts as the negative electrode. [1] Zinc is more reactive than iron (higher in the reactivity series), so zinc loses electrons more readily. Electrons flow from zinc to iron through the external circuit, making zinc the negative electrode. [1]
(b) Half-equation: Zn(s) → Zn²⁺(aq) + 2e⁻ [1]
Marking note: For (a), students must state that zinc is more reactive/higher in the reactivity series. Simply naming zinc without explanation receives 1 mark only.
12. [2 marks]
Application: Electroplating / Extraction of aluminium / Chlor-alkali industry (manufacture of chlorine, sodium hydroxide, and hydrogen) [1]
Explanation: Electroplating uses electrolysis to deposit a layer of metal onto an object. The object to be plated is made the cathode, the plating metal is the anode, and a solution of the plating metal's ions is used as the electrolyte. During electrolysis, the anode dissolves and metal ions are deposited on the cathode. [1]
Acceptable alternative: Extraction of aluminium – Al₂O₃ is dissolved in molten cryolite and electrolysed; Al³⁺ ions are reduced at the cathode to form aluminium metal.
Marking note: The application and explanation must be consistent. Award 1 mark for a valid application and 1 mark for a correct, relevant explanation.
13. [3 marks]
(a) Species oxidised: Br⁻ (bromide ions) [1]
(b) Chlorine gains electrons (is reduced): Cl goes from oxidation number 0 in Cl₂ to –1 in Cl⁻. [1] Bromide ions lose electrons (are oxidised): Br goes from oxidation number –1 in Br⁻ to 0 in Br₂. Since there is simultaneous electron transfer (loss and gain of electrons), this is a redox reaction. [1]
Marking note: Students must show oxidation number changes. Simply stating "chlorine is reduced and bromide is oxidised" without oxidation number evidence receives 1 mark only for part (b).
14. [3 marks]
Conditions: Iron must be in contact with both oxygen (from air) and water (moisture). [1]
Word equation: Iron + Oxygen + Water → Hydrated iron(III) oxide (rust) [2]
Marking note: Award 1 mark for both conditions (oxygen AND water). Award 2 marks for the correct word equation. "Hydrated iron(III) oxide" or "rust" is acceptable as the product. "Iron oxide" alone receives 1 mark.
15. [4 marks]
(a) Product at anode: Chlorine gas (Cl₂) [1]
(b) Product at cathode: Hydrogen gas (H₂) [1]
(c) Chloride ions (Cl⁻) are preferentially discharged over hydroxide ions (OH⁻) because the concentration of Cl⁻ ions is much higher in concentrated brine. [1] When the concentration of chloride ions is high, they are discharged in preference to hydroxide ions despite hydroxide ions being lower in the electrochemical series. [1]
Marking note: The key concept is that concentration affects the order of discharge. Answers that only state "chloride ions are discharged" without explaining why (concentration effect) receive 1 mark only for part (c). Reference to the electrochemical series alone is insufficient.
Section C: Structured Response Questions
16. [7 marks]
(a) Metals above carbon in the reactivity series are more reactive than carbon. [1] This means carbon cannot displace these metals from their compounds (cannot reduce their oxides). A less reactive element cannot reduce the oxide of a more reactive element. [1]
(b)(i) The zinc metal dissolves / decreases in mass. A grey solid (silver) is deposited first, followed by a brown/pink solid (copper). [2]
Marking note: Award 1 mark for zinc dissolving and 1 mark for correct description of deposits. If the student says only "silver is deposited" without mentioning copper, award 1 mark.
(b)(i alt) If the student states that silver is deposited first because silver ions are more easily reduced: Award full marks if the reasoning is correct.
(b)(ii) Ionic equation: Zn(s) + 2Ag⁺(aq) → Zn²⁺(aq) + 2Ag(s) [1]
(b)(iii) Ag⁺ is more easily reduced than Cu²⁺. [1] Silver is below copper in the reactivity series, meaning silver is less reactive than copper. The less reactive a metal, the more easily its ions are reduced (the metal ion is a stronger oxidising agent). Therefore, Ag⁺ ions are more easily reduced than Cu²⁺ ions. [1]
Marking note: The explanation must link reactivity series position to ease of reduction of the metal ion. Simply stating "silver is less reactive" without explaining the consequence for reduction receives 1 mark only.
17. [6 marks]
(a) Electrode X is the anode [1] (connected to the positive terminal).
(b) Gas at electrode X: Oxygen (O₂) [1]
(c) Gas at electrode Y: Hydrogen (H₂) [1]
(d) Half-equation at electrode Y: 2H⁺(aq) + 2e⁻ → H₂(g) [1]
(e) During electrolysis of dilute sulfuric acid, water is decomposed into hydrogen and oxygen. [1] The H⁺ and OH⁻ ions come from water, and as water is removed, the concentration of H₂SO₄ (which provides the conducting ions) increases. Sulfuric acid is not consumed in the overall process — it acts as an electrolyte and catalyst. [1]
Marking note: For (e), the key point is that water is electrolysed, not sulfuric acid. The acid remains and becomes more concentrated as water is removed. Answers that say "sulfuric acid is produced" are incorrect.
18. [6 marks]
(a) The spoon should be connected to the negative terminal (cathode). [1]
(b) The other electrode should be made of silver. [1]
(c) A suitable electrolyte: Silver nitrate solution (AgNO₃) / Any soluble silver salt solution [1]
(d) Half-equation at the spoon: Ag⁺(aq) + e⁻ → Ag(s) [1]
(e) At the anode, silver metal dissolves: Ag(s) → Ag⁺(aq) + e⁻. [1] At the cathode (spoon), silver ions are deposited: Ag⁺(aq) + e⁻ → Ag(s). The rate at which silver ions are produced at the anode equals the rate at which they are deposited at the cathode, so the concentration of silver ions in the electrolyte remains approximately constant. [1]
Marking note: For (e), students must explain that the anode dissolves at the same rate as deposition at the cathode. Simply stating "silver ions are replaced" without explanation receives 1 mark only.
19. [6 marks]
(a) Oxidation number of iron in Fe₂O₃: +3 [1]
Working: 2x + 3(–2) = 0 → 2x = +6 → x = +3
(b) Oxidation number of carbon:
In CO: +2 [1] — x + (–2) = 0 → x = +2
In CO₂: +4 [1] — x + 2(–2) = 0 → x = +4
(c) Oxidising agent: Fe₂O₃ (iron(III) oxide) [1]
Explanation: Iron decreases in oxidation number from +3 in Fe₂O₃ to 0 in Fe, meaning it is reduced. The substance that is reduced is the oxidising agent. [1]
Reducing agent: CO (carbon monoxide) [1]
Explanation: Carbon increases in oxidation number from +2 in CO to +4 in CO₂, meaning it is oxidised. The substance that is oxidised is the reducing agent. [1]
Marking note: Students must identify the compound (Fe₂O₃), not just the element. "Iron" alone is not sufficient — it must be Fe₂O₃. Similarly, "carbon" alone is not sufficient for the reducing agent — it must be CO.
20. [6 marks]
(a) Decreasing reactivity: S > R > Q > P [1]
(b) From the table:
- S reacts with PSO₄, QSO₄, and RSO₄, so S is more reactive than P, Q, and R. [1]
- R reacts with PSO₄ and QSO₄ (but not SO₄), so R is more reactive than P and Q but less reactive than S. [1]
- Q reacts with PSO₄ (but not RSO₄ or SO₄), so Q is more reactive than P but less reactive than R and S.
- P does not react with any solution, so P is the least reactive. [1]
Marking note: Award 1 mark for each valid deduction. Students may present the reasoning in a different order but must clearly explain the relative reactivity of each metal using table evidence.
(c) No reaction would occur. [1] Copper is below hydrogen in the reactivity series, meaning copper is less reactive than hydrogen. A metal below hydrogen cannot displace hydrogen from dilute acids. Therefore, copper does not react with dilute hydrochloric acid. [1]
Marking note: The answer must state that no reaction occurs AND explain why using the reactivity series. Simply stating "no reaction" without explanation receives 1 mark only.
Mark Summary
| Section | Questions | Marks |
|---|---|---|
| A | 1–5 | 10 |
| B | 6–15 | 15 |
| C | 16–20 | 15 |
| Total | 20 questions | 40 marks |
Common Mistakes & Marking Notes
-
Oxidation vs. Reduction confusion: Students frequently confuse oxidation and reduction. Remind them of OIL RIG (Oxidation Is Loss, Reduction Is Gain of electrons).
-
Oxidising/reducing agent identification: Students often identify the wrong substance. Emphasise that the oxidising agent is the one that is REDUCED (it causes oxidation in another substance but is itself reduced).
-
Electrolysis products in aqueous vs. molten: Students must consider whether the electrolyte is molten or aqueous. In aqueous solutions, the concentration of ions and the electrochemical series both affect which ions are discharged.
-
Half-equations: Students should include state symbols and balance both atoms and charges. Common errors include forgetting to balance charges with electrons.
-
Reactivity series reasoning: When explaining displacement reactions, students must refer to the reactivity series explicitly — a more reactive metal displaces a less reactive metal from its compound.
-
Oxidation number calculations: Remind students that the sum of oxidation numbers in a compound equals zero, and in an ion, it equals the charge of the ion.