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Secondary 4 Pure Chemistry Redox Electrochemistry Quiz
Free Sec 4 Pure Chemistry Redox Electrochemistry quiz, LongCat Exam version, with questions, answers, and O Level-style practice for Singapore students.
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Secondary 4 Pure Chemistry Quiz - Redox Electrochemistry
Answer Key
Section A: Multiple Choice Questions
1. B
Oxidation is defined as the loss of electrons (OIL RIG: Oxidation Is Loss, Reduction Is Gain).
2. A
Zinc (Zn) is the reducing agent because it donates electrons and is itself oxidised (oxidation state increases from 0 to +2).
3. C
K has oxidation state +1, O has −2. Let Cr = x. Then: 2(+1) + 2x + 7(−2) = 0 → 2 + 2x − 14 = 0 → 2x = 12 → x = +6.
4. C
In an electrolytic cell, the cathode is connected to the negative terminal of the battery. Reduction occurs at the cathode.
5. B
During electrolysis of molten NaCl, Cl⁻ ions are discharged at the anode: 2Cl⁻ → Cl₂ + 2e⁻. Sodium metal forms at the cathode.
6. D
Reduction involves gain of electrons. Fe³⁺ + e⁻ → Fe²⁺ (gain of 1 electron) and Fe³⁺ + 3e⁻ → Fe (gain of 3 electrons) are both valid reduction half-equations.
7. B
Magnesium is more reactive than copper, so magnesium is the negative electrode and dissolves. Hydrogen ions are reduced at the copper electrode (positive terminal), producing bubbles of H₂ gas. Electrons flow from magnesium to copper.
8. C
Zinc is more reactive than iron (higher in the electrochemical series), so it acts as a sacrificial anode, corroding preferentially and protecting the iron. While silver and gold can be used for decorative plating, zinc provides the best corrosion protection.
9. A
In MnO₄⁻: x + 4(−2) = −1 → x = +7. In Mn²⁺, the oxidation state is +2. The change is from +7 to +2.
10. B
NaOH + HCl → NaCl + H₂O is a neutralisation reaction. There is no change in oxidation state of any element. All other options involve changes in oxidation states (redox reactions).
Section B: Short Answer and Structured Questions
11.
(a) Oxidation is the loss of electrons by a substance. [1]
(b) Reduction is the gain of electrons by a substance. [1]
12.
(a) Aluminium (Al) is oxidised. [1] Its oxidation state increases from 0 (in Al) to +3 (in AlCl₃). [1]
(b) Cu²⁺(aq) + 2e⁻ → Cu(s) [1]
13.
(a) 2Cl⁻(aq) → Cl₂(g) + 2e⁻ [1]
(b) Greenish-yellow gas / bubbles of chlorine gas are produced. [1]
(c) Copper(II) ions are preferentially discharged over hydrogen ions because Cu²⁺ is lower in the electrochemical series (less reactive / has a higher position in the discharge series) than H⁺. [1] Therefore, Cu²⁺ has a greater tendency to gain electrons and be reduced at the cathode. [1]
14.
(a) Zinc will be the negative electrode. [1] Zinc is more reactive than iron (higher in the electrochemical series), so zinc loses electrons more readily and acts as the negative terminal. [1]
(b) Electrons flow from the zinc electrode to the iron electrode through the external circuit. [1]
15.
(a) The anode should be made of silver. [1]
(b) The spoon should be connected to the negative terminal (cathode). [1]
(c) The electrolyte should be a soluble silver salt solution, e.g., silver nitrate (AgNO₃) solution. [1]
16. Aluminium is more reactive than carbon (higher in the electrochemical series). [1] Therefore, carbon cannot reduce aluminium oxide. Electrolysis is required because aluminium is a reactive metal that can only be extracted by electrolysis of its molten compound. [1]
Section C: Data-Based and Calculation Questions
17.
(a) To protect the key from corrosion / rusting and/or to improve its appearance. [1]
(b) Cu(s) → Cu²⁺(aq) + 2e⁻ [1]
(c) The mass of copper deposited on the key is 0.64 g. [1] The copper ions dissolved from the anode (Cu → Cu²⁺ + 2e⁻) go into solution and are then deposited at the cathode (Cu²⁺ + 2e⁻ → Cu). Since the same number of moles of copper is transferred, the mass lost by the anode equals the mass gained by the cathode. [1]
18.
(a) S > Q > P > R [2]
Working:
- S displaces P, Q, and R → S is the most reactive.
- Q displaces P and R → Q is more reactive than P and R.
- P displaces R → P is more reactive than R.
- R displaces none → R is the least reactive.
(b) Metal R does not displace any of the other metals from their salt solutions, [1] which means R is the least reactive of the four metals.
19.
(a) Cryolite is added to lower the melting point of aluminium oxide, reducing the energy cost / making the process more economical. [1]
(b) Al³⁺(l) + 3e⁻ → Al(l) [1]
(c) The anodes are made of carbon (graphite). [1] During electrolysis, the oxygen produced at the anode reacts with the carbon anodes to form carbon dioxide (C + O₂ → CO₂), causing the anodes to gradually wear away and need regular replacement. [1]
20.
(a) 2H⁺(aq) + 2e⁻ → H₂(g) [1]
(b) 4OH⁻(aq) → O₂(g) + 2H₂O(l) + 4e⁻ [1]
(c) From the half-equations:
- Cathode: 2H⁺ + 2e⁻ → H₂ (2 moles of electrons produce 1 mole of H₂)
- Anode: 4OH⁻ → O₂ + 2H₂O + 4e⁻ (4 moles of electrons produce 1 mole of O₂)
The ratio of H₂ : O₂ = 2 : 1 by volume. [1]
If H₂ = 48 cm³, then O₂ = 48 ÷ 2 = 24 cm³. [1]
Mark Summary:
| Section | Marks |
|---|---|
| A (Q1–10) | 10 |
| B (Q11–16) | 20 |
| C (Q17–20) | 10 |
| Total | 40 |