Secondary 4 Pure Chemistry Redox Electrochemistry Quiz

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Secondary 4 Pure Chemistry Quiz - Redox Electrochemistry

Name: ____________________
Class: ____________________
Date: ____________________
Score: ________ / 50

Duration: 60 Minutes
Total Marks: 50

Instructions:

• Answer all questions in the spaces provided.
• Show all working for calculations.
• State symbols must be included in chemical equations where required.
• Use a ruler for any diagrams.

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Section A: Fundamentals of Redox (Questions 1–7)

1. Define "oxidation" in terms of electron transfer. [1]
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2. In the reaction: $\text{CuO(s)} + \text{H}_2\text{(g)} \rightarrow \text{Cu(s)} + \text{H}_2\text{O(l)}$, identify the reducing agent. [1]
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3. Assign oxidation numbers to the underlined elements in the following compounds: [2] (a) $\underline{\text{Mn}}\text{O}_4^-$ : ___________ (b) $\text{K}_2\underline{\text{Cr}}_2\text{O}_7$ : ___________

4. A student adds potassium manganate(VII) solution to a solution containing $\text{Fe}^{2+}$ ions. (a) State the color change observed. [1] \

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   (b) Explain this observation in terms of redox. [2]
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5. Write the ionic half-equation for the oxidation of $\text{Zn}$ to $\text{Zn}^{2+}$. [1]
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6. Which of the following is a strong oxidizing agent? [1] A) $\text{Na}$ B) $\text{Mg}$ C) $\text{Cl}_2$ D) $\text{I}_2$ \

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7. Explain why the reaction between $\text{Mg}$ and $\text{CuSO}_4$ is a redox reaction. [2]
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Section B: Electrolysis (Questions 8–14)

8. State the function of the electrolyte in an electrolytic cell. [1]
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9. During the electrolysis of molten lead(II) bromide ($\text{PbBr}_2$): (a) Name the product formed at the cathode. [1] \

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   (b) Describe the observation at the anode. [1] \

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10. In the electrolysis of concentrated aqueous sodium chloride: (a) Which gas is evolved at the anode? [1] \

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    (b) Write the half-equation for the reaction occurring at the cathode. [2]
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11. Explain why hydrogen gas is produced at the cathode instead of sodium metal during the electrolysis of dilute aqueous $\text{NaCl}$. [2]
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12. A piece of graphite is used as an electrode in the electrolysis of aqueous $\text{CuSO}_4$. (a) Why is graphite preferred over a reactive metal like iron? [1] \

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    (b) State the observation at the anode. [1] \

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13. Describe the process of electroplating a copper spoon with silver. Include the identity of the anode, cathode, and electrolyte. [3]
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14. In the purification of copper, an impure copper anode is used. Explain why the anode decreases in mass over time. [2]
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Section C: Chemical Cells & Fuel Cells (Questions 15–20)

15. In a simple electrochemical cell consisting of a zinc strip and a copper strip in their respective sulfate solutions: (a) Which metal acts as the anode? [1] \

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    (b) In which direction do electrons flow through the external wire? [1] \

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16. Write the half-equation for the reduction of $\text{Cu}^{2+}$ ions at the cathode in a $\text{Zn-Cu}$ cell. [1]
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17. Explain the role of a salt bridge in a galvanic cell. [2]
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18. A hydrogen fuel cell is used to power a vehicle. (a) State the two reactants used in the fuel cell. [1] \

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    (b) State the only product formed by the reaction. [1] \

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19. Write the overall balanced equation for the reaction occurring in a hydrogen fuel cell. [2]
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20. Compare a hydrogen fuel cell with a traditional combustion engine in terms of environmental sustainability. [3]
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Answer Key - Redox Electrochemistry Quiz

1. Oxidation is the loss of electrons. (1m)

2. $\text{H}_2$ (or Hydrogen). (1m) - It removes oxygen from $\text{CuO}$.

3. (a) $+7$ (1m) (b) $+6$ (1m)

4. (a) Purple to colorless. (1m) (b) $\text{MnO}_4^-$ is reduced to $\text{Mn}^{2+}$ (colorless) while $\text{Fe}^{2+}$ is oxidized to $\text{Fe}^{3+}$. (2m)

5. $\text{Zn(s)} \rightarrow \text{Zn}^{2+}\text{(aq)} + 2\text{e}^-$ (1m)

6. C) $\text{Cl}_2$ (1m)

7. $\text{Mg}$ is oxidized (loses electrons) and $\text{Cu}^{2+}$ is reduced (gains electrons). (2m)

8. To allow the movement of ions to complete the circuit / conduct electricity. (1m)

9. (a) Lead (Pb). (1m) (b) Red-brown fumes of bromine gas evolved. (1m)

10. (a) Chlorine gas ($\text{Cl}_2$). (1m) (b) $2\text{H}^+\text{(aq)} + 2\text{e}^- \rightarrow \text{H}_2\text{(g)}$. (2m)

11. $\text{H}^+$ ions are lower in the reactivity series (easier to reduce) than $\text{Na}^+$ ions, so $\text{H}^+$ is selectively discharged. (2m)

12. (a) Graphite is inert and will not react with the electrolyte/products. (1m) (b) Bubbles of oxygen gas evolved. (1m)

13.

• Anode: Pure silver ($\text{Ag}$) electrode. (1m)
• Cathode: Copper spoon. (1m)
• Electrolyte: Silver nitrate ($\text{AgNO}_3$) solution. (1m)

14. Impure copper atoms at the anode are oxidized to $\text{Cu}^{2+}$ ions and enter the solution. (2m)

15. (a) Zinc ($\text{Zn}$). (1m) (b) From Zinc (anode) to Copper (cathode). (1m)

16. $\text{Cu}^{2+}\text{(aq)} + 2\text{e}^- \rightarrow \text{Cu(s)}$ (1m)

17. It maintains electrical neutrality by allowing ions to migrate between the two half-cells. (2m)

18. (a) Hydrogen ($\text{H}_2$) and Oxygen ($\text{O}_2$). (1m) (b) Water ($\text{H}_2\text{O}$). (1m)

19. $2\text{H}_2\text{(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{H}_2\text{O(l)}$ (2m)

20.

• Fuel cells produce only water, whereas combustion engines produce $\text{CO}_2$ and $\text{NO}_x$ (greenhouse gases/pollutants). (2m)
• Fuel cells are more efficient/cleaner. (1m)