Secondary 4 Combined Science Chemistry Redox Electrochemistry Quiz

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Secondary 4 Combined Science Chemistry Quiz - Redox Electrochemistry

Name: ____________________
Class: ____________________
Date: ____________________
Score: ________ / 45

Duration: 60 Minutes
Total Marks: 45

Instructions:

• Answer all questions.
• For structured questions, write your answers in the spaces provided.
• Show all working for calculations.
• 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 $\text{Mg(s)} + \text{CuSO}_4\text{(aq)} \rightarrow \text{MgSO}_4\text{(aq)} + \text{Cu(s)}$, identify the substance being reduced. [1]

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3. Determine the oxidation state of the underlined element in the following compounds: [3] (a) $\underline{\text{S}}$ in $\text{SO}_2$: __________ (b) $\underline{\text{Mn}}$ in $\text{KMnO}_4$: __________ (c) $\underline{\text{Cr}}$ in $\text{Cr}_2\text{O}_3$: __________

4. State whether the following processes are oxidation or reduction: [2] (a) $\text{Fe}^{2+} \rightarrow \text{Fe}^{3+} + \text{e}^-$ : ____________________ (b) $\text{Cl}_2 + 2\text{e}^- \rightarrow 2\text{Cl}^-$ : ____________________

5. Explain why the reaction between zinc granules and copper(II) sulfate solution is described as a redox reaction. [2]

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6. Identify the oxidizing agent in the reaction: $\text{CuO(s)} + \text{H}_2\text{(g)} \rightarrow \text{Cu(s)} + \text{H}_2\text{O(l)}$. [1]

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7. Write the ionic half-equation for the oxidation of aluminum to aluminum ions. [2]

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Section B: The Reactivity Series & Displacement (Questions 8-13)

8. Which of the following metals is the strongest reducing agent: Magnesium, Iron, Copper, or Gold? [1]

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9. Predict the observation when a piece of iron nail is placed in a solution of magnesium nitrate. Explain your answer. [2] Observation: _______________________________________________________________ Explanation: _______________________________________________________________

10. A metal $\text{M}$ displaces copper from $\text{CuSO}_4$ but does not displace zinc from $\text{ZnSO}_4$. Arrange $\text{M}$, $\text{Cu}$, and $\text{Zn}$ in order of increasing reactivity. [1]

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11. Explain why copper cannot be extracted from its ore by heating with carbon. [2]

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12. Describe a chemical test to confirm the presence of $\text{Cu}^{2+}$ ions in a solution. State the observation. [2] Test: ______________________________________________________________________ Observation: _______________________________________________________________

13. Given the reactivity series, why is aluminum used to protect iron pipes from rusting (sacrificial protection)? [2]

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Section C: Electrochemistry (Questions 14-20)

14. In an electrolytic cell, define the term "anode" and state the charge of the electrode. [2] Definition: ________________________________________________________________ Charge: ___________________________________________________________________

15. During the electrolysis of molten lead(II) bromide ($\text{PbBr}_2$): [3] (a) Name the product formed at the cathode: ____________________ (b) Name the product formed at the anode: ____________________ (c) State the observation at the anode: ____________________

16. Draw a labeled diagram of a simple electrolytic cell used to refine copper. Label the anode, cathode, electrolyte, and power source. [4] (Space for diagram)

17. Explain why the anode in the electrolysis of copper(II) sulfate using copper electrodes gradually decreases in mass. [2]

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18. Compare the electrolysis of molten $\text{NaCl}$ and concentrated aqueous $\text{NaCl}$. Which gas is produced at the cathode in both cases? [2]

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19. In the electroplating of a steel spoon with silver: [3] (a) Which electrode should the steel spoon be connected to? ____________________ (b) What should be used as the anode? ____________________ (c) What should be the electrolyte? ____________________

20. Describe the process of "electrolytic refining" of a metal. Why is this process necessary for high-conductivity wires? [3]

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Answer Key - Secondary 4 Combined Science Chemistry Quiz (Redox Electrochemistry)

1. Oxidation Definition

• Loss of electrons. (1)

2. Substance Reduced

• $\text{CuSO}_4$ (or $\text{Cu}^{2+}$ ions). (1)

3. Oxidation States

• (a) $+4$ (1)
• (b) $+7$ (1)
• (c) $+3$ (1)

4. Process Identification

• (a) Oxidation (1)
• (b) Reduction (1)

5. Redox Explanation

• It involves both oxidation and reduction simultaneously. Zinc is oxidized (loses electrons) while copper(II) ions are reduced (gain electrons). (2)

6. Oxidizing Agent

• $\text{CuO}$ (Copper(II) oxide). (1)

7. Half-Equation

• $\text{Al} \rightarrow \text{Al}^{3+} + 3\text{e}^-$ (2)

8. Strongest Reducing Agent

• Magnesium (most reactive metal in the list). (1)

9. Iron in Magnesium Nitrate

• Observation: No visible change / No reaction. (1)
• Explanation: Iron is less reactive than magnesium; it cannot displace magnesium from its salt. (1)

10. Reactivity Order

• $\text{Cu} < \text{M} < \text{Zn}$ (1)

11. Copper Extraction

• Copper is lower than carbon in the reactivity series. Carbon is not a strong enough reducing agent to displace copper from its oxide/ore. (2)

12. $\text{Cu}^{2+}$ Test

• Test: Add aqueous ammonia (excess) or sodium hydroxide. (1)
• Observation: Deep blue solution (with ammonia) or blue precipitate (with $\text{NaOH}$). (1)

13. Sacrificial Protection

• Aluminum is more reactive than iron. It will be oxidized (lose electrons) in preference to iron, protecting the iron from rusting. (2)

14. Anode

• Definition: The electrode where oxidation occurs. (1)
• Charge: Positive. (1)

15. Molten $\text{PbBr}_2$

• (a) Lead (1)
• (b) Bromine (1)
• (c) Brownish-red fumes/gas. (1)

16. Diagram

• Correct labels for Anode (impure Cu), Cathode (pure Cu), Electrolyte ($\text{CuSO}_4$), and Battery/DC source. (4)

17. Anode Mass Decrease

• Copper atoms at the anode are oxidized to $\text{Cu}^{2+}$ ions ($\text{Cu} \rightarrow \text{Cu}^{2+} + 2\text{e}^-$), which dissolve into the electrolyte. (2)

18. $\text{NaCl}$ Electrolysis

• Hydrogen gas ($\text{H}_2$) is produced at the cathode in aqueous $\text{NaCl}$ (due to water reduction), whereas Sodium ($\text{Na}$) is produced in molten $\text{NaCl}$. Wait, the question asks which gas is produced in both—actually, only aqueous produces gas at cathode. Correction: In molten, Na (metal) is produced. In aqueous, $\text{H}_2$ gas is produced. (2)

19. Silver Plating

• (a) Cathode (1)
• (b) Pure silver rod (1)
• (c) Silver nitrate solution / $\text{AgNO}_3$ (1)

20. Electrolytic Refining

• Process: Impure metal is the anode, pure metal is the cathode. Metal ions move from anode to cathode. (2)
• Necessity: To remove impurities that increase electrical resistance, ensuring high conductivity for wires. (1)