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O Level Chemistry Redox Electrochemistry Quiz

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Questions

O-Level Chemistry Quiz - Redox Electrochemistry

Name: ________________________
Class: ________________________
Date: ________________________
Score: ______ / 40

Duration: 45 minutes
Total Marks: 40

Instructions:

Answer all questions.
Write your answers in the spaces provided.
The number of marks is given in brackets [ ] at the end of each question or part question.
You may use a calculator.

Section A: Multiple Choice & Short Concepts (10 Marks)

1. Which statement correctly describes oxidation in terms of electron transfer?
[1]
A. Gain of electrons
B. Loss of electrons
C. Gain of hydrogen
D. Loss of oxygen

2. In the electrolysis of molten lead(II) bromide, which product is formed at the anode?
[1]
A. Lead metal
B. Bromine gas
C. Hydrogen gas
D. Oxygen gas

3. Which of the following ions is discharged preferentially at the cathode during the electrolysis of dilute aqueous sodium chloride?
[1]
A. Na⁺
B. H⁺
C. Cl⁻
D. OH⁻

4. What is the oxidation state of chromium in potassium dichromate, K₂Cr₂O₇?
[1]
A. +3
B. +6
C. +7
D. +12

5. In a simple cell consisting of magnesium and copper electrodes in dilute sulfuric acid, which metal acts as the negative terminal?
[1]
A. Copper, because it is more reactive.
B. Magnesium, because it is more reactive.
C. Copper, because it loses electrons.
D. Magnesium, because it gains electrons.

6. Identify the reducing agent in the following reaction:
$\text{ZnO} + \text{C} \rightarrow \text{Zn} + \text{CO}$
[1]
A. ZnO
B. C
C. Zn
D. CO

7. Which observation confirms that a substance is an oxidising agent when tested with aqueous potassium iodide?
[1]
A. The solution turns from colourless to brown.
B. The solution turns from brown to colourless.
C. A white precipitate is formed.
D. Effervescence is observed.

8. During the electrolysis of concentrated aqueous copper(II) chloride using inert electrodes, what is formed at the anode?
[1]
A. Oxygen
B. Chlorine
C. Copper
D. Hydrogen

9. Which equation represents the half-equation for the reduction of copper(II) ions?
[1]
A. $\text{Cu} \rightarrow \text{Cu}^{2+} + 2e^-$
B. $\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}$
C. $\text{Cu}^{2+} \rightarrow \text{Cu} + 2e^-$
D. $\text{Cu} + 2e^- \rightarrow \text{Cu}^{2+}$

10. Why is graphite commonly used as an electrode in electrolysis?
[1]
A. It is a good insulator.
B. It is unreactive and conducts electricity.
C. It reacts with the electrolyte to form products.
D. It has a low melting point.

Section B: Structured Questions (20 Marks)

11. The diagram below shows the setup for the electrolysis of molten sodium chloride.

(Imagine a diagram showing a crucible with molten NaCl, two graphite electrodes connected to a DC power supply.)

(a) Label the anode and the cathode on the diagram.
[1]

(b) Write the ionic half-equations for the reactions occurring at each electrode.
Anode: _________________________________________________________ [1]
Cathode: ______________________________________________________ [1]

(d) State one industrial use of the product formed at the anode in part (b).
[1]

12. A student investigates the reactivity of metals P, Q, and R by setting up simple cells. The voltmeter readings are recorded below. The electrolyte used is dilute sulfuric acid.

Cell Combination	Voltage (V)	Direction of Electron Flow
P and Q	0.5	P to Q
Q and R	1.2	R to Q
P and R	1.7	R to P

(a) Arrange metals P, Q, and R in order of decreasing reactivity (most reactive first).
[1]
Order: ________________________

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

(d) Predict the voltage and direction of electron flow if a cell is made using Magnesium and Copper. (Given: Mg is much more reactive than Cu).
Voltage: __________ V (approximate relative magnitude compared to 1.7V)
Direction: ________________ to ________________
[2]

13. Hydrogen fuel cells are considered a cleaner alternative to internal combustion engines.

(a) Write the overall chemical equation for the reaction in a hydrogen fuel cell.
[1]

(b) State one advantage of using hydrogen fuel cells over petrol engines.
[1]

(d) In a hydrogen fuel cell, hydrogen is oxidised at the anode. Write the half-equation for this reaction in an acidic electrolyte.
[1]

14. Iron(II) sulfate solution is added to acidified potassium manganate(VII) solution. The purple colour of the manganate(VII) disappears.

(a) What type of reagent is iron(II) sulfate in this reaction?
[1]

(b) Explain the colour change in terms of electron transfer.
[2]

15. Copper purification is achieved through electrolysis. The impure copper is used as the anode, and pure copper is used as the cathode. The electrolyte is aqueous copper(II) sulfate.

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

(b) Explain what happens to the mass of the anode and the cathode during the process.
[2]
Anode: ______________________________________________________________
Cathode: _____________________________________________________________

(c) Impurities such as silver and gold fall to the bottom of the cell as "anode sludge". Explain why these metals do not dissolve into the solution.
[2]

(d) A student attempts to electroplate an iron key with silver.
(i) What should be used as the anode?
[1]

(ii) What electrolyte should be used?
[1]

Section C: Free Response & Application (10 Marks)

16. Electroplating is a common application of electrolysis.

(a) Explain why the object to be plated must be connected to the negative terminal.
[1]

(b) Suggest one reason why the object is rotated during electroplating.
[1]

17. Consider the reaction between zinc and copper(II) sulfate solution: $\text{Zn}(s) + \text{CuSO}_4(aq) \rightarrow \text{ZnSO}_4(aq) + \text{Cu}(s)$

(a) Identify the oxidising agent in this reaction.
[1]

(b) Write the ionic half-equation for the reduction process.
[1]

18. A student sets up an electrolytic cell using aqueous copper(II) sulfate and carbon electrodes.

(a) Name the product formed at the anode.
[1]

(b) Describe the observation at the cathode.
[1]

19. Define the term "electrolyte" in the context of electrolysis.
[2]

20. State two conditions necessary for electrolysis to occur.
[2]

Answers

O-Level Chemistry Quiz - Redox Electrochemistry (Answer Key)

Total Marks: 40

Section A: Multiple Choice & Short Concepts

1. B
Loss of electrons is oxidation (OIL RIG). [1]

2. B
Bromide ions ( $Br^-$ ) are oxidised to bromine ( $Br_2$ ) at the anode. [1]

3. B
In dilute aqueous solutions, $H^+$ is preferentially discharged over $Na^+$ because hydrogen is lower in the reactivity series. [1]

4. B
K is +1, O is -2. $2(+1) + 2(Cr) + 7(-2) = 0 \Rightarrow 2 + 2Cr - 14 = 0 \Rightarrow 2Cr = 12 \Rightarrow Cr = +6$ . [1]

5. B
Magnesium is more reactive than copper, so it loses electrons more readily and acts as the negative terminal (anode in a cell). [1]

6. B
Carbon gains oxygen (or loses electrons effectively in the redox sense relative to Zn), causing ZnO to reduce. Carbon is the reducing agent. [1]

7. A
Oxidising agents oxidise iodide ions ( $I^-$ ) to iodine ( $I_2$ ), which is brown in solution. [1]

8. B
In concentrated solutions, halide ions ( $Cl^-$ ) are preferentially discharged over hydroxide ions ( $OH^-$ ) at the anode. [1]

9. B
Reduction is gain of electrons. $Cu^{2+}$ gains 2 electrons to form Cu. [1]

10. B
Graphite is inert (does not react easily) and is a good conductor of electricity. [1]

Section B: Structured Questions

11.
(a) Anode: Positive electrode; Cathode: Negative electrode. [1]
(b) Anode: $2Cl^- \rightarrow Cl_2 + 2e^-$ [1]
Cathode: $Na^+ + e^- \rightarrow Na$ [1]
(c) In aqueous solution, $H^+$ ions from water are present. $H^+$ is lower in the reactivity series than $Na^+$ , so $H^+$ is preferentially discharged at the cathode to form hydrogen gas. [2]
(d) Bleaching agent / Disinfectant / Manufacture of PVC / Hydrochloric acid. [1]

12.
(a) R > P > Q [1]
(b) R is the most reactive as it pushes electrons to both P and Q (highest tendency to lose electrons). Q is the least reactive as it accepts electrons from both. [2]
(c) Zinc / Iron / Lead (Any metal less reactive than R but more reactive than Cu, or simply a metal above H in reactivity series if R is very reactive like Mg/Zn). Accept any metal more reactive than Copper. [1]
(d) Voltage: Greater than 1.7 V (since Mg is more reactive than R likely is, or simply "High"). Direction: Magnesium to Copper. [2]

13.
(a) $2H_2 + O_2 \rightarrow 2H_2O$ [1]
(b) Only water is produced (no greenhouse gases / no pollutants). [1]
(c) Hydrogen is difficult to store / Explosive / Expensive to produce / Lack of infrastructure. [1]
(d) $H_2 \rightarrow 2H^+ + 2e^-$ [1]

14.
(a) Reducing agent. [1]
(b) Iron(II) ions lose electrons (are oxidised) to Iron(III). Manganate(VII) ions gain electrons (are reduced) to Manganese(II), causing the purple colour to fade/disappear. [2]
(c) $Fe^{2+} \rightarrow Fe^{3+} + e^-$ [1]

15.
(a) $Cu^{2+} + 2e^- \rightarrow Cu$ [1]
(b) Anode: Mass decreases (copper dissolves). [1]
Cathode: Mass increases (copper is deposited). [1]
(c) Silver and gold are less reactive than copper (lower in the reactivity series). They do not lose electrons easily to form ions, so they remain as solid metals and fall off. [2]
(d) (i) Silver metal / Ag. [1]
(ii) Silver nitrate solution / $AgNO_3(aq)$ . [1]

Section C: Free Response & Application

16.
(a) To attract positive metal ions (cations) to the object, where they gain electrons and are reduced to metal atoms. [1]
(b) To ensure an even coating / uniform thickness of the plated metal. [1]

17.
(a) Copper(II) sulfate / Copper(II) ions / $Cu^{2+}$ . [1]
(b) $Cu^{2+} + 2e^- \rightarrow Cu$ [1]

18.
(a) Oxygen gas. [1]
(b) A reddish-brown solid (copper) is deposited. [1]

19.
A substance that conducts electricity when molten or dissolved in water, and is decomposed by the electric current. [2]
(1 mark for conducting electricity in molten/aqueous state, 1 mark for being decomposed)

20.

The electrolyte must be molten or in aqueous solution (ions must be free to move). [1]
A direct current (DC) power supply must be used. [1]