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Secondary 4 Combined Science Chemistry Redox Electrochemistry Quiz

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Questions

Secondary 4 Combined Science 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 Questions (Questions 1–10)

Choose the correct answer for each question. Each question carries 1 mark.

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

2. In the reaction $Zn(s) + Cu^{2+}(aq) \rightarrow Zn^{2+}(aq) + Cu(s)$ , which species acts as the reducing agent? A. $Zn(s)$ B. $Cu^{2+}(aq)$ C. $Zn^{2+}(aq)$ D. $Cu(s)$

3. What is the oxidation state of manganese in potassium manganate(VII), $KMnO_4$ ? A. +2 B. +5 C. +6 D. +7

4. During the electrolysis of molten lead(II) bromide, $PbBr_2$ , which product is formed at the anode? A. Lead metal B. Bromine gas C. Hydrogen gas D. Oxygen gas

5. Which of the following ions will be preferentially discharged at the cathode during the electrolysis of dilute aqueous sodium chloride? A. $Na^+$ B. $H^+$ C. $Cl^-$ D. $OH^-$

6. In a simple chemical cell made of magnesium and copper strips dipped in dilute sulfuric acid, which electrode is the negative terminal? A. Copper, because it is less reactive. B. Magnesium, because it loses electrons more easily. C. Copper, because it gains electrons. D. Magnesium, because it is the cathode.

7. What is the observation at the anode during the electrolysis of concentrated aqueous copper(II) chloride using inert carbon electrodes? A. A brown solid is deposited. B. A colourless gas is evolved. C. A greenish-yellow gas is evolved. D. The blue colour of the solution fades.

8. Which equation represents the reduction half-equation occurring at the cathode during the electrolysis of molten sodium chloride? A. $Na^+ + e^- \rightarrow Na$ B. $2Cl^- \rightarrow Cl_2 + 2e^-$ C. $Na \rightarrow Na^+ + e^-$ D. $Cl_2 + 2e^- \rightarrow 2Cl^-$

9. Iron rusts in the presence of water and oxygen. Which substance can prevent rusting by sacrificial protection? A. Copper B. Tin C. Zinc D. Silver

10. In the electrolysis of aqueous copper(II) sulfate using copper electrodes, what happens to the concentration of copper(II) ions in the electrolyte? A. It increases. B. It decreases. C. It remains unchanged. D. It becomes zero.

Section B: Structured Questions (Questions 11–15)

Answer all questions in the spaces provided.

11. Consider the following reaction between iron(III) oxide and carbon monoxide in a blast furnace: $Fe_2O_3(s) + 3CO(g) \rightarrow 2Fe(s) + 3CO_2(g)$

(a) Identify the substance that is oxidised. Explain your answer in terms of oxygen transfer. [2]

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

12. The diagram below shows the setup for the electrolysis of concentrated aqueous sodium chloride (brine) using inert graphite electrodes.

Image pending generation for this question.

(a) Name the gas produced at the anode. [1]

(b) Write the ionic half-equation for the reaction at the anode. [2]

(d) Explain why sodium metal is not produced at the cathode in this aqueous solution. [2]

13. A student sets up a simple chemical cell using a strip of zinc and a strip of metal X dipped in their respective sulfate solutions, connected by a salt bridge and a voltmeter. The voltmeter reads +1.10 V, and the zinc strip is the negative terminal.

(a) State the direction of electron flow in the external circuit. [1] From _______________ to _______________

(b) If metal X is copper, write the half-equation for the reaction occurring at the copper electrode. [1]

(c) Suggest what would happen to the voltmeter reading if the zinc strip is replaced by a magnesium strip. Explain your answer. [2]

14. Chlorine gas is bubbled through aqueous potassium iodide. The solution turns from colourless to brown.

(a) Write the ionic equation for this reaction. [2]

(b) Identify the reducing agent in this reaction. [1]

15. Describe the observations and write the half-equations for the electrolysis of dilute sulfuric acid using inert platinum electrodes.

(a) At the Cathode: Observation: ______________________________________________________ [1] Half-equation: ____________________________________________________ [2]

(b) At the Anode: Observation: ______________________________________________________ [1] Half-equation: ____________________________________________________ [2]

Section C: Free Response Questions (Questions 16–20)

Answer all questions in the spaces provided.

16. Electroplating is an application of electrolysis used to coat an object with a thin layer of metal. A steel spoon is to be electroplated with silver.

(a) State the material used for the anode. [1]

(b) State the electrolyte used. [1]

(d) Explain why the steel spoon must be connected to the negative terminal of the power supply. [2]

17. The extraction of aluminium from its ore, bauxite (aluminium oxide), involves electrolysis of molten aluminium oxide mixed with cryolite.

(a) Why is cryolite added to the aluminium oxide? [1]

(b) Write the half-equation for the formation of aluminium at the cathode. [1]

18. A student investigates the reactivity of metals P, Q, and R by setting up simple cells. The results are shown below:

Cell Combination	Voltage (V)	Negative Terminal
P and Q	0.50	P
Q and R	1.20	Q
P and R	1.70	P

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

(b) Explain how you determined the order of reactivity. [2]

(c) Predict the voltage and the negative terminal if a cell is made using metals R and a metal S that is more reactive than P. Assume the difference in reactivity between S and P is similar to P and Q. [2] Voltage: __________ V Negative Terminal: __________

19. Rusting is an electrochemical process.

(a) State the two essential conditions required for iron to rust. [1]

(b) In the rusting process, iron acts as the anode. Write the half-equation for the oxidation of iron. [1]

(c) Galvanising involves coating iron with zinc. Explain how zinc protects iron from rusting even if the coating is scratched. [3]

20. Consider the electrolysis of aqueous copper(II) sulfate using inert carbon electrodes.

(a) As electrolysis proceeds, the blue colour of the solution fades. Explain why. [2]

(b) Write the half-equation for the reaction at the anode. [2]

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Answers

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

Total Marks: 40

Section A: Multiple Choice Questions

1. B

Oxidation is the loss of electrons (OIL RIG).

2. A

Zn loses electrons to become $Zn^{2+}$ . The substance that loses electrons is the reducing agent.

3. D

$K (+1) + Mn (x) + 4 \times O (-2) = 0 \Rightarrow 1 + x - 8 = 0 \Rightarrow x = +7$ .

4. B

At the anode (positive), negative ions are attracted. $Br^-$ loses electrons to form $Br_2$ (bromine gas).

5. B

In aqueous solutions, $H^+$ is preferentially discharged over $Na^+$ because hydrogen is lower in the electrochemical series (easier to reduce).

6. B

Magnesium is more reactive than copper. It loses electrons more easily, becoming the negative terminal (anode in a cell).

7. C

Chloride ions ( $Cl^-$ ) are discharged at the anode in concentrated solutions to form chlorine gas ( $Cl_2$ ), which is greenish-yellow.

8. A

Reduction occurs at the cathode. $Na^+$ gains an electron to form Na metal.

9. C

Zinc is more reactive than iron. It oxidises (sacrifices) itself to protect the iron.

10. C

With copper electrodes, the anode dissolves ( $Cu \rightarrow Cu^{2+} + 2e^-$ ) at the same rate that copper deposits at the cathode ( $Cu^{2+} + 2e^- \rightarrow Cu$ ). The concentration remains constant.

Section B: Structured Questions

11. (a) Carbon monoxide (CO) [1]. It gains oxygen to form carbon dioxide [1]. (b) Iron(III) oxide ( $Fe_2O_3$ ) [1]. (c) From +3 to 0 [1]. ( $Fe$ in $Fe_2O_3$ is +3, elemental $Fe$ is 0).

12. (a) Chlorine [1]. (b) $2Cl^- \rightarrow Cl_2 + 2e^-$ [2] (1 for formulae, 1 for balancing/states). (c) Hydrogen [1]. (d) Hydrogen ions ( $H^+$ ) are lower in the electrochemical series than sodium ions ( $Na^+$ ) [1]. Therefore, $H^+$ ions are preferentially discharged/reduced at the cathode [1].

13. (a) From Zinc to Metal X (Copper) [1]. (b) $Cu^{2+} + 2e^- \rightarrow Cu$ [1]. (c) The voltage would increase [1]. Magnesium is further away from copper in the reactivity series than zinc is, resulting in a larger potential difference [1].

14. (a) $Cl_2 + 2I^- \rightarrow 2Cl^- + I_2$ [2] (1 for correct species, 1 for balancing). (b) Potassium iodide / Iodide ions ( $I^-$ ) [1]. (c) Chlorine is more reactive than iodine [1]. Chlorine accepts electrons from iodide ions (oxidising iodide to iodine) [1].

15. (a) Cathode: * Observation: Colourless gas bubbles evolved [1]. * Half-equation: $2H^+ + 2e^- \rightarrow H_2$ [2]. (b) Anode: * Observation: Colourless gas bubbles evolved [1]. * Half-equation: $4OH^- \rightarrow O_2 + 2H_2O + 4e^-$ (or $2H_2O \rightarrow O_2 + 4H^+ + 4e^-$ ) [2].

Section C: Free Response Questions

16. (a) Silver [1]. (b) Silver nitrate solution ( $AgNO_3$ ) or any soluble silver salt [1]. (c) $Ag^+ + e^- \rightarrow Ag$ [1]. (d) The spoon needs to be coated with silver. Silver ions ( $Ag^+$ ) are positive and are attracted to the negative electrode (cathode) where they gain electrons and deposit as solid silver on the spoon [2].

17. (a) To lower the melting point of aluminium oxide, saving energy/costs [1]. (b) $Al^{3+} + 3e^- \rightarrow Al$ [1]. (c) Oxygen gas is produced at the anode ( $2O^{2-} \rightarrow O_2 + 4e^-$ ) [1]. The oxygen reacts with the hot graphite (carbon) anodes to form carbon dioxide ( $C + O_2 \rightarrow CO_2$ ), causing them to burn away [1].

18. (a) P > Q > R [1]. (b) The negative terminal is the more reactive metal. P is negative against Q (P>Q). Q is negative against R (Q>R). Therefore P > Q > R [2]. (c) Voltage: 2.20 V (1.70 + 0.50) [1]. Negative Terminal: S [1]. (Since S is more reactive than P, and P is more reactive than R, S is the most reactive and will be the negative terminal).

19. (a) Water (moisture) and Oxygen (air) [1]. (b) $Fe \rightarrow Fe^{2+} + 2e^-$ [1]. (c) Zinc is more reactive than iron [1]. When scratched, zinc acts as the anode and loses electrons (oxidises) preferentially [1]. The electrons flow to the iron, preventing the iron from losing electrons and rusting [1].

20. (a) Copper(II) ions ( $Cu^{2+}$ ) are responsible for the blue colour [1]. They are discharged at the cathode to form copper metal and are not replaced at the anode (since inert electrodes are used and $OH^-$ is discharged instead) [1]. (b) $4OH^- \rightarrow O_2 + 2H_2O + 4e^-$ [2]. (c) Hydrogen [1]. (Once $Cu^{2+}$ is depleted, $H^+$ from water is discharged).