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Secondary 3 Chemistry Redox Electrochemistry Quiz

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Questions

Secondary 3 Chemistry Quiz - Redox Electrochemistry

Name: __________________________
Class: __________________________
Date: __________________________
Score: ______ / 45

Duration: 45 minutes
Total Marks: 45

Instructions:

Answer all questions 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.
A copy of the Periodic Table is provided in the data booklet (assumed available).

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

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

Answer: _________________________________________________________________________

2. In the reaction below, which species acts as the reducing agent? [1] $Zn(s) + Cu^{2+}(aq) \rightarrow Zn^{2+}(aq) + Cu(s)$

A. $Zn(s)$ B. $Cu^{2+}(aq)$ C. $Zn^{2+}(aq)$ D. $Cu(s)$

Answer: _________________________________________________________________________

3. During the electrolysis of molten lead(II) bromide, what is observed at the anode? [1] A. Silvery grey liquid forms B. Red-brown vapour forms C. Bubbles of colourless gas form D. No visible change

Answer: _________________________________________________________________________

4. Which ion is preferentially discharged at the cathode during the electrolysis of dilute aqueous sodium chloride? [1] A. $Na^+$ B. $H^+$ C. $Cl^-$ D. $OH^-$

Answer: _________________________________________________________________________

5. State the oxidation state of manganese in potassium manganate(VII), $KMnO_4$ . [1]

Answer: _________________________________________________________________________

6. Identify the oxidising agent in the following reaction: [1] $2Fe^{2+} + Cl_2 \rightarrow 2Fe^{3+} + 2Cl^-$

Answer: _________________________________________________________________________

7. In a simple chemical cell made of magnesium and copper strips dipped in dilute sulfuric acid, which metal acts as the negative terminal? [1]

Answer: _________________________________________________________________________

8. Why is graphite often used as an electrode in electrolysis? [1]

Answer: _________________________________________________________________________

9. What is the product formed at the anode during the electrolysis of concentrated aqueous copper(II) chloride? [1]

Answer: _________________________________________________________________________

10. Define the term "electrolyte". [1]

Answer: _________________________________________________________________________

Section B: Structured Questions (25 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-equation for the reaction occurring at the: (i) Cathode: __________________________________________________________________ [1] (ii) Anode: ___________________________________________________________________ [1]

(d) If aqueous sodium chloride were used instead of molten sodium chloride, the products would be different. Name the product formed at the cathode in this case. [1]

Answer: _________________________________________________________________________

12. A student investigates the reactivity of three metals, P, Q, and R, by setting up simple chemical cells. The voltage recorded for each pair is shown below.

Metal Pair	Voltage (V)	Negative Terminal
P and Q	0.50	P
Q and R	0.80	Q
P and R	1.30	P

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

Answer: _________________________________________________________________________

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

(c) Predict the voltage and the negative terminal if a cell is made using metals P and a new metal S, where S is more reactive than P but less reactive than Magnesium. Assume the voltage between Mg and P is 1.0V and Mg and S is 0.4V. (i) Voltage: _______________ V [1] (ii) Negative Terminal: _______________ [1]

13. Iron(II) sulfate solution is pale green. When chlorine gas is bubbled through it, the solution turns yellow/brown.

(a) Identify the ion responsible for the yellow/brown colour. [1]

Answer: _________________________________________________________________________

(b) Write the ionic equation for the conversion of iron(II) ions to iron(III) ions. [1]

Answer: _________________________________________________________________________

(c) State whether iron(II) ions are oxidised or reduced in this reaction. Explain your answer in terms of electron transfer. [2]

(d) Suggest a chemical test to confirm the presence of iron(III) ions in the final solution, including the observation. [2]

Test: __________________________________________________________________________ Observation: ____________________________________________________________________

14. Electroplating is used to coat a steel spoon with silver.

(a) Name the material used for the: (i) Anode: __________________________ [1] (ii) Cathode: ________________________ [1]

(b) Suggest a suitable electrolyte for this process. [1]

Answer: _________________________________________________________________________

(d) Explain why the concentration of the electrolyte remains constant during the electroplating process. [2]

15. Hydrogen fuel cells are considered a clean alternative to fossil fuels.

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

Answer: _________________________________________________________________________

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

Answer: _________________________________________________________________________

(c) In a hydrogen fuel cell, hydrogen is fed to the anode and oxygen to the cathode. (i) Write the half-equation for the reaction at the anode (acidic medium). [1]

Answer: _________________________________________________________________________

(ii) Write the half-equation for the reaction at the cathode (acidic medium). [1]

Answer: _________________________________________________________________________

(d) Despite being clean, hydrogen fuel cells are not yet widely used in cars. State one major disadvantage or challenge associated with the use of hydrogen fuel cells. [1]

Answer: _________________________________________________________________________

Section C: Free Response / Application (10 Marks)

16. The extraction of aluminium from its ore, bauxite ( $Al_2O_3$ ), involves electrolysis.

(a) Explain why aluminium oxide is dissolved in molten cryolite before electrolysis. [2]

(b) The electrodes are made of graphite. Explain why the anodes need to be replaced regularly. [2]

Answer: _________________________________________________________________________

(d) Aluminium extraction is very expensive. Apart from the cost of electricity, give one other reason why aluminium is more expensive to produce than iron. [1]

Answer: _________________________________________________________________________

17. A student sets up an electrochemical cell using Zinc and Copper electrodes in their respective sulfate solutions.

(a) Which electrode acts as the anode in this voltaic cell? [1]

Answer: _________________________________________________________________________

(b) Write the ionic half-equation for the reaction occurring at the copper electrode. [1]

Answer: _________________________________________________________________________

(d) If the zinc electrode is replaced with an iron electrode, the voltage of the cell decreases. Explain what this indicates about the reactivity of iron compared to zinc. [1]

Answer: _________________________________________________________________________

18. Consider the electrolysis of dilute sulfuric acid using inert platinum electrodes.

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

Answer: _________________________________________________________________________

(b) Write the ionic half-equation for the reaction at the anode. [1]

Answer: _________________________________________________________________________

(d) What is the ratio of the volume of gas produced at the cathode to the volume of gas produced at the anode? [1]

Answer: _________________________________________________________________________

19. Rusting is an electrochemical process involving iron.

(a) State the two essential substances required for iron to rust. [2]

Answer: _________________________________________________________________________

(b) Explain how attaching a block of magnesium to an iron ship hull prevents the iron from rusting. [2]

Answer: _________________________________________________________________________

20. Displacement reactions are redox reactions.

(a) When zinc powder is added to copper(II) sulfate solution, the blue colour fades. Explain why the colour fades. [1]

Answer: _________________________________________________________________________

(b) Write the ionic equation for this reaction. [1]

Answer: _________________________________________________________________________

(d) Explain why copper metal does not react with zinc sulfate solution. [1]

Answer: _________________________________________________________________________

Answers

Secondary 3 Chemistry Quiz - Redox Electrochemistry (Answer Key)

Total Marks: 45

Section A: Multiple Choice & Short Answer

1. B [1]

Oxidation is the loss of electrons (OIL RIG).

2. A [1]

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

3. B [1]

Bromide ions ( $Br^-$ ) are oxidised to bromine ( $Br_2$ ), which is a red-brown vapour at high temperatures or liquid.

4. B [1]

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

5. +7 [1]

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

6. $Cl_2$ [1]

Chlorine gains electrons to become $Cl^-$ . The species gaining electrons is the oxidising agent.

7. Magnesium [1]

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

8. Graphite is inert (unreactive) and a good conductor of electricity. [1]

Accept: "It does not react with the products/electrolyte" or "It conducts electricity."

9. Chlorine gas [1]

In concentrated solution, $Cl^-$ is discharged in preference to $OH^-$ .

10. An electrolyte is a substance that conducts electricity when molten or dissolved in water, due to the movement of free ions. [1]

Section B: Structured Questions

11. Electrolysis of NaCl (a) Anode: Positive electrode; Cathode: Negative electrode. [1] (b) (i) Cathode: $Na^+ + e^- \rightarrow Na$ [1] (ii) Anode: $2Cl^- \rightarrow Cl_2 + 2e^-$ [1] (c)

Solid NaCl: Ions are held in a fixed lattice position and cannot move. [1]
Molten NaCl: Ions are free to move and carry charge. [1] (d) Hydrogen gas [1] Note: In aqueous NaCl, $H^+$ is discharged at the cathode.

12. Reactivity Series from Cells (a) P, Q, R [1] Logic: P & Q: Neg = P $\rightarrow$ P > Q. Q & R: Neg = Q $\rightarrow$ Q > R. P & R: Neg = P $\rightarrow$ P > R. Order: P > Q > R. (b)

Electrons flow from the more reactive metal (negative terminal) to the less reactive metal. [1]
Since P is negative against Q, and Q is negative against R, P is most reactive. [1] (c) (i) 0.60 V [1] Logic: $E_{Mg} - E_P = 1.0$ $E_{Mg} - E_S = 0.4$ Difference between S and P = $1.0 - 0.4 = 0.6V$ . (ii) S [1] (S is more reactive than P, so it is the negative terminal).

13. Redox of Iron (a) Iron(III) ion / $Fe^{3+}$ [1] (b) $Fe^{2+} \rightarrow Fe^{3+} + e^-$ [1] (c)

Oxidised. [1]
Because it loses an electron (oxidation state increases from +2 to +3). [1] (d)
Test: Add aqueous sodium hydroxide (or ammonia). [1]
Observation: Red-brown precipitate forms (insoluble in excess NaOH). [1]

14. Electroplating (a) (i) Anode: Silver [1] (ii) Cathode: Steel spoon [1] (b) Silver nitrate solution (or any soluble silver salt like $AgNO_3$ ). [1] (c) $Ag^+ + e^- \rightarrow Ag$ [1] (d)

At the anode, silver atoms oxidise to $Ag^+$ ions ( $Ag \rightarrow Ag^+ + e^-$ ), entering the solution. [1]
At the cathode, $Ag^+$ ions are reduced to silver atoms ( $Ag^+ + e^- \rightarrow Ag$ ), leaving the solution. [1]
The rate of dissolution equals the rate of deposition, keeping concentration constant.

15. Hydrogen Fuel Cells (a) $2H_2 + O_2 \rightarrow 2H_2O$ [1] (b) The only product is water (no carbon dioxide / greenhouse gases / pollutants). [1] (c) (i) Anode: $H_2 \rightarrow 2H^+ + 2e^-$ [1] (ii) Cathode: $O_2 + 4H^+ + 4e^- \rightarrow 2H_2O$ [1] (d) Any one:

Difficulty in storing hydrogen (requires high pressure/low temp). [1]
Lack of refuelling infrastructure.
High cost of production (if from non-renewable sources).
Safety concerns (flammability).

Section C: Free Response / Application

16. Aluminium Extraction (a)

To lower the melting point of aluminium oxide. [1]
This saves energy/costs as lower temperatures are required for electrolysis. [1] (b)
Oxygen is produced at the anode ( $2O^{2-} \rightarrow O_2 + 4e^-$ ). [1]
The oxygen reacts with the graphite (carbon) anode to form carbon dioxide ( $C + O_2 \rightarrow CO_2$ ), causing the anode to burn away. [1] (c) $Al^{3+} + 3e^- \rightarrow Al$ [1] (d)
Aluminium is more reactive than iron, so it cannot be extracted by reduction with carbon (coke) easily; it requires electrolysis. [1]
OR: The ore processing (Bayer process) is complex.

17. Zinc-Copper Cell (a) Zinc [1] Zinc is more reactive than copper, so it oxidises and acts as the anode. (b) $Cu^{2+} + 2e^- \rightarrow Cu$ [1] (c) From Zinc (anode) to Copper (cathode) through the wire. [1] (d) Iron is less reactive than zinc. [1] The voltage difference between Fe and Cu is smaller than between Zn and Cu.

18. Electrolysis of Dilute Sulfuric Acid (a) Oxygen [1] (b) $4OH^- \rightarrow O_2 + 2H_2O + 4e^-$ OR $2H_2O \rightarrow O_2 + 4H^+ + 4e^-$ [1] (c) Hydrogen [1] (d) 2:1 [1] 2 volumes of Hydrogen for every 1 volume of Oxygen.

19. Rusting (a) Oxygen (from air) and Water. [2] (1 mark each) (b)

Magnesium is more reactive than iron. [1]
Magnesium loses electrons more readily (oxidises) in preference to iron, protecting the iron from oxidation. [1] (c) Sacrificial protection. [1]

20. Displacement Reactions (a) The blue colour is due to $Cu^{2+}$ ions. As they are removed from the solution (reduced to Cu metal), the colour fades. [1] (b) $Zn + Cu^{2+} \rightarrow Zn^{2+} + Cu$ [1] (c) Copper(II) ion / $Cu^{2+}$ [1] (d) Copper is less reactive than zinc, so it cannot displace zinc ions from the solution. [1]