Secondary 4 Combined Science Chemistry Redox Electrochemistry Quiz

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Secondary 4 Combined Science Chemistry Quiz - Redox Electrochemistry

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Duration: 60 minutes
Total Marks: 40
Instructions: Answer all 20 questions. Section A is short answer, Section B is structured response, Section C is data/interpretation. Show all working where calculation is required.

Section A: Short Answer (Questions 1–5)

1. State the meaning of the term oxidation in terms of electrons. [1]

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2. State the meaning of the term reduction in terms of electrons. [1]

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3. Write the colour of the cathode in a simple copper sulfate electrolysis cell using copper electrodes. [1]

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4. Name the gas produced at the anode during electrolysis of aqueous sodium chloride using inert electrodes. [1]

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5. State one use of electroplating in everyday life. [1]

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Section B: Structured Response (Questions 6–15)

6. (a) Define redox reaction. [1]
(b) State what happens to the oxidation state of an element that is oxidised. [1]

(a) _______________________________________________________
(b) _______________________________________________________

7. (a) Write the ionic half-equation for the discharge of copper(II) ions at the cathode. [1]
(b) Write the ionic half-equation for the discharge of bromide ions at the anode. [1]

(a) _______________________________________________________
(b) _______________________________________________________

8. A student places a clean iron nail into copper(II) sulfate solution.
(a) State the observation on the iron nail. [1]
(b) Explain why this reaction occurs using the reactivity series. [2]

(a) _______________________________________________________
(b) _______________________________________________________

9. (a) State the products at the anode and cathode during electrolysis of molten lead(II) bromide using inert electrodes. [2]
(b) Write the equation for the overall reaction. [1]

(a) Anode: ______________ Cathode: ______________
(b) _______________________________________________________

10. (a) State two conditions necessary for rusting of iron to occur. [2]
(b) Name one method to prevent rusting. [1]

(a) _______________________________________________________
(b) _______________________________________________________

11. In the electrolysis of acidified water using platinum electrodes:
(a) State the gas produced at the cathode and a test for it. [2]
(b) State the gas produced at the anode and a test for it. [2]

(a) _______________________________________________________
(b) _______________________________________________________

12. (a) Explain why zinc is used to galvanise iron. [2]
(b) State what happens to the zinc when the coated iron is scratched. [1]

(a) _______________________________________________________
(b) _______________________________________________________

13. A cell is set up with zinc and copper electrodes in contact with their own ions.
(a) State which metal is the anode. [1]
(b) State the direction of electron flow in the external wire. [1]
(c) Write the overall cell reaction. [1]

(a) _______________________________________________________
(b) _______________________________________________________
(c) _______________________________________________________

14. (a) State the colour change of litmus paper in chlorine gas produced at anode during electrolysis of aqueous sodium chloride. [1]
(b) Explain why this change occurs. [2]

(a) _______________________________________________________
(b) _______________________________________________________

15. (a) Calculate the charge in coulombs passed when a current of $0.50\ \text{A}$ flows for $200\ \text{s}$. [2]
(b) State the formula linking charge, current and time. [1]

(a) _______________________________________________________
(b) _______________________________________________________

Section C: Data and Interpretation (Questions 16–20)

16. The reactivity series (most to least reactive) is: Potassium, Sodium, Calcium, Magnesium, Aluminium, Zinc, Iron, Tin, Lead, Copper, Silver, Gold.
(a) Which of iron or copper is more easily oxidised? [1]
(b) Write a word equation for the reaction when iron is placed in copper(II) nitrate solution. [2]

(a) _______________________________________________________
(b) _______________________________________________________

17.

Image pending generation: experimental_setup for Q17.

Using the diagram, state:
(a) the colour of the solution after some time. [1]
(b) the product at the cathode. [1]
(c) the product at the anode. [1]

(a) _______________________________________________________
(b) _______________________________________________________
(c) _______________________________________________________

18. A battery is connected to two inert electrodes in potassium iodide solution.
(a) State the product at the anode and its observation. [2]
(b) State the product at the cathode. [1]

(a) _______________________________________________________
(b) _______________________________________________________

19. The table shows results when metal X and metal Y are placed in solutions of their ions.

Metal in solution of	X ions	Y ions
X strip	No change	X dissolves
Y strip	No change	No change

(a) State which metal is more reactive. [1]
(b) Explain your answer using redox ideas. [2]

(a) _______________________________________________________
(b) _______________________________________________________

20. (a) State the oxidation state of Mn in $KMnO_4$. [2]
(b) State the oxidation state of Cr in $K_2Cr_2O_7$. [2]

(a) _______________________________________________________
(b) _______________________________________________________

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Secondary 4 Combined Science Chemistry Quiz - Redox Electrochemistry: Answer Key

Total Marks: 40
Topic: Redox Electrochemistry

Section A

1. Oxidation is loss of electrons (OIL). [1]
Teaching note: Recall OIL RIG: Oxidation Is Loss, Reduction Is Gain of electrons.

2. Reduction is gain of electrons (RIG). [1]
Teaching note: Opposite of Q1; electrons are gained by the species reduced.

3. Copper (pink/brown) – cathode remains copper coloured or gains copper deposit. [1]
Teaching note: In CuSO₄ with Cu electrodes, Cu²⁺ deposits as Cu metal on cathode, same colour.

4. Chlorine gas, $Cl_2$. [1]
Teaching note: In aqueous NaCl with inert electrodes, Cl⁻ is discharged preferentially over OH⁻ in concentrated solution forming Cl₂.

5. Any one: protect objects from corrosion (e.g. chrome plating on taps), decorative coating (silver plating jewellery), reduce friction. [1]
Teaching note: Electroplating uses electrolysis to coat a metal object with a thin layer of another metal.

Section B

6. (a) Redox reaction is a reaction where both reduction and oxidation occur simultaneously. [1]
(b) Its oxidation state increases. [1]
Teaching note: Oxidation state tracks electron loss; loss of electrons → number becomes more positive.

7. (a) $Cu^{2+} + 2e^- \rightarrow Cu$ [1]
(b) $2Br^- \rightarrow Br_2 + 2e^-$ [1]
Teaching note: Cations go to cathode and gain electrons; anions go to anode and lose electrons.

8. (a) Iron nail becomes coated with a reddish-brown (copper) layer. [1]
(b) Iron is above copper in reactivity series, so iron is more reactive and displaces Cu²⁺ from solution: $Fe + Cu^{2+} \rightarrow Fe^{2+} + Cu$. Iron is oxidised. [2]
Marking: 1 for reactivity reason, 1 for displacement/oxidation explanation.

9. (a) Anode: bromine ($Br_2$); Cathode: lead ($Pb$). [2]
(b) $Pb^{2+} + 2Br^- \rightarrow Pb + Br_2$ or molten $PbBr_2 \rightarrow Pb + Br_2$. [1]

10. (a) Presence of oxygen (air) and water (moisture). [2]
(b) Painting, greasing, galvanising, alloying, sacrificial protection. [1]

11. (a) Cathode: hydrogen ($H_2$); test: lighted splint gives pop sound. [2]
(b) Anode: oxygen ($O_2$); test: glowing splint relights. [2]

12. (a) Zinc is more reactive than iron, so it acts as sacrificial anode; oxidised first, protecting iron. [2]
(b) Zinc is oxidised (corrodes) instead of iron. [1]

13. (a) Zinc. [1]
(b) From zinc (anode) to copper (cathode). [1]
(c) $Zn + Cu^{2+} \rightarrow Zn^{2+} + Cu$. [1]

14. (a) Litmus turns red (then may be bleached white). [1]
(b) Chlorine reacts with water to form HCl and HOCl (acidic), turning litmus red; bleaching by HOCl. [2]

15. (a) $Q = I \times t = 0.50 \times 200 = 100\ \text{C}$. [2]
(b) $Q = I t$. [1]

Section C

16. (a) Iron. [1]
(b) Iron + copper(II) nitrate → iron(II) nitrate + copper. [2]

17. (a) Blue colour fades (becomes paler). [1]
(b) Copper metal ($Cu$). [1]
(c) Oxygen ($O_2$) from water (carbon anode inert, OH⁻ discharged). [1]
Image note: Diagram must show carbon electrodes, blue CuSO₄, battery; correct polarity confirms products.

18. (a) Iodine ($I_2$); brown solution / black solid at anode. [2]
(b) Hydrogen ($H_2$). [1]

19. (a) Y is more reactive. [1]
(b) Y strip dissolves in X ions, meaning Y is oxidised and X²⁺ reduced; Y loses electrons more readily. [2]

20. (a) K = +1, O = −2; let Mn = x: $+1 + x + 4(-2) = 0 \Rightarrow x = +7$. [2]
(b) K = +1, O = −2; $2(+1) + 2y + 7(-2) = 0 \Rightarrow 2y = 12 \Rightarrow y = +6$. [2]