Part VII Redox Reactions, Chemical Cells and Electrolysis

Which of the following is a redox reaction?

• A. 2FeSO₄(aq) + 2H₂SO₄(l) → Fe₂(SO₄)₃(aq) + SO₂(g) + 2H₂O(l)
• B. Mg(NO₃)₂(aq) + 2NaOH(aq) → Mg(OH)₂(s) + 2NaNO₃(aq)
• C. Na₂Cr₂O₇(aq) + 2NaOH(aq) → 2Na₂CrO₄(aq) + H₂O(l)
• D. CaCO₃(s) + CO₂(g) + H₂O(l) → Ca(HCO₃)₂(aq)

Refer to the set-up. It is found that the voltage of the cell drops rapidly when the cell has operated for some time. This is because

• A. many gas bubbles form on the surface of the copper electrode.

• B. the concentration of the copper(II) sulphate solution decreases.

• C. a layer of copper forms on the surface of the magnesium electrode.

• D. an oxide layer forms on the surface of the magnesium electrode.

Consider the half equation below:

SO₄²⁻(aq) + xH⁺(aq) + ye⁻ → HS⁻(aq) + zH₂O(l)

Which of the following combinations about x, y and z is correct?

• A

• B

• C

• D

Consider the set-up. Which of the following statements concerning the set-up is/are correct?

(1) Zinc ions move into the copper(II) sulphate solution through the porous device.

(2) Gas bubbles form at the copper container.

(3) An electric current flows from the zinc strip to the copper container in the external circuit.

• A. (1) only

• B. (2) only

• C. (1) and (3) only

• D. (2) and (3) only

Which of the following solutions, upon mixed with an aqueous solution of sulphur dioxide respectively, would produce a colour change?

(1) Br₂(aq)

(2) Fe(NO₃)₃(aq)

(3) K₂Cr₂O₇(aq)/H⁺(aq)

• A. (1) and (2) only

• B. (1) and (3) only

• C. (2) and (3) only

• D. (1), (2) and (3)

Which of the following statements about 1.5 V zinc carbon cells is correct?

• A. They have a button shape.

• B. They can work over a wide range of temperatures.

• C. The negative electrode is zinc.

• D. The electrolyte is potassium hydroxide.

Gas M is bubbled into solution N as shown in the diagram. Which of the following combinations about M and N will give an observable change?

• A

• B

• C

• D

The following reduction half equations are extracted from the Electrochemical Series (E.C.S.).

X₂(aq) + 2e⁻ → 2X⁻(aq)

Y³⁺(aq) + e⁻ → Y²⁺(aq)

Z₂(aq) + 2e⁻ → 2Z⁻(aq)

(Note: these equations are arranged according to their order in the E.C.S.)

With reference to the above equations, which of the following reactions is/are feasible?

(1) 2Z⁻(aq) + 2Y³⁺(aq) → Z₂(aq) + 2Y²⁺(aq)

(2) Z₂(aq) + 2X⁻(aq) → X₂(aq) + 2Z⁻(aq)

(3) X₂(aq) + 2Y²⁺(aq) → 2X⁻(aq) + 2Y³⁺(aq)

• A. (1) only

• B. (2) only

• C. (1) and (3) only

• D. (2) and (3) only

The diagram shows a set-up for electroplating copper metal onto an iron key. Which of the following statements about this electroplating is/are correct?

(1) The colour intensity of the electrolytic solution remains unchanged during electroplating.

(2) The iron key is connected to the positive pole of the battery.

(3) Copper(II) nitrate solution can be used as the electrolyte in place of copper(II) sulphate solution.

• A. (1) only

• B. (2) only

• C. (1) and (3) only

• D. (2) and (3) only

The diagram shows a simple chemical cell connected to a voltmeter. Which of the following statements concerning this cell is/are correct?

(1) Electrode Q is the anode.

(2) Oxidation occurs at electrode P.

(3) Electrode P becomes bigger and thicker gradually.

• A. (1) only

• B. (2) only

• C. (1) and (3) only

• D. (2) and (3) only