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CIE iGCSE Co-Ordinated Science C4.1 Electrolysis Exam Style Questions Paper 1

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Question

Concentrated aqueous sodium chloride is electrolysed using inert electrodes.

Which row shows the products at the electrodes?

▶️ Answer/Explanation

In the electrolysis of concentrated aqueous sodium chloride using inert electrodes:

  • At the cathode (negative electrode): Hydrogen ions (\( \text{H}^+ \)) from the water are preferentially discharged over sodium ions (\( \text{Na}^+ \)) because hydrogen is less reactive than sodium. The hydrogen ions gain electrons to form hydrogen gas:
    \( 2\text{H}^+ + 2e^- \rightarrow \text{H}_2 \)
  • At the anode (positive electrode): Chloride ions (\( \text{Cl}^- \)) are preferentially discharged over hydroxide ions (\( \text{OH}^- \)) because the solution is concentrated. The chloride ions lose electrons to form chlorine gas:
    \( 2\text{Cl}^- \rightarrow \text{Cl}_2 + 2e^- \)

Therefore, the products are chlorine at the anode and hydrogen at the cathode, which matches row B.

Sodium is not produced because sodium ions remain in solution (sodium is too reactive to be discharged in aqueous solution).

✅ Answer: (B)

Question

The apparatus used in the electrolysis of concentrated aqueous sodium chloride is shown.

Which row identifies the electrolyte and the cathode?

▶️ Answer/Explanation
The electrolyte is the solution that undergoes electrolysis, which is the concentrated aqueous sodium chloride (4). The cathode is the negative electrode, connected to the negative terminal of the battery (2). Therefore, row C correctly identifies 4 as the electrolyte and 2 as the cathode.
✅ Answer: (C)

Question

Molten lead(II) bromide is electrolysed using inert electrodes. Which row describes the products of this electrolysis?

▶️ Answer/Explanation
In the electrolysis of molten lead(II) bromide, lead ions (\(\mathrm{Pb^{2+}}\)) are attracted to the cathode (negative electrode) where they gain electrons to form grey lead metal.
Bromide ions (\(\mathrm{Br^{-}}\)) are attracted to the anode (positive electrode) where they lose electrons to form red-brown bromine gas.
Therefore, lead forms at the negative electrode (not positive) and bromine forms at the positive electrode (not negative). The only products are lead and bromine.
✅ Answer: (C)

Question

The apparatus for the electrolysis of solution R is shown.

What are the names of P, Q and R?

▶️ Answer/Explanation
In electrolysis, the cathode is the negative electrode where reduction occurs, and the anode is the positive electrode where oxidation occurs. The electrolyte is the ionic compound (in molten or aqueous solution) that conducts electricity and undergoes decomposition. In the diagram, P is connected to the negative terminal of the battery (cathode), Q is connected to the positive terminal (anode), and R is the solution being electrolysed (electrolyte). Therefore, P = cathode, Q = anode, R = electrolyte.
✅ Answer: (D)

Question

The electrolysis of concentrated aqueous sodium chloride is shown.

Which statement describes the product at the cathode?

A. It is a colourless gas that pops with a lighted splint.
B. It is a colourless gas that relights a glowing splint.
C. It is a grey-black solid.
D. It is a pale yellow-green gas that bleaches litmus paper.

▶️ Answer/Explanation
At the cathode (negative electrode), hydrogen ions (\(\mathrm{H^+}\)) from water are discharged in preference to sodium ions because hydrogen is less reactive. Hydrogen gas is produced, which is colourless and gives a ‘pop’ with a lighted splint. Oxygen would relight a glowing splint, chlorine is a pale yellow-green gas, and a grey-black solid is not formed here.
✅ Answer: (A)
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