Electrolysis
7 marks · HardA student investigates the products formed at the cathode when electrolysing sodium chloride solution and copper(II) sulfate solution, each with inert electrodes.
State the product formed at the cathode during the electrolysis of sodium chloride solution using inert electrodes.
Explain why hydrogen, not sodium, is produced at the cathode during the electrolysis of sodium chloride solution.
State the product formed at the cathode during the electrolysis of copper(II) sulfate solution using inert electrodes.
Explain why copper is deposited at the cathode during the electrolysis of copper(II) sulfate solution with inert electrodes.
In both experiments, describe one observation that could be made at the cathode, and explain how it relates to the product formed.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Hydrogen · must show Hydrogen, H2
(b) Hydrogen is less reactive than sodium and is more easily discharged at the cathode. Sodium ions remain in solution because sodium is more reactive and is harder to reduce. · must show Hydrogen discharged because less reactive, Sodium remains because more reactive
(c) Copper · must show Copper, Cu
(d) Copper is below hydrogen in the reactivity series, so copper ions (Cu²⁺) are reduced to copper metal more easily than hydrogen ions are reduced to hydrogen gas. · must show Copper below hydrogen in reactivity series, Cu2+ ions reduced at cathode
(e) Sodium chloride: bubbles (gas seen) at the cathode – hydrogen produced. Copper(II) sulfate: pink/brown layer appears on the cathode – copper deposited. · must show Gas at cathode for NaCl, Colour change (pink/brown deposit) for CuSO4
Common mistakes: Confusing products (e.g., sodium instead of hydrogen; hydrogen instead of copper); Incomplete explanations of reactivity; Lack of observation–product link
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Hydrogen (or H2) identified as cathode product for sodium chloride solution
M1 (b) · 1 mark · Correctly refers to difference in reactivity between hydrogen and sodium OR notes that sodium is more reactive and thus harder to discharge
A1 (b) · 1 mark · Explains that hydrogen ions are discharged/reduced at the cathode because it is easier and sodium ions remain in solution
B2 (c) · 1 mark · Copper (or Cu) identified as cathode product for copper(II) sulfate solution
M2 (d) · 1 mark · Refers to position of copper in the reactivity series below hydrogen
A2 (d) · 1 mark · Explains that Cu²⁺ ions are more easily reduced/discharged to copper metal than H⁺ ions
B3 (e) · 1 mark · Links correct cathode observation to product (gas for hydrogen, coloured deposit for copper)
Worked solution
(a) The product at the cathode during the electrolysis of sodium chloride solution is hydrogen.
(b) Sodium ions (Na⁺) and hydrogen ions (H⁺) are attracted to the cathode. Sodium is more reactive than hydrogen, so it is harder to reduce sodium ions than hydrogen ions. Hydrogen ions are more easily discharged at the cathode, forming hydrogen gas.
(c) The product at the cathode during the electrolysis of copper(II) sulfate solution is copper.
(d) Copper ions (Cu²⁺) are lower than hydrogen in the reactivity series. This means they are easier to reduce—Cu²⁺ ions gain electrons more readily than H⁺ ions—so copper is deposited at the cathode.
(e) For sodium chloride, bubbles (gas) appear at the cathode, showing hydrogen gas is produced. For copper(II) sulfate, a pink/brown layer forms on the cathode, showing copper metal is deposited.