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Electrolysis: Question 3

Syllabus 4.1, 4.2

Structured Core 7 marks

A school chemistry club investigates two uses of electrolysis.

In the first investigation, the club electrolyses concentrated aqueous sodium chloride solution using two inert graphite electrodes connected to a d.c. supply.

(a) Identify the gas produced at the anode, and describe a test that would confirm its identity. [2]

(b) Identify the gas produced at the cathode. [1]

(c) Name the compound that remains dissolved in the solution once electrolysis is complete. [1]

In the second investigation, the club's teacher demonstrates how to electroplate a brass belt buckle with a thin, even layer of nickel.

(d) Describe how the buckle could be electroplated with nickel, referring to the electrodes used, the electrolyte, and how the buckle is connected in the circuit. [2]

(e) State one reason why manufacturers electroplate objects such as this buckle with nickel. [1]

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Worked solution

Part (a): Gas at the anode

Concentrated aqueous sodium chloride contains a high concentration of chloride ions, Cl−\text{Cl}^-, alongside a much lower concentration of hydroxide ions from water. At the positive anode, chloride ions are discharged preferentially, releasing chlorine gas.

Test: hold damp blue litmus paper in the gas. Chlorine bleaches it white.

Part (b): Gas at the cathode

The solution also contains sodium ions, Na+\text{Na}^+, and hydrogen ions from water. Sodium is far more reactive than hydrogen, so at the negative cathode, hydrogen ions are discharged instead, releasing hydrogen gas (sodium ions remain in solution).

Part (c): Compound remaining in solution

Since chloride ions are removed as chlorine gas and hydrogen ions are removed as hydrogen gas, the sodium ions and the hydroxide ions left behind combine to leave sodium hydroxide dissolved in the solution.

Part (d): Electroplating the buckle with nickel

To electroplate the buckle:

  • Connect the brass buckle as the cathode (negative electrode).
  • Connect a strip of pure nickel as the anode (positive electrode).
  • Use an electrolyte solution containing nickel ions, such as aqueous nickel(II) sulfate.

When a direct current passes through the cell, nickel atoms dissolve from the anode into the solution as Ni2+\text{Ni}^{2+} ions, while nickel ions already in solution are deposited as solid nickel onto the buckle at the cathode, building up an even coating.

Part (e): Reason for electroplating

Electroplating with nickel gives the buckle a shinier, more attractive appearance, and/or protects the brass underneath from corrosion.

Final answers

  • (a) Chlorine; damp blue litmus paper is bleached white.
  • (b) Hydrogen.
  • (c) Sodium hydroxide.
  • (d) Buckle = cathode, pure nickel = anode, electrolyte contains nickel ions; nickel dissolves at the anode and deposits on the buckle at the cathode.
  • (e) Improves appearance and/or resists corrosion.