Electrolysis (Singapore N(A)-Level Science Chemistry 5107): breaking down molten and aqueous compounds with electricity, predicting the products at each electrode, and the everyday uses of electrolysis

What this topic is really about

Electrolysis is the use of electricity to break a compound down into simpler substances. It is the reverse of a spontaneous reaction: instead of energy coming out, electrical energy is put in to force the ions apart. The whole topic rests on one idea, that free-moving ions carry charge to the electrodes, where they gain or lose electrons. This guide ties the three dot points together and links to each one for the worked answers and practice.

The complete set of dot-point pages for this topic lives at /sg-n-level/chemistry/syllabus/electrolysis.

Electrolysis of molten compounds

Electrolysis of molten compounds is the simplest case, because only the ions of the compound are present. A molten binary ionic compound splits into its metal at the cathode and its non-metal at the anode. The compound must be molten so the ions are free to move.

Electrolysis of aqueous solutions

Electrolysis of aqueous solutions adds a complication: water supplies hydrogen and hydroxide ions, so there is competition at each electrode. At the cathode, the less reactive of the metal and hydrogen is released, so a reactive metal stays in solution and hydrogen comes off instead. At the anode, a concentrated halide solution gives the halogen, while a sulfate or nitrate solution gives oxygen.

The uses of electrolysis

Uses of electrolysis shows why this matters in industry: extracting very reactive metals such as aluminium from their molten ores, purifying metals such as copper, and electroplating objects with a thin protective or decorative metal coat. In electroplating, the object is the cathode and the plating metal is the anode.

How this topic is examined

• State the electrode rules every time. Cathode negative, reduction; anode positive, oxidation. Examiners reward the correct half-equation and the matching observation.
• Handle the aqueous competition. For solutions, decide the cathode product from reactivity and the anode product from whether the solution is a concentrated halide or a sulfate/nitrate.
• Describe electroplating as a set-up. Object as cathode, plating metal as anode, salt solution of the plating metal as electrolyte.

Check your knowledge

A mix of recall, reasoning and prediction questions covering Electrolysis. Attempt them under timed conditions, then check against the solutions.

1. Explain why a solid ionic compound cannot be electrolysed but a molten one can. (2 marks)
2. Name the products at the cathode and anode when molten sodium chloride is electrolysed. (2 marks)
3. Write the cathode half-equation for the electrolysis of molten lead(II) bromide. (1 mark)
4. State the product at the cathode when dilute copper(II) sulfate solution is electrolysed, and explain why. (2 marks)
5. Describe how an iron spoon can be electroplated with silver. (3 marks)