Edexcel IGCSE Chemistry · Spec 1.58C-1.60C
Electrolysis Experiments and Half-Equations (Paper 2)
Covers recall the products for four named electrolytes, Write balanced electrode half-equations and Describe a safe, reproducible aqueous electrolysis practical.
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Electrolysis Experiments and Half-Equations (Paper 2)
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Explained
Predicting the products, and writing the half-equations
Electrolysis splits an ionic compound using electricity, and it only works when the ions can move, which means the compound must be molten or dissolved.
Positive ions are attracted to the negative electrode, the cathode, where they gain electrons. Negative ions go to the positive electrode, the anode, where they lose electrons. Reduction happens at the cathode and oxidation at the anode, every time.
Molten compounds
With a molten compound there are only two ions present, so the products are simply the metal at the cathode and the non metal at the anode.
Molten lead bromide gives lead at the cathode and bromine at the anode. Molten aluminium oxide gives aluminium and oxygen.
Aqueous solutions
Water complicates this, because it supplies hydrogen ions and hydroxide ions as well, so there is a competition at each electrode.
At the cathode, the less reactive of the metal and hydrogen is produced. So copper sulfate solution gives copper, because copper is below hydrogen, while sodium chloride solution gives hydrogen, because sodium is above it.
At the anode, a halide gives the halogen if it is concentrated, and otherwise oxygen is produced from the hydroxide ions. So concentrated sodium chloride solution gives chlorine, while dilute sodium sulfate solution gives oxygen.
Half-equations
A half-equation shows what happens at one electrode, with the electrons written in.
At the cathode the ion gains electrons, so they appear on the left: copper ions plus two electrons give copper. At the anode the ion loses electrons, so they appear on the right: two chloride ions give chlorine plus two electrons.
Balance the charges as well as the atoms. If the charges on the two sides do not match, the number of electrons is wrong.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme asks what happens to chloride ions at the anode. It credits that chloride ions lose electrons and allows the passive phrasing that electrons are lost. Then it rejects chlorine, Cl or Cl2 losing electrons.
The distinction is exact. Chloride ions are what is present in the electrolyte and what loses the electrons; chlorine is what is produced afterwards. Naming the product as the thing that reacted describes the process backwards.
Elsewhere the same series is equally strict about the state. Asked how sodium is extracted, a mark scheme credits electrolysis, then adds: reject electrolysis of an aqueous solution. It also makes the second mark, that sodium is more reactive than carbon, depend on the first.
Aqueous is wrong because in solution the water would be discharged in preference to a reactive metal, so no sodium would form at all. Molten is the word that makes the answer correct.
On a related redox question the mark scheme wants both halves stated together, that one substance loses electrons and the other gains them. Oxidation and reduction always happen as a pair, and naming only one of them is half an answer.
A safe aqueous practical
Use a small volume of solution in a beaker with two carbon electrodes held apart by a lid, connected to a low voltage d.c. supply. Collect any gases in inverted test tubes over the electrodes.
Test the products to identify them: a lighted splint pops for hydrogen, a glowing splint relights for oxygen, and damp litmus paper is bleached white by chlorine.
The safety points are worth stating. Chlorine is toxic, so use a fume cupboard or a very dilute solution and a short run. Keep the voltage low, and use carbon electrodes because they are inert and will not react with the products.
Spec 1.58C-1.60C
What you need to know
- Recall the products for four named electrolytes
- Write balanced electrode half-equations
- Describe a safe, reproducible aqueous electrolysis practical
Active recall
Quick check
Answer each question before opening the answer.
What forms at the cathode in copper sulfate solution?
Copper, with inert electrodes
Write the hydrogen cathode half-equation
2H+ + 2e- -> H2
Why are chlorine experiments ventilated?
Chlorine is toxic and must not be inhaled
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