Electrolysis of aqueous solutions: At the positive electrode (anode), oxygen is produced unless the solution contains halide ions when the halogen is produced.

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At the positive electrode (anode), negative ions lose electrons (oxidation).

2 key terms

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

Identify the positive electrode as the anode, where negative ions lose electrons (oxidation).
List the negative ions present, including OH⁻ from water and any halide ions from the dissolved compound.
State that oxygen is produced when no halide ions are present, from discharge of OH⁻ ions.
State that the halogen is produced when halide ions are present, for example chlorine from Cl⁻.
Write a correct anode half-equation, such as 4OH⁻ → O₂ + 2H₂O + 4e⁻ or 2Cl⁻ → Cl₂ + 2e⁻.
Describe a suitable test or observation to identify the product, such as relighting a glowing splint for oxygen or bleaching damp litmus for chlorine.

Common mistakes and how to put them right

Assuming oxygen is always produced at the anode.
Put it right by checking for halide ions first, because halide ions are discharged in preference to OH⁻.
Confusing the anode with the cathode.
Put it right by recalling that the anode is the positive electrode and attracts negative ions.
Writing chlorine as Cl rather than Cl₂.
Put it right by showing that two chloride ions lose electrons to form one chlorine molecule.

Exam tips

Name the ions present before predicting the product, so your reasoning is clear and checkable.
Quote the rule directly: oxygen unless halide ions are present, then the halogen.
Include the state symbol or observation where possible, for example 'chlorine gas bleaches damp litmus paper'.

Try the lesson

The start of the Electrolysis of aqueous solutions: At the positive electrode (anode), oxygen is produced unless the solution contains halide ions when the halogen is produced. lesson, made from checked teaching notes for this specification point.

Have a go first

Aqueous sodium sulfate is electrolysed. Which gas is produced at the positive electrode (anode)?

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Listen

At the positive electrode (anode), negative ions lose electrons (oxidation). Aqueous solutions contain OH⁻ ions from water and possibly halide ions such as Cl⁻, Br⁻ or I⁻. If no halide ions are present, OH⁻ ions are discharged and oxygen gas is produced: 4OH⁻ → O₂ + 2H₂O + 4e⁻. If halide ions are present, the halogen is produced instead, for example 2Cl⁻ → Cl₂ + 2e⁻. So sodium chloride solution gives chlorine at the anode, while sodium sulfate solution gives oxygen. Observations help: chlorine bleaches damp litmus paper, while oxygen relights a glowing splint. A strong answer earns marks for these points. Identify the positive electrode as the anode, where negative ions lose electrons (oxidation). List the negative ions present, including OH⁻ from water and any halide ions from the dissolved compound. State that oxygen is produced when no halide ions are present, from discharge of OH⁻ ions. State that the halogen is produced when halide ions are present, for example chlorine from Cl⁻. A common slip: Assuming oxygen is always produced at the anode. Instead, checking for halide ions first, because halide ions are discharged in preference to OH⁻. Exam tip: Name the ions present before predicting the product, so your reasoning is clear and checkable.

Key words

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Anode

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Halide ions

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See every definition

Anode: The positive electrode, where negative ions lose electrons (oxidation).
Halide ions: Negative ions such as Cl⁻, Br⁻ or I⁻; when present, the halogen is produced instead of oxygen.

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