AQA · GCSE · Combined Science

Course version: 8464 · Version from Sept 2018

Representation of reactions at electrodes as half equations (HT only): At the anode (positive electrode), negatively charged ions lose electrons and so the reactions are oxidations.

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Q: Is this Representation of reactions at electrodes as half equations (HT only): At the anode (positive electrode), negatively charged ions lose electrons and so the reactions are oxidations. guide aligned to the AQA specification?

Yes. This guide is linked to the Representation of reactions at electrodes as half equations (HT only): At the anode (positive electrode), negatively charged ions lose electrons and so the reactions are oxidations. topic in the AQA GCSE specification.

Electrolysis uses a direct current supply to drive non-spontaneous changes.

2 key terms

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

The anode is the positive electrode in electrolysis.
Negatively charged ions (anions) move towards the anode.
At the anode, anions lose electrons to the electrode.
Loss of electrons is oxidation, so anode reactions are oxidations.
A half equation at the anode shows electrons as products, for example 2Br⁻ → Br₂ + 2e⁻.
In aqueous solutions, hydroxide ions may be oxidised to oxygen rather than a simple anion.

Common mistakes and how to put them right

Thinking the anode is negative because negative ions go there.
Put it right: Anions are attracted to the positive anode.
Writing electrons as reactants in an anode half equation.
Put it right: Oxidation releases electrons, so they appear as products.
Confusing oxidation with gain of oxygen only.
Put it right: In electrode reactions, oxidation is loss of electrons.

Exam tips

State the electrode sign first, then name the ion and the electron change.
Check that charge and mass balance in any half equation you write.
Use the phrase 'loss of electrons is oxidation' to justify why an anode reaction is an oxidation.
Remember that writing half equations for electrode reactions is a Higher Tier only skill.

Try the lesson

The start of the Representation of reactions at electrodes as half equations (HT only): At the anode (positive electrode), negatively charged ions lose electrons and so the reactions are oxidations. lesson, made from checked teaching notes for this specification point.

Have a go first

In electrolysis, which type of reaction happens at the anode (positive electrode)?

Tip: press 1 to 4 to choose

Listen

Electrolysis uses a direct current supply to drive non-spontaneous changes. The anode is the positive electrode, so negatively charged ions (anions) are attracted towards it. When an anion reaches the anode, it transfers one or more electrons to the electrode. Loss of electrons is oxidation, so every anode reaction is an oxidation. For example, in molten lead(II) bromide, bromide ions are oxidised: 2Br⁻ → Br₂ + 2e⁻. In aqueous solutions, hydroxide ions may be oxidised instead, producing oxygen: 4OH⁻ → O₂ + 2H₂O + 4e⁻. The electrons flow through the external circuit from anode to cathode. Note that representing these reactions as half equations is assessed at Higher Tier only. A strong answer earns marks for these points. The anode is the positive electrode in electrolysis. Negatively charged ions (anions) move towards the anode. At the anode, anions lose electrons to the electrode. Loss of electrons is oxidation, so anode reactions are oxidations. A common slip: Thinking the anode is negative because negative ions go there. Instead, anions are attracted to the positive anode. Exam tip: State the electrode sign first, then name the ion and the electron change.

See it

A top-down flowchart. The first box reads 'Anode: positive electrode' and points to 'Negatively charged ions (anions) attracted towards anode'. This points to 'Anion reaches the anode', which points to 'Anion loses electrons to the electrode'. That box points to two things. The first is 'Loss of electrons is oxidation', which points to 'Anode reactions are oxidations'. The second is 'Half equation shows electrons as products', which points to two example boxes: 'Example: 2Br⁻ → Br₂ + 2e⁻' and 'In aqueous solutions, hydroxide ions may be oxidised: 4OH⁻ → O₂ + 2H₂O + 4e⁻'. — It shows how anions are attracted to the positive anode, lose electrons there, and so give an oxidation shown as a half equation with electrons as products.

Key words

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Electrolysis

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Half equations

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See every definition

Electrolysis: A process that uses a direct current supply to drive non-spontaneous changes.
Half equations: A representation of reactions at electrodes, showing electrons as products at the anode, assessed at Higher Tier only.

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The first box reads 'Anode: positive electrode' and points to 'Negatively charged ions (anions) attracted towards anode'. This points to 'Anion reaches the anode', which points to 'Anion loses electrons to the electrode'. That box points to two things. The first is 'Loss of electrons is oxidation', which points to 'Anode reactions are oxidations'. The second is 'Half equation shows electrons as products', which points to two example boxes: 'Example: 2Br⁻ → Br₂ + 2e⁻' and 'In aqueous solutions, hydroxide ions may be oxidised: 4OH⁻ → O₂ + 2H₂O + 4e⁻'.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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