AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Representation of reactions at electrodes as half equations (HT only): 4OH⁻ → O₂ + 2H₂O + 4e⁻
This Higher Tier half equation describes oxidation at the anode during the electrolysis of aqueous solutions when hydroxide ions (OH⁻) are discharged.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This Higher Tier half equation describes oxidation at the anode during the electrolysis of aqueous solutions when hydroxide ions (OH⁻) are discharged. Four hydroxide ions lose one electron each, giving four electrons in total, and rearrange to form one oxygen molecule (O₂) plus two water molecules (2H₂O). The equation is balanced for atoms: four oxygen and four hydrogen atoms appear on both sides. It is also balanced for charge: the left side has four negative charges, while the right side has four electrons, each with a 1⁻ charge, so both sides total 4⁻. Because electrons are released, this is oxidation, occurring at the positive anode. Always count atoms and total charge separately to ensure the half equation is complete.
What earns the marks
- Recognises that OH⁻ ions are oxidised at the anode, releasing electrons and forming O₂ and H₂O.
- Balances oxygen and hydrogen atoms so that 4OH⁻ gives O₂ and 2H₂O.
- Balances charge by including 4e⁻ on the product side, giving a total charge of 4⁻ on each side.
- States that loss of electrons is oxidation and that this half equation represents the anode reaction.
- Uses the equation to explain why oxygen gas is observed at the positive electrode in aqueous electrolysis.
Common mistakes and how to put them right
Writing 2OH⁻ → O₂ + H₂O + 2e⁻.
Put it right: This is not atom-balanced because the oxygen and hydrogen counts do not match; correct it to 4OH⁻ → O₂ + 2H₂O + 4e⁻.
Placing electrons on the left, as in 4OH⁻ + 4e⁻ → O₂ + 2H₂O.
Put it right: This would represent reduction; move the electrons to the product side because hydroxide ions lose electrons.
Omitting the state or charge symbols, such as writing OH or e instead of OH⁻ and e⁻.
Put it right: Include the negative charge on hydroxide ions and on electrons so the charge balance is clear.
Exam tips
- Check atom balance first, then charge balance; a Higher Tier half equation must satisfy both.
- Link the side on which electrons appear to oxidation or reduction: electrons on the right means oxidation at the anode.
Try the lesson
The start of the Representation of reactions at electrodes as half equations (HT only): 4OH⁻ → O₂ + 2H₂O + 4e⁻ lesson, made from checked teaching notes for this specification point.
Have a go first
In the half equation for the discharge of hydroxide ions, 4OH⁻ → O₂ + 2H₂O + 4e⁻, what does the '4e⁻' represent?
Listen
This Higher Tier half equation describes oxidation at the anode during the electrolysis of aqueous solutions when hydroxide ions (OH⁻) are discharged. Four hydroxide ions lose one electron each, giving four electrons in total, and rearrange to form one oxygen molecule (O₂) plus two water molecules (2H₂O). The equation is balanced for atoms: four oxygen and four hydrogen atoms appear on both sides. It is also balanced for charge: the left side has four negative charges, while the right side has four electrons, each with a 1⁻ charge, so both sides total 4⁻. Because electrons are released, this is oxidation, occurring at the positive anode. Always count atoms and total charge separately to ensure the half equation is complete. A strong answer earns marks for these points. Recognises that OH⁻ ions are oxidised at the anode, releasing electrons and forming O₂ and H₂O. Balances oxygen and hydrogen atoms so that 4OH⁻ gives O₂ and 2H₂O. Balances charge by including 4e⁻ on the product side, giving a total charge of 4⁻ on each side. States that loss of electrons is oxidation and that this half equation represents the anode reaction. Exam tip: Check atom balance first, then charge balance; a Higher Tier half equation must satisfy both.
Key words
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Oxidation
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Half equation
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Anode
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Hydroxide ions
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See every definition
- Oxidation
- Loss of electrons, occurring at the positive anode, as when hydroxide ions release electrons in this half equation.
- Half equation
- A representation of the reaction at an electrode that must be balanced for both atoms and total charge.
- Anode
- The positive electrode at which this reaction occurs, where hydroxide ions are oxidised and oxygen gas is observed.
- Hydroxide ions
- Ions with the formula OH⁻ that lose one electron each and rearrange to form oxygen and water.
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