AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
(HT only) write balanced half equations and ionic equations where…
Full specification wording
Atoms, elements and compounds: (HT only) write balanced half equations and ionic equations where appropriate.
Higher tier students extend equation writing to half equations and ionic equations.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 10 of 10
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Higher tier students extend equation writing to half equations and ionic equations. A half equation shows electrons gained or lost by one species, for example Mg → Mg²⁺ + 2e⁻ for oxidation or Cl₂ + 2e⁻ → 2Cl⁻ for reduction. An ionic equation shows only the ions that change, omitting spectator ions, for example Ag⁺(aq) + Cl⁻(aq) → AgCl(s) for a precipitation reaction. You balance both atoms and charge: the total charge on each side must be equal, and the number of electrons must match the charge change. These skills are used in electrolysis, displacement, redox and precipitation contexts.
What earns the marks
- Writes half equations that balance atoms and charge, including the correct number of electrons, for example 2H⁺ + 2e⁻ → H₂ or Cu → Cu²⁺ + 2e⁻.
- Identifies oxidation as loss of electrons and reduction as gain of electrons from the position of electrons in a half equation.
- Writes ionic equations by removing spectator ions from a full equation, for example Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
- Balances ionic equations for both atoms and charge, ensuring the sum of charges is the same on both sides.
- Selects the appropriate representation for the context, using half equations for electrode processes and ionic equations for reactions in solution.
- Includes state symbols in ionic equations where they clarify which species are aqueous and which form a solid, liquid or gas.
Common mistakes and how to put them right
Writing electrons on the wrong side of a half equation, which reverses the direction of electron transfer.
Put it right: Place electrons on the product side for oxidation and on the reactant side for reduction.
Failing to balance charge in an ionic equation, for example leaving unequal total charges.
Put it right: Adjust coefficients until the charge sum matches on both sides.
Including spectator ions in an ionic equation.
Put it right: Cancel ions that appear unchanged on both sides and write only the reacting species.
Exam tips
- Check charge balance separately from atom balance; a half equation can have equal atoms but unequal charge.
- Underline or list spectator ions before cancelling so you do not remove ions that actually react.
- For electrolysis half equations, decide first whether the species is oxidised or reduced, then place electrons accordingly.
Try the lesson
The start of the Atoms, elements and compounds: (HT only) write balanced half equations and ionic equations where appropriate. lesson, made from checked teaching notes for this specification point.
Have a go first
In the reaction Zn + Cu²⁺ → Zn²⁺ + Cu, which half equation correctly represents the reduction of copper ions?
Listen
Higher tier students extend equation writing to half equations and ionic equations. A half equation shows electrons gained or lost by one species, for example Mg → Mg²⁺ + 2e⁻ for oxidation or Cl₂ + 2e⁻ → 2Cl⁻ for reduction. An ionic equation shows only the ions that change, omitting spectator ions, for example Ag⁺(aq) + Cl⁻(aq) → AgCl(s) for a precipitation reaction. You balance both atoms and charge: the total charge on each side must be equal, and the number of electrons must match the charge change. These skills are used in electrolysis, displacement, redox and precipitation contexts. A strong answer earns marks for these points. Writes half equations that balance atoms and charge, including the correct number of electrons, for example 2H⁺ + 2e⁻ → H₂ or Cu → Cu²⁺ + 2e⁻. Identifies oxidation as loss of electrons and reduction as gain of electrons from the position of electrons in a half equation. Writes ionic equations by removing spectator ions from a full equation, for example Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s). Balances ionic equations for both atoms and charge, ensuring the sum of charges is the same on both sides. A common slip: Writing electrons on the wrong side of a half equation, which reverses the direction of electron transfer. Instead, place electrons on the product side for oxidation and on the reactant side for reduction. Exam tip: Check charge balance separately from atom balance; a half equation can have equal atoms but unequal charge.
See it
Key words
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Half equation
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Ionic equation
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Spectator ions
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Oxidation
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See every definition
- Half equation
- Shows electrons gained or lost by one species, for example Mg → Mg²⁺ + 2e⁻ for oxidation or Cl₂ + 2e⁻ → 2Cl⁻ for reduction.
- Ionic equation
- Shows only the ions that change, omitting spectator ions, for example Ag⁺(aq) + Cl⁻(aq) → AgCl(s) for a precipitation reaction.
- Spectator ions
- Ions that appear unchanged on both sides of a full equation and are cancelled so that only the reacting species are written.
- Oxidation
- Loss of electrons, identified from the position of electrons in a half equation.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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