AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Oxidation and reduction in terms of electrons (HT only): write ionic equations for displacement reactions
This Higher Tier only statement requires converting a full displacement equation into an ionic equation showing only the particles that change.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This Higher Tier only statement requires converting a full displacement equation into an ionic equation showing only the particles that change. In displacement, a more reactive element takes the place of a less reactive one. For example, zinc displaces copper from copper sulfate: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s). Sulfate ions are spectator ions as they remain unchanged in solution. Cancelling them gives the ionic equation: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). To succeed, identify the ions, cancel spectator ions appearing unchanged on both sides, and write the remaining species with correct state symbols and charges. This also applies to halogens: Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq) becomes Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq).
What earns the marks
- Write the full balanced equation first, including correct formulae and state symbols, before converting to an ionic equation.
- Identify spectator ions as those that appear unchanged in form, charge and state on both sides of the equation.
- Cancel spectator ions to leave only the reacting ions and atoms in the final ionic equation.
- Balance the final ionic equation for both atoms and total charge, including coefficients where needed.
- Apply the method to both metal displacement and halogen displacement reactions.
Common mistakes and how to put them right
Including spectator ions such as sulfate or nitrate in the final ionic equation.
Put it right by checking each ion appears on both sides unchanged and removing it.
Forgetting state symbols or writing them incorrectly.
Put it right by labelling aqueous ions as (aq) and solid metals or halogens as (s) or as appropriate.
Failing to balance charge after cancelling ions.
Put it right by adding coefficients and checking that total charge is equal on both sides.
Exam tips
- Write the full equation first and underline the ions that change; this makes spectator ions obvious.
- Check that the charges on both sides of the ionic equation are equal, not just the number of atoms.
Try the lesson
The start of the Oxidation and reduction in terms of electrons (HT only): write ionic equations for displacement reactions lesson, made from checked teaching notes for this specification point.
Have a go first
Magnesium ribbon is placed in iron(II) sulfate solution and displaces the iron. Which ionic equation represents this displacement reaction?
Listen
This Higher Tier only statement requires converting a full displacement equation into an ionic equation showing only the particles that change. In displacement, a more reactive element takes the place of a less reactive one. For example, zinc displaces copper from copper sulfate: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s). Sulfate ions are spectator ions as they remain unchanged in solution. Cancelling them gives the ionic equation: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). To succeed, identify the ions, cancel spectator ions appearing unchanged on both sides, and write the remaining species with correct state symbols and charges. This also applies to halogens: Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq) becomes Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq). A strong answer earns marks for these points. Write the full balanced equation first, including correct formulae and state symbols, before converting to an ionic equation. Identify spectator ions as those that appear unchanged in form, charge and state on both sides of the equation. Cancel spectator ions to leave only the reacting ions and atoms in the final ionic equation. Balance the final ionic equation for both atoms and total charge, including coefficients where needed. A common slip: Including spectator ions such as sulfate or nitrate in the final ionic equation. Instead, checking each ion appears on both sides unchanged and removing it. Exam tip: Write the full equation first and underline the ions that change; this makes spectator ions obvious.
See it
Put it in order
Put these steps in the order they happen.
- 1. Identify the ions
- 2. Find spectator ions: unchanged in form, charge and state on both sides
- 3. Cancel spectator ions
- 4. Write remaining species with correct state symbols and charges
- 5. Balance for atoms and total charge
- 6. Write the full balanced equation with formulae and state symbols
Key words
Say what each word means before you turn the card.
Spectator ions
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Ionic equation
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Displacement
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See every definition
- Spectator ions
- Ions that appear unchanged in form, charge and state on both sides of the equation.
- Ionic equation
- An equation showing only the reacting ions and atoms, with correct state symbols and charges, after spectator ions are cancelled.
- Displacement
- A reaction in which a more reactive element takes the place of a less reactive one.
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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