Knowledge Organiser: Displacement Reactions
Key Terms
- Displacement: more reactive replaces less reactive
- OIL RIG: Oxidation Is Loss, Reduction Is Gain
- Spectator ion: unchanged throughout the reaction
- Ionic equation: spectator ions removed
- Exothermic: displacement reactions release heat
Must-Know Facts
- More reactive metal always displaces less reactive
- Less reactive added = no reaction
- Displacing metal is oxidised (loses electrons)
- Displaced metal ion is reduced (gains electrons)
- Mg + CuSO₄ → MgSO₄ + Cu (key example)
- Colour change: blue CuSO₄ → colourless; brown Cu forms
- Halogen displacement: Cl₂ > Br₂ > I₂
Key Equations
- Mg + CuSO₄ → MgSO₄ + Cu (metal displacement)
- Fe + CuSO₄ → FeSO₄ + Cu (iron displaces copper — second key example)
- Cu + MgSO₄ → NO REACTION (Cu less reactive than Mg)
- Mg → Mg²⁺ + 2e⁻ (oxidation half-equation)
- Cu²⁺ + 2e⁻ → Cu (reduction half-equation)
Common Mistakes
- Getting OIL RIG backwards: The displacing metal is OXIDISED (loses electrons); the metal ion being displaced is REDUCED (gains electrons)
- Saying a less reactive metal can displace a more reactive one: Displacement only works one way — more reactive always displaces less reactive
- Forgetting to balance ionic equations: Check that charge is balanced on both sides as well as atoms
- Missing spectator ions in ionic equations: Spectator ions (e.g. SO₄²⁻) do not appear in the ionic equation — only the species that change
Memory Aid
- The displacement rule: "More reactive metal KICKS OUT less reactive." Think of a school queue: the bigger (more reactive) student always gets to the front.
- OIL RIG: The most famous chemistry mnemonic — write it on the top of your exam paper as soon as you turn it over. Oxidation Is Loss. Reduction Is Gain. The metal going IN (displacing) is oxidised; the metal coming OUT is reduced.
- Colour change memory: "CuSO₄ is BLUE, Cu metal is BROWN." Blue disappears, brown appears — a clear sign displacement has occurred. Iron(III) compounds are orange/rust coloured; iron(II) is pale green.
- Spectator ion tip: Spectators WATCH but don't PLAY. They appear in the same form on both sides of the equation. Cancel them out to get the ionic equation.