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2.3.4Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols

Ionic Equations — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 5 parts Lesson 24 of 53 Unit quiz

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Part 1 of 5Half-Equations

2.3.4 Ionic Equations

A half-equation shows the electrons lost or gained by one species only.

Oxidation
  • Electrons appear on the right-hand side, for example Fe²⁺ → Fe³⁺ + e⁻ and 2Br⁻ → Br₂ + 2e⁻.
Reduction
  • Electrons appear on the left-hand side, for example Cl₂ + 2e⁻ → 2Cl⁻ and Zn²⁺ + 2e⁻ → Zn.
Acidified oxyanions
  • Species such as MnO₄⁻ and Cr₂O₇²⁻ need water and hydrogen ions to balance oxygen and hydrogen.
Check
  • Both atoms and total charge must balance on each side of every half-equation.

Part 2 of 5Building and Combining a Redox Equation

Follow the same order each time so that no step is missed.

1
Write the species that changes and its product, balancing the atom being oxidised or reduced first.
2
Add H₂O to balance oxygen atoms, then add H⁺ to balance the hydrogen atoms in an acidic solution.
3
Add electrons to the more positive side so that the total charges on both sides are equal.
4
Multiply the half-equations so each contains the same number of electrons, then add them and cancel the electrons.
5
Cancel any species, such as H⁺ or H₂O, that appears on both sides, then recheck atoms and charge.

Part 3 of 5Ionic Equations and Spectator Ions

Ionic Equations and Spectator Ions
Ionic equation
  • It shows only the ions or molecules that actually change, written with state symbols.
Spectator ions
  • Ions that are present unchanged on both sides are left out, such as Na⁺ in a precipitation reaction.
Precipitation
  • Ag⁺(aq) + Cl⁻(aq) → AgCl(s) represents silver nitrate with any soluble chloride.
Neutralisation
  • H⁺(aq) + OH⁻(aq) → H₂O(l) represents any strong acid reacting with any strong alkali.

Part 4 of 5Worked Example

Question Write the ionic equation for dichromate(VI) ions oxidising iodide ions in acid, giving chromium(III) ions and iodine.

  1. ReductionCr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O.
  2. Oxidation2I⁻ → I₂ + 2e⁻, which is multiplied by 3 to give 6I⁻ → 3I₂ + 6e⁻.
  3. Add the equations; the 6e⁻ cancel: Cr₂O₇²⁻ + 14H⁺ + 6I⁻ → 2Cr³⁺ + 7H₂O + 3I₂.
  4. Charge check
    • left = -2 + 14 - 6 = +6;
      • right = 2 × 3 = +6.

Answer Cr₂O₇²⁻(aq) + 14H⁺(aq) + 6I⁻(aq) → 2Cr³⁺(aq) + 7H₂O(l) + 3I₂(aq).

Part 5 of 5Exam Checklist & Tips

2.3.4 – Key Points for the Exam

  • Half-equation An equation showing only the oxidation or only the reduction part of a redox reaction.
  • Ionic equation An equation showing only the particles that take part in the reaction.
  • Spectator ion An ion that is present but unchanged during a reaction.
  • Acidified Containing excess H⁺ ions, needed for many oxyanion half-equations.

Common Mistakes

  • Leaving electrons in the final overall equation instead of cancelling them.
  • Adding H⁺ and H₂O in the wrong order, which leaves oxygen or hydrogen unbalanced.
  • Forgetting to multiply the half-equations to match electron numbers before adding them.
  • Writing a spectator ion in the ionic equation or omitting state symbols.

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