Half Equations & Ionic Equations (AQA GCSE Combined Science: Trilogy): Revision Note

Half equations & ionic equations

Higher tier only

What is a half equation?

• A half equation shows what happens to one species during a redox reaction

• Each half equation shows:

  • Atoms balanced

  • Charges balanced

  • The gain or loss of electrons

• You need two half equations to make a full redox equation

How to write a half equation

Steps to write a half equation:

1. Identify the species involved, e.g. a metal ion being reduced or a non-metal ion being oxidised

2. Write the correct formula and charge for the species before and after the change

3. Balance the atoms on both sides of the equation

4. Add electrons (e⁻) to one side to balance the charges

5. Check that both atoms and charges are balanced

Examples of half equations

• Copper(II) ions gains electrons to form copper metal

1. The species involved are:

   • Copper(II) ions as a reactant

   • Copper as a product

2. The formulas and charges are:

   • Copper(II) ions = Cu²⁺

   • Copper = Cu

3. Balancing the atoms on both sides of the equation gives:

   • Cu²⁺ → Cu

4. Two electrons are required, on the reactant side, to balance the equation:

   • Cu²⁺ + 2e^- → Cu

5. Check that the atoms and charges are balanced:

   • Atoms

     • Each side has one Cu

   • Charges

     • A 2+ charge on the copper(II) ion

     • A 1- charge on each electron = 2- charge overall

     • The 2+ and 2- charges cancel

• Bromide ions lose electrons to form bromine

1. The species involved are:

   • Bromide ions as a reactant

   • Bromine as a product

2. The formulas and charges are:

   • Bromide ions = Br^-

   • Bromine = Br₂

3. Balancing the atoms on both sides of the equation gives:

   • 2Br^- → Br₂

4. Two electrons are required to balance the equation:

   • 2Br^- - 2e^- → Br₂

   • This is more commonly written as:

   • 2Br^- → Br₂ + 2e^-

5. Check that the atoms and charges are balanced:

   • Atoms

     • Each side has two Br

   • Charges

     • Bromide ions each have a 1− charge

       • So, two bromide ions give a total charge of 2−

     • Two electrons (1− each) are released to balance this

What is an ionic equation?

• An ionic equation shows only the particles that change during a reaction

  • Two half equations can be combined to form an ionic equation

• They are useful for reactions in solution such as:

  • Neutralisation

  • Precipitation

How to write an ionic equation

Steps to write an ionic equation:

1. Write the full balanced chemical equation

2. Split aqueous compounds into ions

   • Only aqueous substances are split into ions

   • Solids, liquids, and gases stay in their molecular form

3. Write the ionic form of the equation

4. Cancel the spectator ions

   • Spectator ions do not take part in the chemical change

5. Write the final ionic equation

   • Ionic equations must be balanced for both atoms and charge

Examples of ionic equations

• The neutralisation reaction of hydrochloric acid and sodium hydroxide

1. The full balanced chemical equation is:

   • HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l)

2. The aqueous ions are:

   • HCl → H⁺ and Cl^-

   • NaOH → Na⁺ and OH^-

   • NaCl → Na⁺ and Cl^-

   • H₂O stays as a molecule

3. The ionic form of the equation is:

   • H⁺ + Cl^- + Na⁺ + OH^- → Na⁺ + Cl^- + H₂O

4. Cancelling out the spectator ions:

   • Na⁺ and Cl^- are spectator ions because they appear unchanged on both sides

   • H⁺ + Cl^- + Na⁺ + OH^- → Na⁺ + Cl^- + H₂O

5. The final ionic equation is:

   • H⁺ (aq) + OH^- (aq) → H₂O (l)

• Half equations and ionic equations feature in:

  • Metal displacement reactions

  • Electrolysis

  • Precipitation reactions

    • Testing for metal hydroxides

    • Testing for halide ions

    • Testing for sulfate ions