Half Equations & Ionic Equations (AQA GCSE Chemistry) : Revision Note

Stewart Hird

Last updated

Half equations & ionic equations

Higher tier only

How to write a half equation

  • Half equations and ionic equations are specific types of equations for showing some of the fine details going on in chemical reactions

  • Half equations are used to show what happens to the electrons in reactions where atoms, molecules or ions are gaining or losing electrons

  • They are called half equations, because they represent only half of what is happening in a reaction involving electron transfer

    • One species gains electrons

    • Another species loses electrons

  • Examples of half equations are:

Pb2+ + 2e- → Pb

2Br-  → Br2 + 2e-

How to write an ionic equation

  • Ionic equations are used to indicate what happens to ions during reactions

  • They help to simplify complicated processes where many substances are present, but only certain ions are actually reacting with each other

  • For example, we can use ionic equations to show what happens when an acid neutralises and an alkali:

HCl + NaOH  → NaCl + H2O

  • Written out as an ionic equation would be

H+ + OH- → H2O

  • This is because sodium and chloride ions were present at the beginning and also at the end of the reaction, so they are unchanged

  • Ions which are present but do not take part in reactions are called spectator ions

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Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.

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