Ionic Half Equations (Cambridge International IGCSE Science)

Spec point — C4.1.8 · Construct ionic half-equations for reactions at the anode (to show oxidation) and at the cathode (to show reduction)

Ionic half equations

Extended tier only

• Electrochemistry is concerned with the transfer of electrons

  • This is why the definitions of oxidation and reduction are in terms of losing or gaining electrons, not oxygen
  • Oxidation is the loss of electrons
  • Reduction is the gain of electrons

• As the ions come into contact with the electrode:

  • Electrons are lost or gained
  • The ions form neutral s ubstances
  • These substances are discharged as products at the electrodes

• At the anode, negatively charged ions lose electrons

  • So, oxidation occurs at the anode

• At the cathode, the positively charged ions gain electrons

  • So, reduction occurs at the cathode

• Ionic half-equations only show half of what is happening in a reaction involving electron transfer

  • The ionic half-equation for oxidation shows the loss of electrons
  • The ionic half-equation for reduction shows the gain of electrons

• Ionic half-equations must have the atoms and charges balanced

Writing ionic half-equations

Metals

• Metals are positive ions, e.g. Li⁺, Cu²⁺, Al³⁺

• If a metal is produced during electrolysis:

  • The metal ions will gain electrons to form metal atoms
  • The metal ions will be reduced

• The charge on the metal ion indicates the number of electrons that will be gained

  • For example:

Li⁺+ e^–→ Li

Cu²⁺+ 2e^–→ Cu

Al³⁺+ 3e^–→ Al

Non-metals

• Non-metals are typically negative ions, e.g. Cl^–, Br^–, OH^–

  • The hydrogen ion, H⁺, is an exception to this

Hydrogen ion / hydrogen half-equation:

• If hydrogen is formed during electrolysis:

  • Two hydrogen ions will gain 2 electrons to form hydrogen, H₂
  • Hydrogen ions are reduced

2H⁺+ 2e^–→ H₂

Halide ion / halogen half-equation:

• If a halogen is formed during electrolysis:

  • Two halide ions will lose 2 electrons to form the halogen
  • Halide ions are oxidised

2X^–→ X₂+ 2e^–

Hydroxide ion / oxygen half-equation:

• The ionic half-equation for hydroixde ions forming oxygen is more challenging to balance

  • Hydroxide ions lose electrons to form oxygen, O₂, and water, H₂O
  • Hydroxide ions are oxidised

4OH^–→ O₂+ 2H₂O + 4e^–

Table of reduction and oxidation reactions at the electrodes

Exam Hint

• To help you remember the definitions of oxidation and reduction, use OIL RIG

  • Oxidation Is Loss (of electrons) Reduction Is Gain (of electrons)

• To help you remember where oxidation and reduction take place, use a RED CAT and AN OX

  • REDuction at the CAThode
  • ANode for OXidation