GCSE Combined Science (AQA)

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Half Equations

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Chemical changes Electrolysis

Half Equations

19:31 Electrodes, Ions and Half Equations
Spec 4.4.3.1 4.4.3.5 4.4.1.4
Previous pH Scale Next Process of Electrolysis
  • Half equations represent reactions at electrodes during electrolysis.
  • Electrolysis separates molten or aqueous ions.
  • Half equations show ions gaining or losing electrons to become atoms or molecules.
  • Negative ions move to the positive electrode (anode), positive ions to the negative electrode (cathode).
  • Negative ions have excess electrons; positive ions lack electrons.
  • Exchange of electrons neutralizes ion charges, forming neutral atoms.
  • Example: Electrolysis of aluminum oxide forms aluminum at the cathode.
  • Aluminum ions gain electrons at the cathode, forming aluminum atoms.
  • Half equation: Al����� + 3e��� ��� Al.
  • Reduction is gain of electrons (OIL RIG: Oxidation is loss, reduction is gain).
  • Electrons must be on the left with positive ions in half equations.
  • Non-metal forms at the anode; oxygen ions become oxygen atoms.
  • Oxygen atoms exist as diatomic molecules; need two ions for one molecule.
  • Half equation for oxygen: 2O����� ��� O��� + 4e���.
  • Oxidation is loss of electrons; oxygen ions lose electrons.
  • In aqueous solutions, if metal is more reactive than hydrogen, hydrogen forms at the cathode.
  • Water molecules break down, releasing hydrogen ions.
  • Reactivity series determines if hydrogen or metal forms at cathode.
  • Example: Zinc is more reactive than hydrogen; hydrogen gas forms at cathode.
  • Half equation for hydrogen: 2H��� + 2e��� ��� H���.
  • If metal is less reactive than hydrogen, metal forms at cathode.
  • Example: Copper is less reactive; copper forms at cathode.
  • Half equation for copper: Cu����� + 2e��� ��� Cu.
  • Halide ions produce halogen gases at anode.
  • Example: Iodide ions form iodine gas at anode.
  • Half equation for iodine: 2I��� ��� I��� + 2e���.
  • Without halide ions, oxygen and water form at anode.
  • Hydroxide ions lose electrons, forming oxygen and water.
  • Half equation for hydroxide: 4OH��� ��� O��� + 2H���O + 4e���.
  • Predict reactions and deduce half equations for exams.
  • Example: Electrolysis of molten calcium bromide.
  • Calcium ions form calcium at cathode; bromide ions form bromine at anode.
  • Half equation for bromine: 2Br��� ��� Br��� + 2e���.
  • Half equation for calcium: Ca����� + 2e��� ��� Ca.
  • Example: Electrolysis of magnesium nitrate solution.
  • Hydroxide ions form oxygen and water at anode.
  • Hydrogen ions form hydrogen gas at cathode.
  • Half equation for hydroxide: 4OH��� ��� O��� + 2H���O + 4e���.
  • Half equation for hydrogen: 2H��� + 2e��� ��� H���.

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