Half equations in electrolysis

• Positively charged ions gain electrons at the negative cathode

  The gain of electrons is called reduction
• Negatively charged ions lose electrons at the anode

  The loss of electrons is called oxidation
• Oxidation is the loss of electrons
• Reduction is the gain of electrons
• • In the electrolysis of molten lead(II) bromide the half equation at the negative electrode (cathode) is:

  Pb2+ + 2e– ⟶ Pb
• • At the positive electrode (anode) bromine gas is produced by the discharge of bromide ions:

  2Br- ⟶ Br2 + 2e-
• • Aluminium ions are discharged at the negative electrode (cathode) and the aluminium is collected at the bottom of the cell:

  Al3 + 3e- ⟶ Al
• • At the positive electrode (anode) oxygen gas is produced:

  2O2- ⟶ O2 + 4e-
• At the negative electrode (cathode), when the metal is more reactive than hydrogen, hydrogen is discharged and the half equation is:

  2H + 2e-⟶ H2 


  When the metal is less reactive than hydrogen, the metal is discharged, e.g.

  Cu2 + 2e- ⟶ Cu   
• At the positive electrode (anode), if a halide ion is present, the corresponding halogen is formed e.g.

  2Cl– ⟶ Cl2 + 2e-

  When a halide ion is not present, oxygen is formed as hydroxide ions are discharged, e.g.

  4OH– ⟶ O2 + 2H2O + 4e-