Electrolysis of solutions

Ions in water

Pure water can conduct electricity because a small proportion of its molecules dissociate into ions. The two ions formed in water are, hydrogen ions, H+, and hydroxide ions, OH-.

During the electrolysis of water:

  • H+ ions are attracted to the cathode, gain electrons and form hydrogen gas
  • OH- ions are attracted to the anode, lose electrons and form oxygen gas

The overall balanced equation for the process is:

2H2O(l) → 2H2(g) + O2(g)

The volume of hydrogen given off is twice the volume of oxygen given off.

Electrolysis of dissolved ionic compounds

An electrolyte formed by dissolving an ionic compound contains two pairs of negative and positive ions:

  • positive hydrogen ions from the water, and positive ions from the compound
  • negative hydroxide ions from the water, and negative ions from the compound

The ions compete at each electrode to gain or lose electrons.

At the cathode

Whether hydrogen or a metal is produced at the cathode depends on the position of the metal in the metal reactivity series:

  • the metal is produced at the cathode if it is less reactive than hydrogen
  • hydrogen is produced at the cathode if the metal is more reactive than hydrogen
The relative reactivity of selected elements from most to least: potassium, sodium, calcium, magnesium, aluminium, carbon, zinc, iron, tin, lead, hydrogen, copper, silver, gold, platinum.The reactivity series of metals - carbon and hydrogen are non-metals, shown for comparison

At the anode

Either oxygen or a non-metal from the electrolyte can be produced at the anode:

  • for the most common compounds oxygen is produced (from the hydroxide ions)
  • if halide ions (chloride, bromide or iodide ions) are present, then the negatively charged halide ions lose electrons to form the corresponding non-metal halogen (chlorine, bromine or iodine)

The table summarises the product formed at the anode during the electrolysis of different electrolytes in solution.

Negative ionElement given off at anode
Chloride, Cl-Chlorine, Cl2
Bromide, Br-Bromine, Br2
Iodide, I-Iodine, I2
Sulfate, SO42-Oxygen, O2

Example

Predict the product formed at the positive electrode during the electrolysis of concentrated sodium chloride solution.

Chlorine will be produced.

Question

Predict the product formed at the positive electrode during the electrolysis of concentrated sodium sulfate solution.

Oxygen will be produced.