Electrolysis of solutions

Water is a poor conductor of electricity, but it does contain some hydrogen ions, H+, and hydroxide ions, OH-. These ions are formed when a small proportion of water molecules naturally dissociate.

Electrolysis of dissolved ionic compounds

An electrolyte formed by dissolving an ionic compound contains:

  • positive hydrogen ions from the water, and positive metal 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 will be produced at the cathode if it is less reactive than hydrogen
  • hydrogen will be 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

Example

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

Hydrogen will be produced because sodium is more reactive than hydrogen.

Question

Predict the product formed at the negative electrode during the electrolysis of copper chloride solution.

Copper will be produced because copper is less reactive than hydrogen.

At the anode

Oxygen is produced (from hydroxide ions) at the anode, unless halide ions (chloride, bromide or iodide ions) are present. In that case, the negatively charged halide ions lose electrons and form the corresponding halogen (chlorine, bromine or iodine).

The balanced half equation (higher tier only) for the formation of oxygen from hydroxide ions is:

4OH- → 2H2O + O2 + 4e-

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

Negative ion from electrolyteElement given off at anode
Chloride, Cl-Chlorine, Cl2
Bromide, Br-Bromine, Br2
Iodide, I-Iodine, I2
Sulfate, SO42-Oxygen, O2
Nitrate, NO3-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.