Electrolysis of solutions

Electrolysis of acidified water

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. If water is acidified with a little dilute sulfuric acid:

  • 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:

  • hydrogen ions from the water and positive ions from the compound
  • 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)

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(II) chloride solution.

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

At the anode

Oxygen is produced (from hydroxide ions), 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 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
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.

Very dilute solutions of halide compounds

If a halide solution is very dilute, oxygen is given off instead of the halogen. This is because the number of halide ions in the solution is very small. Therefore, very few halide ions will travel to the anode and lose electrons to form a halogen. Instead, water molecules in the solution, some of which break down to give hydroxide ions, produce oxygen at the anode.