Electrolysis of Aqueous solutions

An aqueous solution is a solution in which the solvent is liquid water. Therefore, all aqueous solutions will contain water. The water molecules in aqueous solutions dissociate during electrolysis, forming H⁺ and OH ions:

H2O ⇌ H⁺ + OH⁻

When an ionic compound dissolves in water, the positive ions from the ionic compound also become part of the solution and are attracted to the OH⁻ of the water molecule. Similarly, the negative ions from the ionic compound are attracted to the H⁺ of the water molecule.

Reactions at the Negative Electrode (Cathode)

The cathode attracts positively charged metal ions and hydrogen ions (H⁺), so electrolysis will either produce the metal or hydrogen at the cathode. The product you get at the cathode depends on the position of the metal in the reactivity series.

  • If the metal is above hydrogen in the reactivity series, then hydrogen gas will be produced. You will see bubbles of hydrogen gas
  • If the metal is below hydrogen in the reactivity series, the metal will be produced
The reactivity series with the bottom section labelled to be less reactive than hydrogen and the middle section labelled to be less reactive than carbon. An upward pointing arrow indicates the reactivity increases as you go up the series.

  • Carbon and hydrogen are not metals, but they are often used for comparison

For example, the electrolysis of an aqueous solution of lead (II) bromide will produce hydrogen gas at the cathode. This is because lead is above hydrogen in the reactivity series.

Reactions at the Positive Electrode (Anode)

The positive anode attracts hydroxide ions (OH⁻) and negatively charged non-metal ions. At the anode, oxygen will be produced, unless the other negative ion is a halide, such as Cl⁻, Br⁻ or I⁻. In that case, the halogen produced will be Cl2, Br2 or I2.

Common Electrolytes

Here are some common electrolytes and their corresponding products:

Aqueous solutionProduct at the cathode (-)Product at the anode (+)
Sodium chloride (NaCl)Hydrogen (H2)Chlorine (Cl2)
Copper (II) sulfate (CuSO4)Copper (Cu)Oxygen (O2)
Sulfuric acid (H2SO4)Hydrogen (H2)Oxygen (O2)
Hydrochloric acid (HCl)Hydrogen (H2)Chlorine (Cl2)