An aqueous solution is a solution in which the solvent is liquid water. So, 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.
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.
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.
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.
Here are some common electrolytes and their corresponding products:
|Aqueous solution||Product at the cathode (-)||Product at the anode (+)|
|Sodium chloride (NaCl)||Hydrogen (H2)||Chlorine (Cl2)|
|Copper (II) sulfate (H2SO4)||Copper (Cu)||Oxygen (O2)|
|Sulfuric acid (H2SO4)||Hydrogen (H2)||Oxygen (O2)|
|Hydrochloric acid (HCl)||Hydrogen (H2)||Chlorine (Cl2)|