Predicting the products of electrolysis

Ionic substances in solution break down into elements during electrolysis. Different elements are released depending on the particular ionic substance.

At the negative electrode

At the negative electrode, positively charged ions gain electrons. This is reduction, and you say that the ions have been reduced.

Metal ions and hydrogen ions are positively charged. Whether you get the metal or hydrogen during electrolysis depends on the position of the metal in the reactivity series:

• The metal will be produced if it is less reactive than hydrogen
• Hydrogen will be produced if the metal is more reactive than hydrogen

The reactivity series of metal - carbon and hydrogen are not metals, but they are shown for comparison

So the electrolysis of copper chloride solution produces copper at the negative electrode. But the electrolysis of sodium chloride solution produces hydrogen.

At the positive electrode

At the positive electrode, negatively charged ions lose electrons. This is oxidation, and you say that the ions have been oxidised. The table summarises some of the elements you should expect to get during electrolysis.

 

Negative ion in solution	Element given off at positive electrode
Chloride, Cl–	Chlorine, Cl2
Bromide, Br–	Bromine, Br2
Iodide, I–	Iodine, I2
Sulfate, SO42–	Oxygen, O2

Putting it together

The table shows some common ionic compounds, and the elements released when their solutions are electrolysed.

 

Ionic substance in solution	Element at the negative electrode	Element at the positive electrode
Copper chloride, CuCl2	Copper	Chlorine
Copper sulfate, CuSO4	Copper	Oxygen
Sodium chloride, NaCl	Hydrogen	Chlorine
Hydrochloric acid, HCl	Hydrogen	Chlorine
Sulfuric acid, H2SO4	Hydrogen	Oxygen