Electrolysis – O Level Chemistry

In this post, we’ll look at electrolysis as tested in O Level Chemistry.

During electrolysis, electricity is passed through an electrolyte (aqueous or molten ionic compound), resulting in the decomposition of the electrolyte.

When do ionic compounds conduct electricity

Solid ionic compounds cannot undergo electrolysis, as the ions are held strongly together in the lattice, and are unable to move about to conduct electricity. Hence, in the solid state, ionic compounds cannot undergo electrolysis.

Only when dissolved in water or when molten then ionic compounds can undergo electrolysis, as the ions are now free to move about to conduct electricity

Setup of a simple electrolytic cell

The diagram below shows a simple setup of an electrolytic cell:

The anode and cathode are called the electrode. They are used to provide electrical connectivity between the electrolyte and the external circuit (i.e. batteries and wires).

The anode is connected to the positive terminal of the battery (and is positively charged), while the cathode is connected to the negative terminal of the battery (and is negatively charged).

An inert electrode (e.g graphite, and platinum) is one that does not react or take part in the electrolytic reaction. A reactive electrode is one that will take part in the electrolytic reaction.

Determine Products at the cathode and anode of an electrolytic cell

Product at the cathode

• Cathode or negative electrode is connected to the negative terminal of the battery. Cathode will attract positive ions or cations. Reduction takes place at the cathode, as the cation gains electrons.
• if there are more than 1 cations in the solution, the one that is lowest in the reactivity series will be preferentially discharge (or reduced).

Product at the anode

• Anode or positive electrode is connected to the positive terminal of the battery. Anode will attract negative ions or anions. Oxidation takes place at the anode, as the anion loses electrons.
• If there are more than one anion in the solution, use the following to decide which anion will be discharged:- OH^– will be preferentially discharged in dilute/ aqueous solutions, unless solution is concentrated halide solutions. 
• When solution is a concentrated halide solution (e.g concentrated sodium chloride solution), then the anion that will be preferentially discharged will be the halide (e.g. chloride).- NO₃^– SO₄^2- and CO₃^2- will not be discharged (for your syllabus).