IB Chemistry - Oxidation

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Syllabus ref: 9.2 Syllabus ref: 19.1

Electrical energy is used to break compounds apart in electrolytic cells. This section looks at the general components of electrolytic cells and the processes that occur within.

Electrolytic cells

An electrolytic cell is the apparatus used to pass electricity through an electrolyte (a conducting liquid, either a molten ionic compound, or an ionic solution). The electrolytic cell has two (usually inert) electrodes to pass the electric current into the electrolyte. The negative electrode is called the cathode and the positive electrode is called the anode.

The inert electrodes are usually made of graphite or platinium wire. Chemical reactions occur at the surface of the electrodes.

The power supply is usually symbolised by a short fat line (negative) and a long thin line (positive)


Reactions at the electrodes

The power pack (battery) supplies electrons for the negative electrode (the cathode), which the positive ions in the electrolyte can pick up to become other species, usually elements. At the same time the positive potential at the positive elctrode can remove electrons from the ions in solution to make new species, also usually elements.

The consequence of these two processes is that electrons are removed from negative ions at the the anode (by the powerful positive potential there) and they are added to positive ions at the cathode (where there are many available electrons). The overall effect is that the anode gains lectrons as the cathode loses them.

As far as the battery is concerned there is a regular flow of electrons out of the negative terminal and into the positive terminal.

Let's look at the reactions occurring at both electrodes in more detail.


Reaction at the anode

The anode is the positive electrode, connected directly to the positive terminal of the battery. This is strongly attractive to negative ions. Once a negative ion arrives at the anode its electron is removed by the strong positive potential and the ion becomes a neutral species. Hence the process that occurs is oxidation.

For example:

Cl-(aq) Cl + 1e

The chlorine atom is too reactive to exist for long on its own and pairs up with another chlorine atom to form a molecule.

2Cl Cl2(g)

Usually both steps are written as one:

2Cl-(aq) Cl2(g) + 2e

Anode reaction


Reactions at the cathode

The cathode is supplied with electrons by the negative terminal of the battery. This attracts positive ions from the electrolyte, which pick up the electrons neutralising their positive charge. Thus the ions become atoms, or molecules depending on the type of ion.

For example, a sodium ion in a molten sodium chloride electrolyte will approach the cathode and pick up an electron:

Na+ + 1e Na

This results in the fomation of metallic sodium. It is the way that sodium is manufactured in industry.

Cathode reaction