Oxidation & Reduction

Oxidation and reduction are key concepts in redox chemistry and are essential to understanding many chemical processes. You must be able to define these terms clearly in different contexts.

Oxidation can be defined as:

• The loss of electrons.

• An increase in oxidation state.

• Addition of oxygen or removal of hydrogen.

Reduction can be defined as:

• The gain of electrons.

• A decrease in oxidation state.

• Removal of oxygen or addition of hydrogen.

Redox reactions involve both oxidation and reduction occurring simultaneously.

Basic Principles

Electron Transfer Model:

• Oxidation: electrons are lost.

• Reduction: electrons are gained.

• This is often summarised as OIL RIG:

  Oxidation Is Loss, Reduction Is Gain (of electrons).
  

Oxidation State:

• Oxidation and reduction can also be defined in terms of changes in oxidation number.

• An increase in oxidation number = oxidation.

• A decrease in oxidation number = reduction.

• Further explanation of oxidation states can be found in the ‘Oxidation States’ note.

Oxidation states

You may be asked to use oxidation states to explain what is being oxidised and reduced in a given reaction.

Example:

• Mg goes from an oxidation state of 0 to +2, so has been oxidised.

• Cl goes from 0 to -1, so has been reduced.
  

Electron transfer

OIL RIG (Oxidation Is Loss, Reduction Is Gain (of electrons)) can be used to identify and explain oxidation and reduction in terms of electron transfer.

Example:

gains electrons to produce neutral elemental iron and so has been reduced. has lost electrons to produce neutral elemental chlorine and so has been oxidised.

This can be demonstrated in half equations which show reduction and oxidation processes separately (explained fully in the ‘Redox Equations’ note).

Electrons balance the charges and therefore clearly show whether a species has lost or gained electrons.

Electrons on the left-hand side show they are acting as a reactant and therefore gained.

Electrons on the right-hand side show they are acting as a product and therefore have been lost as a result of the reaction taking place.

Combining the two half equations gives the original overall equation for the redox reaction.

Agents

Oxidising agents = Electron acceptor

Oxidising agents oxidise other species by accepting electrons and are therefore reduced themselves

Reducing agent = Electron donor

Reducing agents reduce other species by donating electrons and are therefore oxidised themselves

e.g. in the previous reaction

is the reducing agent

is the oxidising agent