Topic 9: Redox processes - 9.1 Oxidation and reduction

Nature of science: How evidence is used-changes in the definition of oxidation and reduction from one involving specific elements (oxygen and hydrogen), to one involving electron transfer, to one invoking oxidation numbers is a good example of the way that scientists broaden similarities to general principles. (1.9)


Essential idea: Redox (reduction-oxidation) reactions play a key role in many chemical and biochemical processes.

Oxidation and reduction can be considered in terms of oxygen gain/hydrogen loss, electron transfer or change in oxidation number.

An oxidizing agent is reduced and a reducing agent is oxidized.

Variable oxidation numbers exist for transition metals and for most main-group non-metals.

The activity series ranks metals according to the ease with which they undergo oxidation.

The Winkler Method can be used to measure biochemical oxygen demand (BOD), used as a measure of the degree of pollution in a water sample.

Applications and skills

Deduction of the oxidation states of an atom in an ion or a compound.

Deduction of the name of a transition metal compound from a given formula, applying oxidation numbers represented by Roman numerals

Identification of the species oxidized and reduced and the oxidizing and reducing agents, in redox reactions.

Deduction of redox reactions using half-equations in acidic or neutral solutions.

Deduction of the feasibility of a redox reaction from the activity series or reaction data.

Solution of a range of redox titration problems.

Application of the Winkler Method to calculate BOD.


Oxidation number and oxidation state are often used interchangeably, though IUPAC does formally distinguish between the two terms. Oxidation numbers are represented by Roman numerals according to IUPAC.

Oxidation states should be represented with the sign given before the number, eg +2 not 2+.

The oxidation state of hydrogen in metal hydrides (-1) and oxygen in peroxides (-1) should be covered

A simple activity series is given in the data booklet in section 25.