Oxidation- Reduction (Redox) Reactions – AP Chemistry College Board Revision

Chapter: Oxidation- Reduction (Redox) Reactions

Understanding Basic Definitions

An oxidation-reduction (redox) reaction involves the transfer of electrons between substances.
Oxidation is the process where a substance loses electrons. The oxidation number of a substance increases in this process.
Reduction is the process where a substance gains electrons. The oxidation number of an element decreases during reduction.
Chemical species causing the oxidation of a substance is known as the oxidising agent or oxidant. They themselves get reduced in the process.
Chemical species causing the reduction of a substance is known as the reducing agent or reductant. They get oxidised themselves during the process.
Redox reactions are balanced in terms of both mass and charge. This is achieved by the ion-electron method and the half-reaction method.

Combination Reactions: Two or more substances combine to form a new substance, and electron transfer occurs in the process.
Decomposition Reactions: One substance breaks down into two or more substances, where oxidation and reduction occur.
Displacement Reactions: Redox reactions where a more reactive metal displaces a less reactive metal from its solution.
Disproportionation Reactions: Reactions in which the same substance is oxidised and reduced.

The Oxidation State (or number) is an indicator of the degree of oxidation of an atom in a chemical compound.
It can be a positive, negative, or zero number that is assigned to an atom following a set of rules, such as free elements having zero oxidation state and the sum of the oxidation states in a neutral molecule equalling zero.

Look for changes in oxidation numbers to identify redox reactions. If the oxidation number of any atom changes during a reaction, it’s an indication of a redox reaction.
Examining the substances before and after the reaction can help identify if electrons are gained or lost, signifying a redox reaction.

Rusting of Iron: Oxidation of iron in presence of oxygen and water results in rust (hydrated iron(III) oxide) which is a common example of redox reaction.
Bleaching: Chlorine bleach works by oxidising stains to colourless compounds.
Photosynthesis: This crucial process in plants is a redox reaction where carbon dioxide is reduced to form glucose and water is oxidised to generate oxygen.
Respiration: The process of breaking down glucose to generate energy in living organisms is also based on redox reactions.

Redox reactions are fundamental to many chemical processes and are widely applied in areas such as electricity generation (in batteries and fuel cells), metallurgy, photography, and biological processes.
A thorough understanding of redox reactions is necessary for a range of scientific and industrial roles, including chemists, biologists, environmental scientists and chemical engineers.