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Oxidation and Reduction

Oxidation is a process of addition of oxygen to a substance or removal of hydrogen from a substance, whereas reduction is a process of addition of hydrogen to a substance or removal of oxygen from a substance.
Oxidation and reduction always take place together. In other words, they are complementary.
Consider the following reaction:


In this reaction, S in H2S is oxidised to S because H is removed, and S in SO2 is reduced to S because O is removed.


In this reaction, Fe in Fe2O3 is reduced to Fe because oxygen is removed, and C in CO is oxidised to CO2 because oxygen is added.


Cu in CuO is _________ to Cu because __________ is _____________. H in H2 is __________ to H2O because ___________ is ___________.
In modern parlance, oxidation and reduction have been given broader meanings, which we can observe if we analyse what happens when a metal such as iron is oxidised and its oxide is reduced.
When iron reacts with oxygen, iron atoms lose electrons to become Fe3+ ions and oxygen atoms gain electrons to become O2–. Conversely, when the oxide is resduced to give metallic iron, the reverse process must happen. Hence, Fe3+ ions must gain electrons to give Fe atoms.
4Fe + 3O2 Description: 25436.jpg 2Fe2O3
Thus, oxidation is defined as loss of electrons (de-electronation) and reduction as gain of electrons (electronation).
Oxidation and reduction reactions are also termed as β€˜redox’ reactions. Redox reactions include processes such as burning, rusting, respiration, photosynthesis and many other important processes in nature.
Substances such as oxygen which accept electrons are called oxidising agents and substances which donate electrons are called reducing agents.

Redox Reactions in Terms of Oxidation Number

Oxidation is a chemical change in which there is an increase in oxidation number, whereas reduction is a chemical change in which there is a decrease in oxidation number.
Oxidation: It is a process where there is a loss of electrons or increase in oxidation number.
Reduction: It is a process where there is a gain of electrons or decrease in oxidation number.
Oxidising agent or oxidant: It is the substance whose oxidation number decreases (electron acceptor).
Reducing agent or reductant: It is the substance whose oxidation number increases (electron donor).

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