A chemical reaction is a change which the reactants undergo in order to form products. There are various types of chemical reactions. Redox reactions are a special class among those reactions. Redox reactions is a short way to denote Reduction – Oxidation reactions. So in order to understand redox reactions, we will have to understand what oxidation and reduction reactions are.
During a reaction, it is possible for an atom to lose one or more of its valence electrons easily. When such a situation arises, the atom gains a positive charge equal because the atom gets rid of the negatively charged electrons. The quantity of charge gained is equal to the number of electrons that it loses and it is said to have been oxidized. This reaction is called an oxidation reaction. The atom then becomes ion and is called a cation, a positively charged ion. An atom or species can only lose up to a certain number of electrons, the rest are too strongly attracted by the nucleus to be pulled away. The charge that an atom or ion bears is known as the oxidation state of the species. An oxidation reaction leads to an increase in oxidation state of the oxidized species.
During a reaction, it is also possible for an atom to instead gain one or more valence electrons lost by some other species. In this case, the atom gains a negative charge by virtue of accepting negatively charged electrons. The atom in this case is said to have been reduced in a reduction reaction. The negatively charged ion is identified as an anion. Just like there is a limit imposed on how many electrons a species can lose, there is a limit to how many electrons a species can accept as well. During a reduction reaction, the oxidation state of the reduced species decreases.
Who gained, who lost?
A redox reaction is called so because one cannot take place without the other and both of them happen simultaneously each time. The valence electrons lost by the species which got oxidized is gained by its partner species which gets reduced. The species that gets oxidized is in turn called the reducing agent, because it forces the reduction of its partner species. Similarly, the reduced species is called the oxidizing agent because it helps the oxidized species to lose electrons by willingly accepting them. Unless both the oxidizing and reducing agents are available, the redox reaction cannot take place.
For the ease of understanding, the redox reactions can be segmented into oxidation and reduction half-reactions. A half-reaction shows either the oxidation process or the reduction process. The electrons lost or gained by the species are also explicitly shown in the redox half-reactions. However, there are many redox reactions in which no real electron transfer might take place. This is because oxidation and reduction reactions have more to do with the increase and decrease in oxidation states rather than physical transfer of the electrons.