A redox (reduction-oxidation) reaction is a type of chemical reaction that involves the transfer of electrons between species. It consists of two half-reactions: oxidation and reduction.
Oxidation: Oxidation is a process where a species loses electrons, leading to an increase in its oxidation state or a decrease in its electron density. In other words, it involves the loss of electrons by a substance. The substance that undergoes oxidation is called the reducing agent or the electron donor. During oxidation, there is typically an increase in the number of oxygen atoms, a decrease in the number of hydrogen atoms, or both.
For example, consider the reaction of iron (Fe) with oxygen (O2) to form iron(III) oxide (Fe2O3):
4Fe + 3O2 -> 2Fe2O3
In this reaction, iron is oxidized from an oxidation state of 0 to +3. It loses electrons and transfers them to oxygen, which is reduced.
Reduction: Reduction is the opposite of oxidation and involves the gain of electrons by a species, resulting in a decrease in its oxidation state or an increase in its electron density. During reduction, there is typically a decrease in the number of oxygen atoms, an increase in the number of hydrogen atoms, or both.
Continuing with the previous example, the oxygen molecules are reduced in the reaction:
4Fe + 3O2 -> 2Fe2O3
The oxygen gains electrons from iron, reducing its oxidation state from 0 to -2.
Overall, in a redox reaction, both oxidation and reduction occur simultaneously. One species loses electrons (undergoes oxidation), while another species gains those electrons (undergoes reduction). The transfer of electrons between these species allows for the conservation of charge in the reaction.