Another classification of chemical reactions is as follows:
OXIDATION AND REDUCTION REACTIONS:
It involves the addition of oxygen to a substance or the removal of hydrogen from a substance.
The compound or element from which hydrogen is removed or in which oxygen is added is said to be oxidized.
The elements which remove hydrogen or add oxygen are called oxidizing agents.
The reaction which involves addition of hydrogen or removal of oxygen is called reduction reaction.
The compound or element from which oxygen is removed or in which hydrogen is added is said to be reduced.
The elements which remove hydrogen or add oxygen are called reducing agents.
EXAMPLE OF OXIDATION AND REDUCTION REACTIONS:
Copper oxide is heated with hydrogen, to produce copper metal and water :
In this reaction, CuO is changing into Cu. That is, oxygen is being removed from copper oxide.
As oxygen is being removed in this case, we can say that copper oxide is being reduced to copper.
In this reaction H2 is changing into H2O. That is, oxygen is being added to hydrogen.
Hence, we can say that hydrogen is being oxidised to water.
In redox reaction, a substance gains oxygen or lose hydrogen, i.e., the substance gets oxidized and simultaneously, another substance loses oxygen or gains hydrogen, i.e., the substance gets reduced. In other words, oxidation and reduction occur simultaneously.
So, the reaction where oxidation and reduction occur simultaneously is known as a redox reaction.
For example, the combination of Hydrogen Sulfide and Chlorine results to the drop out of Sulphur and formation of
Thus, in this case, H2S is the reducing agent and the sulphur gets oxidized to S.