Most of the chemical reactions are accompanied by energy changes. In some reactions, energy is given in the form of heat and light. While in some other chemical reactions heat and light energy is absorbed. So, chemical reactions can be classified into following categories depending upon absorption or liberation of heat or light energy.
1. Exothermic reactions
2. Endothermic reactions
3. photochemical reactions
Exothermic reactions
The chemical reactions in which energy is liberated in the form of heat are called exothermic reactions. In these reactions the energy required to break down the chemical bonds in reactants is usually less than the energy released during formation of chemical bonds in products. For example, when methane gas is burnt in the presence air then a lot of heat is produced.
CH4(g) + 2O2 (g) 
CO2(g) + 2H2O (l) + Heat
Another example of exothermic reaction is reaction of iron oxide with aluminium to produce iron and aluminium oxide. This reaction is called thermite reaction and this reaction is used in welding of iron rails.
Endothermic reactions
The chemical reactions in which energy is absorbed are called endothermic reactions. In these reactions the energy required to break down the chemical bonds in reactants is more than the energy released during formation of chemical bonds in products. For example, when barium hydroxide and ammonium thiocynate are mixed together the reaction mixture becomes very cold which shows that it has absorbed energy so it is an endothermic reaction.
Another example of endothermic reaction is evaporation of water which takes place by absorption of heat energy.
Photochemical reactions
The chemical reactions which take place in the presence of light are known as photochemical reactions. Following are the examples of photochemical reactions-
The process of photosynthesis for preparation of food in plants takes place only in the presence of light of sun. in this process plants convert the carbon dioxide and water into starch and oxygen in the presence of sunlight with the help of chlorophyll pigment
Reaction of hydrogen with chlorine also takes place in the presence of sunlight.
The process of photography is also a photochemical reaction. Actually the photographic film is coated with silver bromide (AgBr) which undergoes chemical reaction when exposed to sunlight.
Test your understanding and answer these questions:
- Define endothermic reactions. Give examples.
- Define exothermic reactions. Give examples.
- What do you understand by photochemical reactions. give examples.