181.

Explain the origin of enthalpy change in a direction. interest

During the reformation of chemical bonds, when the energy required to break bonds is less than the energy released in the formation of new bonds, the chemical reaction would be exothermic i.e. heat would be evolved. e.g.
          
Similarly, when the energy required to break bonds is more than the energy released in the formation of new bonds for products, a chemical reaction would be endothermic and ∆H would be positive. For example,

 
In gaseous phase, the enthalpy change (∆H) of a chemical reaction can be written as