Heat Of Reaction

Fig:- Reaction Energy Diagram 

In the above figure, this enthalpy change in energy between reactants and products is called "Heat Of Reaction". The value of the heat of reaction determines whether the reaction is exothermic or endothermic.

Besides activation and threshold energy, another form of energy exists here and that is "Heat Of Reaction". It is also known as "Enthalpy Change". It is defined as the net amount of energy that should be added or released during a chemical reaction.

Figure-1

Figure-2

In the above figure, energy of the reactants and the energy of the products are not the same, but there are some differences between them. And this difference represents the heat of the reaction. 

Hence, "Heat of a reaction is defined as the difference between the average energy of the reactants and the average energy of the products". 

This difference can be positive or negative. And depending on these positive and negative values of the difference, the reaction may be Endothermic or Exothermic. It is denoted by '∆Hr'.

➩ {∆H(reaction) = ∑H(Product) –∑H(Reactant)}

"If the value of heat of reaction is positive, the reaction is called an endothermic reaction. And if the value of heat of reaction is negative, the reaction is called an exothermic reaction". Thus figure-1 shows exothermic reaction while figure-2 shows endothermic reaction.

In general, for a chemical reaction;

A + B ➝ C + D - Heat emitted 

This is a case where heat energy is released by the reaction and this type of reaction is called "Exothermic Reaction".

&

For a chemical reaction;

A + B + Heat absorbed ➝ C + D 

This is a case where heat energy is absorbed by the reaction and thus the reaction is called an "Endothermic Reaction".

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