Heat Changes During Chemical Equilibrium

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There are many reactions in which energy is evolved while in others energy is absorbed. Depending upon evolution or absorption of energy, the chemical reactions can be classified into two types:

Exothermic reactions and endothermic reactions.

Exothermic Reactions

The chemical reactions which proceed with the evolution of heat energy is called exothermic reactions.

The heat evolved is expressed in Joules or kJ or Calories.

Example of an exothermic reaction is 'thermite reaction'. Here Fe₂O₃ is reduced to Iron and the heat produced in the reaction melts the iron which is used to weld iron nails.

For e.g.,

Endothermic Reactions

The chemical reactions which proceed with the absorption of heat energy are called endothermic reactions.

At constant temperature and constant volume,

As E_p > E_R

For e.g.,

From the above discussion it can be concluded that:

Exothermic reaction DE = -ve

Endothermic reaction DE = +ve