Question
What do you understand by exothermic and endothermic reactions? Give two examples of each.
Solution
(i) Exothermic reactions: Reactions which are accompanied by the evolution of heat energy and have a negative value of ∆H are known as exothermic reactions.e.g.
A negative value of ∆H for exothermic reactions: Heat evolved is equal to the enthalpy change when the reaction is carried out at constant pressure. For an exothermic reaction, there is an evolution of heat energy and hence enthalpy for the products (Hp) will become less than the enthalpy of the reactants (HR).
(ii) Endothermic reactions: Reactions which are accompanied by absorption of heat energy and have positive value of ∆H are known as endothermic reactions e.g.
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A positive value of ∆H for endothermic reactions: Heat absorbed is equal to the enthalpy change when the reaction is carried out at constant pressure. For an endothermic reaction, there is an absorption of heat energy and hence enthalpy of the products (Hp) will become more than the enthalpy of the reactants (HR).
A negative value of ∆H for exothermic reactions: Heat evolved is equal to the enthalpy change when the reaction is carried out at constant pressure. For an exothermic reaction, there is an evolution of heat energy and hence enthalpy for the products (Hp) will become less than the enthalpy of the reactants (HR).
(ii) Endothermic reactions: Reactions which are accompanied by absorption of heat energy and have positive value of ∆H are known as endothermic reactions e.g.
A positive value of ∆H for endothermic reactions: Heat absorbed is equal to the enthalpy change when the reaction is carried out at constant pressure. For an endothermic reaction, there is an absorption of heat energy and hence enthalpy of the products (Hp) will become more than the enthalpy of the reactants (HR).
