JEE Main & Advanced Chemistry Thermodynamics / रासायनिक उष्मागतिकी Exothermic And Endothermic Reactions

(1) Exothermic reactions : The chemical reactions which proceed with the evolution of heat energy are called exothermic reactions. The heat energy produced during the reactions is indicated by writing +q or more precisely by giving the actual numerical value on the products side. In general exothermic reactions may be represented as, \[A+B\to C+D+q\] (heat energy)

In the exothermic reactions the enthalpy of the products will be less than the enthalpy of the reactants, so that the enthalpy change is negative as shown below

\[\Delta H={{H}_{p}}-{{H}_{r}}\] ;  \[{{H}_{p}}<{{H}_{r}}\]; \[\Delta H=-\,ve\]

Examples : (i) \[C(s)+{{O}_{2}}(g)\to C{{O}_{2}}(g)+393.5kJ\]

(at constant temperature and pressure)

or \[C(s)+{{O}_{2}}(g)\to C{{O}_{2}}(g);\]  \[\Delta H=-393.5kJ\]

(ii) \[{{H}_{2}}(g)+\frac{1}{2}{{O}_{2}}(g)\to {{H}_{2}}O(l);\]   \[\Delta H=-285.8kJ\]

(iii) Fermentation is also an example of exothermic reaction.

(2) Endothermic reactions : The chemical reactions which proceed with the absorption of heat energy are called endothermic reactions. Since the heat is added to the reactants in these reactions, the heat absorbed is indicated by either putting (–) or by writing the actual numerical value of heat on the reactant side

\[A+B\to C+D-q\] (heat energy)

The heat absorbed at constant temperature and constant pressure measures enthalpy change. Because of the absorption of heat, the enthalpy of products will be more than the enthalpy of the reactants. Consequently, \[\Delta H\] will be positive \[(+ve)\] for the endothermic reactions.

\[\Delta H={{H}_{p}}-{{H}_{r}}\]; \[{{H}_{p}}>{{H}_{r}}\];  \[\,\,\Delta H=+ve\]

Example : (i) \[{{N}_{2}}(g)+{{O}_{2}}(g)\to 2NO(g);\,\Delta H=+180.5\,kJ\]    

(ii) \[C(s)+2S(s)\to C{{S}_{2}}(l)\,\Delta H=+92.0kJ\]

(iii) Preparation of ozone by passing silent electric discharged through oxygen is the example of endothermic reaction.

(iv) Evaporation of water is also the example of endothermic reaction.

For exothermic reaction : \[\Delta H\] or \[\Delta E=-ve\]

For endothermic reaction : \[\Delta H\] or \[\Delta E=+ve\]