- Energy changes occur during chemical reactions because different amounts of energies are involved in breaking a bonds in reactants and making of bonds in products. Depending upon whether heat is evolved or absorbed during a reaction, the reaction can be exothermic or exothermic.
Exothermic reaction - A chemical reaction in which heat energy is given out .
Buring of natural gas , respiration, formation of compost from vegetables matter also exothermic processes.
Examples
C(s)+O2(g)⸻➤CO2(g)+94.00kcal
2H2(g) +O2(g)⸻➤2H2O(l) +136.0kcal
Endothermic reaction - A chemical reaction in which heat energy is absorbed is called endothermic reaction.
Example
C(s)+2S⸻➤CS(l)-22kcal
C(s)+H2O⸻➤CO(s) +H2(s)-31.4 kcal
{ ⸻➤at 1100°C }