Why is heat evolved or absorbed in a chemical reaction ?

Explanation

Because the reactant have a fixed enthalpy content before the reaction and when these are converted into the products which have a different enthalpy content. So, heat gets released or absorbs. Even if temperature of reactions remains constant yet due to change in bonding energies `Delta E != 0`.
If `H_("Product") gt H_("reactants")`
Reaction should be endothermic as we have to give extra heat to reactants to get these converted into products
and if `H_("Product") lt H_("reactants")`
Reaction will be exothermic as extra heat content of reactant will be released during the reaction.
Enthalpy change of a reaction: `Delta H_("reaction") = H_("products")-H_("reactant")`
`{:(DeltaH_("reactions"),H_("products"),-H_("reactants"),,),(,="positive",-,"endothermic",),(,="negative",-,"exothermic",):}`