Define and explain the term, enthalpy or heat of
reactopn.

Explanation

The enthalpy of a chemical reaction is the difference between
the sum of the enthalpies of products and that of the reactants with
every substance in a definite physical state and in the amounts (moles)
represented by the coefficients in the balanced equation.
Explanation : Consider the following general reaction,
`aA+bB rarr cC+dD` ,
The heat of the reaction `triangle H` is the difference in sum of enthalpies of
products and sum of enthalpies of reactants.
`therefore triangle H= SigmaH_("products")-Sigma H_("reactants")`
`=[cH_(C)+dH_(D)]-[aH_(A)+bH_(B)]`
`=Sigma_(P)H-Sigma_(R)H`
where H represents enthalpy of the substance.
For endothermic reaction, `triangleH` is positive, `(triangleH gt 0)`.
For exothermic reaction, `triangleH` is negaitve, `(triangleH lt 0).`