Question

Define and explain the term, enthalpy of reaction.

Answer 1

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 →cC + dD

The heat of the reaction ΔH is the difference in sum of enthalpies of products and sum of enthalpies of reactants.

∴ Δ_rH = ∑H_products - ∑H_reactants 

= [cH_C + dH_D] - [aH_A + bH_B]

= ∑ _PH - ∑ _RH

where H represents enthalpy of the substance.

For endothermic reaction, ΔH is positive, (ΔH > 0).

For exothermic reaction, ΔH is negative, (ΔH < 0).