From a reaction energy diagram, which expression gives ΔH, the enthalpy change?

• A. ΔH = Er - Ep
• B. ΔH = Ep + Er
• C. ΔH = Eac - Er
• D. ΔH = Ep - Er【Ep is energy of products, Er energy of reactants】

Answer: (D)

Enthalpy change is the difference in energy between the final and initial states. On a reaction energy diagram, the energy of the reactants is Er and the energy of the products is Ep, so the net enthalpy change is ΔH = Ep − Er. This difference tells you whether the final state is higher or lower in energy than the initial state: Ep > Er means ΔH is positive (endothermic), Ep < Er means ΔH is negative (exothermic). The activation energy, the energy barrier from reactants to the highest point on the curve, does not determine ΔH. It describes the rate barrier, not the net energy change. Expressions involving a sum Ep + Er aren’t meaningful for ΔH, as enthalpy change is a difference between final and initial energies.