ΔHrxn = sum of ΔHf (products of reaction) - sum of ΔHf (reactants).
Or ΔHrxn = ∑ΔHf (products of reaction) - ∑ΔHf (reactants).
ΔHrxn - enthalpy change of chemical reaction.
ΔHf - enthalpy of formation of reactants or products.
An enthalpy change is difference between the enthalpy after the chemical reaction has completed.