# Why is Standard heat of reaction equal to difference between standard heats of formation of products and reactants?

The knowledge of standard heat of formation of various substances can be used to calculate the heat of reactions under standard conditions. The standard heat of any reaction is equal to the difference between the standard heat of formation of the products and the reactants

Why is that so? I'd prefer physical derivation for this: $$\Delta H^\circ = \sum\Delta H^\circ_{products} - \sum\Delta H^\circ_{reactants}$$

• Technically, I prefer the form where the equation essentially states that the standard enthalpy change of formation is equal to the sum of the standard enthalpies of formation of the products minus the sum of the standard enthalpies of formation of the reactants. Now, the physical (as in measurable) verification becomes problematic when the energy released includes things like light. Note: Hess's Law applies to heat. – AJKOER Oct 6 '20 at 13:01