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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: $$\Delta H^\circ = \sum\Delta H^\circ_{products} - \sum\Delta H^\circ_{reactants}$$
It is the consequence of the Hess's law and that is the consequence of the energy conservation law. Overall it says the enthalpy change does not depend on the path, but only on the start and final point.
Comparing to the real reaction path, you can formally get there also
If the result were not the same, you could create perpetuum mobile switching both states via 2 different paths, gaining the nonzero net energy.
