Hess's Law

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Hess's Law is the chemists way of saying First Law of Thermodynamics, which means that energy cannot be created, or destroyed it only changes from one form to another. In other words, the the total change in chemical potential energy (enthalpy change), must be equal to the energy lost or gained by the system. Hence, the enthalpy change resulting from converting reactants into products is constant regardless of the way or procedure used to make that change. It also must be noted that the principle behind Hess's Law remains usable even if a particular reavtion is only theoretical and cannot be done practically [1].


  1. Green, J and Damaji, S. (2007) Chemistry, 3rd edition, page 141, IBID press.
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