Hess's law states that compatible chemical reaction enthalpies add when their equations add. Reversing a chemical reaction reverses its enthalpy change; multiplying the entire chemical reaction equation by a factor multiplies that change by the same factor. The reason is that enthalpy is a state function: intermediate states cancel from the differences. A cycle supplies one independent balance relation; it cannot determine two unknown inputs without further information.
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Hess's law, also known as Hess's Law of Constant Heat Summation, states that the total enthalpy change for a chemical reaction is the same, regardless of whether the reaction occurs in one step or multiple steps. In other words, the enthalpy change of a reaction is independent of the path taken from reactants to products.