= Hess's law
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<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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