= Hess's law
{c}
{wiki}
<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.
The law is named after <Germain Henri Hess>.
For specified initial and final <thermodynamic states>, the change is $\Delta H=H_{\rm final}-H_{\rm initial}$. Because <enthalpy> is an <extensive variable>, a quoted reaction $\Delta H$ belongs to the <amount of substance> specified for each species in its <chemical equation>. Reversing the equation changes the sign; scaling every coefficient scales $\Delta H$; adding compatible equations adds their $\Delta H$ values. A per-<mole> value must say which <chemical equation> and normalization it uses. Match <temperature>, <pressure>, <thermodynamic phases>, <chemical solution> compositions and reference states before cancellation; the same formula in a <gas> and a <liquid> represents different states.<Chemistry 2e by Paul Flowers and others/5.3 Enthalpy>[Chemistry 2e, §5.3: enthalpy conventions before Example 5.8; Hess’s Law, equation scaling and reversal]{ref}
The value of <Hess's law> is that an inaccessible reaction <enthalpy> can be obtained from measurable transformations. The algebra does not require those transformations to be steps of the actual <reaction mechanism>. Neither an <activation energy> nor a <reaction rate> follows from the endpoint difference: a <catalyst> may change the <reaction rate> while preserving the compatible initial and final <enthalpies>.
Back to article page
