Hess's law

The Hessian heat set ( also set by Hess ) is used to calculate enthalpy changes in chemical reactions. It was put forward in 1840 by Germain Henri Hess.

In many cases, products of a chemical reaction can occur in various ways. The reaction pathway continued to have the energy conservation law does not affect the energy that is absorbed or released in a reaction. After Hess'schen heat set the same is true for the enthalpy of a chemical reaction.

Statement

The change in enthalpy of an overall process is the sum of the enthalpy changes of the individual process steps.

It follows that the reaction enthalpy from the pathway does not, only the initial and final states of the system depends.

Example

Graphite can be burned directly to carbon dioxide (1 ) or indirectly via the intermediate carbon monoxide ( 2), ( 3):

The Gesamtreaktionsenthalpie is same in both cases: