Hess's law says that reaction enthalpy doesn't depend on the route you take from the reactants to the
products. (A more compact way to state Hess's law is "enthalpy is a state function".) The law is a direct consequence
of the fact that energy is conserved. If a reaction could proceed by two different routes that gave two different
reaction enthalpies, reversing the reaction by one route and going forward by the other would give you a way
to create energy out of nothing- and that's not possible.
Hess's law is important because it provides a practical way to combine thermochemical equations for
known 'step' reactions to get a thermochemical equation for some 'target' reaction.
The basic procedure is this:
- Write out the thermochemical equations for the step reactions.
- Write the balanced chemical equation for the target reaction.
- Reverse step reactions so products/reactants match the target reaction.
- Scale step reactions so products/reactants that don't appear in the target reaction will cancel out.
- Add the step reactions.
- Scale the resulting reaction so it matches the target reaction.
Author: Fred Senese email@example.com