Hess's Law states that energy cannot be created or destroyed; it can only be changed from one form to another. Hess's Law is used to predict the enthalpy change for a reaction ΔH_r in enthalpy changes.

The enthalpy change for a chemical reaction leading from reactants to products is the same no matter which route is chosen for the reaction. This is called a state function. That is, the enthalpy change going from reactants to intermediates 1 then to products is the same as for going from reactants to intermediates 2 then to products.

Addition of chemical equations can lead to a net equation. If energy is included for each equation and added, the result will be the energy for the net equation.

Hess's Law says that enthalpy changes are additive. ΔH_net = ΣΔH_r. Two rules:

• If the chemical equation is reversed, the sign of ΔH changes.
• If coefficients are multiplied, multiply ΔH by the same amount.

see also Chemistry