Hess's Law

Hess's law of heat summation states that the enthalpy change of an overall process is the sum of the enthalpy changes of the individual steps of the process.

Because of this, we can select any number of steps with known enthalpy changes that lead us from an initial state to a final state even if in reality the process does not follow the path we have chosen.

Example
If we know how much energy it takes to heat a can of soup by 100 degrees and how much is released when it cools by 10 degrees we can work out how much energy it takes to heat it by 90 degrees. We do this by adding the enthalpy changes of heating by 100 degrees and cooling by 10.

∆H_{(heating by 90)} = ∆H_{(heating by 100)} + ∆H_{(cooling by 10)}

In reality if we heated the soup by 90 degrees we wouldn't first heat it by 100 then cool it by 10, but we can still imagine doing it this way in order to work out how much energy was needed.

(Hess' or Hess's? Hess's.)