Hess's Law

Hess's law states that for a chemical equation that can be written as the sum of more than one step, the enthalpy change for the overall equation equals the sum of the enthalpy changes for the individual steps.  This is because enthalpy is a state function.  The reason this is important to  us is that it gives us a way to calculate heats of certain reactions that do not lend themselves to direct measurements, or even reactions that may not actually occur.

The reaction usually used to illustrate the usefulness of Hess's law is the combustion of graphite to carbon monoxide.

          (1)  2 C (graphite) + O₂(g) ==> 2 CO(g)    H = ?

It is a very difficult reaction to measure H for this reaction, because it is difficult to get the reaction to stop at CO(g).  The CO(g) is likely to react further with O₂(g) and form CO₂(g) as shown in equation (2).

        (2)  2 CO(g) + O₂(g) ==> 2 CO₂(g)    H = -566.0 kJ

Also, C(graphite) can react with  O₂(g) to form CO₂(g) directly according to equation (3).

        (3)  C (graphite) + O₂(g) ==> CO₂(g)    H = -393.5 kJ

By combining equations (2) and (3) in the correct manner, we can get equation (1) and its  H value.

• reverse the direction of equation (2) and change the sign of its  H
• double equation (3) and its H
• add together the results of the above two steps

                                        2 CO₂(g) ==> 2 CO(g) + O₂(g)         H = (-1)(-566.0 kJ) = + 566.0 kJ

               2 C (graphite) + 2 O₂(g) ==> 2 CO₂(g)                     H = (2)(-393.5 kJ) = -787.0 kJ