Hess's Law
Hess’s Law states that the enthalpy change for a chemical reaction is independent of the route taken. This means that the enthalpy change for the overall process will be identical regardless of how many steps are taken.
Consider the following reaction:
\(A + B\,\,\,\rightarrow\,\,\, C + D\,\,\,\,\,\,\,\,\,\,\Delta H_{x}\)
The enthalpy change for the reaction that forms products C+D directly \((\Delta H_{x})\) will be the same as the sum of the enthalpy changes for the production of C+D via an indirect route where intermediate products are formed and subsequently react to produce C+D.
This is shown in the diagram below.
One method of calculating an enthalpy change for a process involves rearranging a set of given reaction equations with known values.
The following points must be noted when manipulating equations:
- The enthalpy change for the process (\(\Delta H\)) is independent of the route taken (This is Hess’s law).
- The enthalpy change (\(\Delta H\)) is proportional to the quantities of reactants and products. For example, burning twice as much fuel will result in twice the enthalpy change for the process.
- If a reaction is reversed then the sign of the enthalpy change must also be reversed.
- Changing the physical state of any reactant (or product) will involve an enthalpy change.
