Hess' Law

What is Hess’ law?

The total enthalpy change for a reaction is independent of the route taken

How do you use Hess’ Law to work out enthalpy change?

• It states that the total enthalpy change for route 1 is the same as for route 2, so if you know the enthalpy changes for the stages of route 2, you can calculate the enthalpy change for route 1.

• You would use either the enthalpies of formation or combustion to find the enthalpy changes for a route.

How do you use the enthalpies of formation to calculate enthalpy change?

• You would need to know the enthalpy of formation for all the reactants and products that are compounds.

• The enthalpy of formation for elements is 0

• You would then add up the enthalpies for each compound depending on the reaction

How do you use the enthalpies of combustion to calculate enthalpy change?

You can use a similar method to the enthalpies of formation but the direction of the arrows would be different.