Hess’s Law can be considered an extension of the Law of Thermodynamics. It states that if a reaction can take place by more than one route from the same initial and final conditions then the total enthalpy change is the same for each route.
A Hess Cycle can be used to work out Enthalpy Changes.
There are textbooks filled with values for Enthalpies of Formation for various compounds.
You can use Hess’s Law along with these values to work out enthalpy changes for many kinds of reactions.
Drawing Hess Cycle
Write all relevant equations as per definition. This means that if ΔHf is given the reaction describing the enthalpy of formation is written.
Once the equations are all written down combine these equations together to create a cycle.
Example 1:
What is the enthalpy change of the following reaction if the ΔHf of MgO is -601 kJ mol-1 and ΔHf of Al2O3 is-1677kJ mol-1?
The first thing that needs to be done is to write the enthalpy of formation as equations. These would be:
Once all the equations are written, it is all about joining these equations into a cycle.
Once the cycle is drawn the values for the clockwise reactions and the values for the anticlockwise reactions are noted.
As per the definition of the Hess cycle, any route taken will have the same amount of energy