Why is Hess' law useful to calculate enthalpies?

Examine the following reaction to see how anhydrous copper (II) sulfate is hydrated:

This reaction is an example of one for which it is not possible to calculate the enthalpy change directly. This is because water would need to serve as both a hydrating agent and a temperature gauge simultaneously, within the same water sample, which is not possible.

However, we are able to measure the enthalpy changes for the solvation of hydrated and anhydrous copper (II) sulfate. We can then use this information to calculate the enthalpy change of our original hydration because of Hess' Law.

Calorimetry frequently has severe inefficiencies, especially in school laboratories, and using data from two reactions instead of one does double the uncertainty; however, this is our only option because we couldn't get the data we needed any other way.

Hess' law is useful for calculating enthalpies because it allows us to determine the overall enthalpy change of a reaction by summing up the enthalpy changes of individual steps, even if those steps cannot be measured directly. This is particularly helpful when direct measurement of the enthalpy change is not feasible or practical.