Unit 13 - Lesson 5: Hess's Law

Hess’s Law:

the overall enthalpy change in a reaction is equal to the sum of enthalpy changes for the individual steps in the process.

It is a consequence…

 First Law of Thermodynamics: matter cannot be created or destroyed, but it can be transformed into different forms. This is another way to describe the Law of Conservation of Mass.

Rule #1:

If a Chemical Reaction has a specific ΔH, then the reverse reaction would lead to a ΔH with the opposite sign.

For example, if X + Y → Z has a ΔH of 125kJ, then Z → X + Y has a ΔH of -125kJ.

Rule #2:

If the coefficients of a Chemical Reaction are raised to a certain number, then the value of ΔH is multiplied by the same value.

For example, if X + Y → Z has a ΔH of 125kJ, then 2X + 2Y → 2Z has a ΔH of 250kJ.

The same is true for making the coefficient “smaller”.

For example, if X + Y → Z has a ΔH of 125kJ, then ½X + ½Y → ½Z has a ΔH of 62.5kJ.

What is the purpose of Hess’s Law?

Hess’s Law is just another way that we can determine the Change of Enthalpy of a reaction; similarly to using the difference in Bond Energies between Covalent Compounds.

We can do some quick calculations to determine if a reaction is incredibly exothermic or endothermic by using known data as opposed to drawing out Lewis Structures every single time.