Reference: Daniel V. Schroeder, An Introduction to Thermal Physics, (Addison-Wesley, 2000) – Problem 5.2.
The Gibbs free energy is defined as
The enthalpy is the total energy required to create the system from nothing, in which the environment at constant pressure must be pushed back to create the volume in which the new system is to be stored.
For a process occurring at constant temperature and pressure, the change in is
We can calculate changes in for a process such as a chemical reaction by considering the values for the reactants. Consider the reaction in which nitrogen and hydrogen combine to form ammonia:
We can look up the relevant values in Schroeder’s book, where he gives values for 1 mole at and a pressure of 1 bar. The tabulated values are
For the reaction 3 we combine 1 mole of with 3 moles of to get 2 moles of , so we have
This value is for 2 moles, so for one mole of we have
which is close to the value of given in Schroeder’s table. (I’m not sure if we’re supposed to be able to get closer with the given data.)