CH353 - Physical Chemistry I
Spring 2012, Unique 52135

Lecture Summary, 15 February 2012

Entropy based on configurations:  Today we continued our discussion of entropy based on the number of configurations available to a thermodynamic system.  We derived an expression for increasing entropy with increasing volume.  We also saw explicitly that the third law of thermodynamics is a direct consequence of Boltzmann's formulation for entropy.  At 0 K, a perfectly pure crystalline substance has only one possible configuration of the atoms in the crystal, and therefore the molar entropy of the system is 0.

Refocus on the system:  Although the 2nd law provides us with a convenient definition of the direction of spontaneous change, it is cumbersome to have to quantify what is going on in the surroundings as well as the system.  We would like to be able to focus only on the system.  So we derived an expression to combine information on changes in energy and change in entropy during a thermodynamic transformation:

   dG = dH - TdS

Furthermore, we found that if we apply our 2nd law condition of a spontaneous process, we saw that dG < 0, the process will proceed without any additional input of work.