CH353 - Physical Chemistry I
Spring 2012, Unique 52135

Lecture Summary, 30 March 2012

Statistical Thermodynamics: Statistical thermodynamics is a branch of physical chemistry that is concerned with connecting macroscopic properties of a system (pressure, temperature, volume, free energy, etc.).  Although we have gone through this semester without caring about the molecular nature of our "system," there must be a reason why a collection of atoms and molecules will display these properties.  Stat mech figures out how properties of individual atoms or molecules combine to create properties of a system, which is nothing more than a very large number of atoms or molecules. 

All energy is quantized.  A collection of atoms or molecules occupies a ladder of energy states.  Once a system contains any significant quantity of species, it will not be possible to just count up the number of possible configurations and determine the most likely state.  We must use the mathematics of statistics and probability the figure out how the system is most likely configured.  As long as we do this correctly, our guess at what the system probably looks like will probably be a pretty good guess. 

To do this, we need to impose two constraints on our system.  1) The total number of atoms or molecules cannot change.  At any point, we must be able to account for every particle in our system.  2) The total energy of the system cannot change (i.e. we are not doing anything to our system; it's static, sitting doing nothing).

The solution to this problem is expressed by the Boltzmann distribution, which is in your book.  We will see how this allows us to connect microscopic to macroscopic properties.