CH353 - Physical Chemistry I
Spring 2013, Unique 52575

Lecture Summary, 18 February 2013


Free Energy:  For the past couple days we have been practicing solving various free energy problems using the natural variables and Maxwell relations.  There are often many ways to solve a particular problem; it is often possible to make a good guess at where to begin, but always remember that if you reach a dead end, all you need to do is go back to the beginning and try another starting point.  Being comfortable with the basic starting points will give you the tools necessary to manipulate those starting points however you need.

Spontaneous transformations:  Now that we know a variety of ways to determine free energy:

  dG = dH - TdS
  dG = VdP - SdT,
 
etc., we can determine the direction of spontaneous change for any thermodynamic transformation.  If the change in free energy of the system is < 0, the reaction will proceed spontaneously.  If deltaG > 0, then the process will not go forward spontaneously, and we need to change the system in some way to make that happen, for example by changing temperature or pressure.  If deltaG = 0, the system is in equilibrium, meaning that free energy considerations in both the forward and backwards direction are balanced, and the system will not change any further.  The magnitude of deltaG does not yet matter to us - a larger negative number does not make that process "more" spontaneous than another.  The only questions we care about now are if deltaG > 0, < 0, or = 0.