- Physical Chemistry I
Spring 2013, Unique 52575
Lecture Summary, 18 February 2013
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.