CH353 - Physical Chemistry I
Spring 2012, Unique 52135

Lecture Summary, 6 February 2012

Second Law of Thermodynamics: We have spent quite a bit of time developing the first law of thermodynamics, which we have expressed as energy = heat + work.  We have found that we can find heat, work, internal energies, and enthalpies for a variety of different systems and different paths.  But clearly we are missing something.  For example, an an isothermal expansion of an ideal gas, deltaU = 0, but under some conditions, w < 0; i.e. the system does work on the surroundings.  How is this possible?  How can we do work without a change in internal energy?  Furthermore, we have seen that some transformations occur spontaneously only in one direction, even if deltaU = 0.  For example, if we open a bottle of perfume in a room, soon perfume molecules will fill the entire room, but the reverse process will never happen.  Finally, we know of simple examples in which a reaction occurs spontaneously even if it requires energy from the surroundings, for example sweating. 

We are clearly missing a kind of energy that will provide a driving force for spontaneous events to occur even with no change in internal energy.  We developed a state function, called entropy, S, which defines the direction of spontaneous change (deltaS > 0), and which will quantify this driving force:

      deltaS >= q/T.