CH353 - Physical Chemistry I
Spring 2012, Unique 52135

Lecture Summary, 8 February 2012

Second Law of Thermodynamics: We have already seen that the second law of thermodynamics defines the direction of spontaneous change of a thermodynamic process.  However, in order to apply this, we have to know the total change in entropy of the universe, not just the change in entropy of our system:

    deltaS(tot) = deltaS(sys) + deltaS(surr) >= 0

The equality applies of the process is reversible, the inequality applies in all other cases.  This means that we have to determine not only the entropy change of the system but also of the surroundings.  Our rules for solving entropy problems are:

   1.  For any reversible process, deltaS(tot) = 0
   2.  To find deltaS(sys), use a reversible path from initial to final states.  You can do this because entropy is a state function and doesn't depend on the actual path.
   3.  Determine deltaS(surr) independently.  If the process is reversible, you already know deltaS(surr) = -deltaS(sys).  If the process is irreversible, deltaS(surr) = q(surr)/T, and you will have to figure out what q(surr) is.