CHEM 1A03 LECTURE 13 – ENTROPYAND FREE ENERGY
Although many exothermic reactions are spontaneous, there is NOT a direct correlation
between ∆H and spontaneity.
Example: rust on a car is SPONTANEOUS – adds to entropy of universe.
Then what criteria do we use?
The Boltzmann Equation: S = k ln(W) W = # of ‘microstate’, k = Boltzmann constant
The greater number of configurations (or microstates) consistent with “macrostate”, the greater
the entropy of the system. Sort of like the degree of disorder.
∆S – gas expansion or mixing causes an increase in entropy
∆S throughout Phase Changes: In decreasing order: Vapor > Liquid > Solid
Absolute molar entropy increases with increasing Temperature
Dissolution: Entropy of a solution > (Entropy of solvent + Entropy of solute)
Calculating ∆S o
Third Law of Thermodynamics: The entropy of a pure, perfect crystal at 0K is zero. Therefore
we have absolute entropies.
∆ S= ∑ vS(products)−∑ vS(reactants)
Second Law of Thermodynamics:
∆S universeSsystem ∆Ssurroundings
All spontaneous processes result in an increase in the entropy of the universe.
Even if a reaction is exothermic, if the ∆n < 0, then work is done ON the sysrxn ∆S
What about ∆S surroundings?
For a process at constant pressure and temperature, recall: qp=∆H sys and qsurr -q p -∆H sys
If we are able to exchange heat reversibly, in infinitesimal