In the first part of this unit, we presented
the first law of thermodynamics, which states energy is conserved
during any process. In this section, we are going to talk about
the second law of thermodynamics, which involves entropy. Entropy
is a quantitative measurement of the degree of disorder in a
chemical system. The second law of thermodynamics states that
changes within a system favor higher entropy. The third law of
thermodynamics states that the entropy of a perfect crysatl at 0K
is 0. At any temperature higher than 0K, vibrations within the
atoms begin, and disorder results. In your book, there should be
a table of standard entropies for most common compounds at 25°C,
symbolized S°. For a chemical reaction, the 
S is calculated by using the following
equation:
S = S°(products) -
S°(reactants)
Reactions are more likely to occur if there is an increase in
entropy. A positive
S for a reaction means that the products have more entropy, thus
being more favorable. However, you must also take enthalpy into
effect to determine whether a reaction will take place.
