Unit 5: Thermochemistry

Section 5: Entropy

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 crystal 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.