CHM 1046
General Chemistry II
Dr. Michael Blaber

Chemical Thermodynamics

A Molecular Interpretation of Entropy

Many processes can lead to an increase in a system's entropy

Some processes can lead to a decrease in the entropy of a system

2NO(g) + O2(g) ® 2NO2(g) DS < 0

What is the molecular basis for the above observations for the change in entropy?

Let's first consider the last example, the decrease in entropy associated with a decrease in the number of gas molecules for a chemical reaction

2NO(g) + O2(g) ® 2NO2(g)


Degrees of Freedom of molecules

Individual molecules have degrees of freedom related to their motions within a substance

These forms of motion are ways in which the molecule can store energy

The Third Law of Thermodynamics

If we lower the temperature of the system, the thermal energy decreases

The Third Law of Thermodynamics: The entropy of a pure crystalline substance at absolute zero (i.e. 0 Kelvin) is 0.

Continued heating of a solid lattice

In general, the entropy is expected to increase for the following types of processes:

  1. The melting of a solid to form a liquid
  2. The vaporization of a liquid (or solid) to produce a gas
  3. Chemical reactions that involve phase changes of solid ® liquid/gas, or liquid ® gas
  4. Chemical reactions that result in an increase in the number of gaseous molecules
  5. Any time the temperature of a susbstance is increased

2000 Dr. Michael Blaber