A Bluffers Guide to Molecular Thermodynamics

Thermodynamics is a mature subject and it is sometimes difficult not to gel bogged down in the details. The basic ideas were laid down in the early 19th century, at a time when atoms and molecules had scarcely been heard of, and when optimization of the steam engine and other marvels of the industrial revolution were of prime concern. What follows here, though less rigorous than you will find in most textbooks, will be enough to place thermodynamics in a molecular context- 

Most things in the everyday world take place at constant pressure, so the appropriate energy quantity to use is enthalpy. H= U+PV, which comprises the internal cnergy(bO plus a pressure volume corrections term (PV) to take account of any work done with the surrounding if there is a change in volume. Changes in enthalpy during any process will be indicated by AH. 

The disrupting effects of thermal motion at the molecular level are expressed in terms of the entropy changes, AS. These effects will tend to get bigger as the temperature rises. 

The balance between these two opposing effects is expressed in the change in Gibbs free energy  

At thermodynamic equilibrium, when these two effects just balance, AG=0, and, although the individual atoms and molecules are still in motion, the overall average situation undergoes no further spontaneous change. 

---