Chapter 11

Thermochemistry

In the last chapter, we introduced the equilibrium constant, which allows us to decribe in a
quantitative way just how favorable a reaction is. Le Châtelier’s principle, in addition, allows us to
predict qualitatively how the equilibrium position of a system would react to changes in external
conditions. A more fundamental question we did not try to answer, however, is: What makes a
reaction favorable in the first place? Why does Mother Nature prefer some reactions to others? In
this chapter we will introduce and examine the concepts of enthalpy, entropy, and Gibbs free
energy, all of which will give us a fuller understanding of why reactions take place.

From HCl dissociating in water to give protons and chloride ions, to the synthesis of polypeptide
chains (proteins) from amino acids in our cells, to the breakdown of ozone in the stratosphere, all
chemical reactions have in common the fact that they are accompanied by energy changes.
Whenever a reaction takes place, the atoms involved find themselves in a different environment
than before. Old bonds have been broken while new ones have been formed; they may now be
surrounded by solvent molecules rather than “one of their own,” and so on. The amount of energy
still residing in the system, and the way that this energy is being distributed among the species in it,
may very well have changed. It is these changes that dictate the favorability of a reaction, and form
the heart of the study of chemical thermodynamics, or thermochemistry.

Systems and Surroundings:   The First   Law of  Thermodynamics

Heat

States  and State   Functions