Many powerful calculations in thermodynamics are based on a few fundamental principles, which are called the laws of
thermodynamics. Begin by reviewing the two main laws of the field.
The first law of thermodynamics asserts that energy is conserved during any process. The three major forms of energy for chemical purposes are the internal energy of each substance, the external work due to changes in pressure or volume, and the exchange of heat with the surroundings.
The internal energy is sometimes called chemical energy because it is the consequence of all the motions of particles and forces between particles: molecules, atoms, nucleons, and electrons.
If no heat flows in or out of a sample of matter, any external work done on or by the sample is precisely offset by the opposite change in internal energy. Expansion against a confining pressure reduces the internal energy, whereas external compression of the system increases the internal energy.
The first law of thermodynamics also tells you that if no work is done on or by the sample—that is, pressure and volume are held constant—any heat flow is counterbalanced by a change in internal energy. An exothermic reaction releasing heat to the surroundings, therefore, is accompanied by a decrease in internal energy, whereas an endothermic reaction has a concomitant increase in internal energy.
The second law of thermodynamics involves entropy, which for our purposes is a statistical measure of the degree of disorder in a chemical system. As an illustration, compare the arrangements of Na + and Cl – ions in both solid and liquid sodium chloride.
Solid sodium chloride has a crystalline structure in which the cations and anions alternate in a repeating pattern. (See Figure 1.)
Figure 1. The crystal structure of NaCl.