Appendix A Thermodynamics

Thermodynamics is the branch of science that deals with the conversion of one form of energy into another. The laws of thermodynamics govern all materials processes and reactions and can be used to predict how a system and its surroundings will behave under various defined conditions. Changes in temperature (T), pressure (p), and volume (V) all play a role in the overall thermodynamic behavior of a system and can be related to one another through thermodynamic principles. The conditions for equilibrium are established by thermodynamics, but the rate at which equilibrium is achieved cannot be determined by thermodynamics [1]. To better understand thermodynamics, it is important first to define some important thermodynamic concepts. A thermodynamic system can be defined as the material and volume of interest. The system is separated from its surroundings by boundaries (which can be real or imaginary) and can be defined as either closed or open. A closed system contains a fixed mass while for an open system mass (typically in the form of a fluid) flows through the boundaries of the system. A system can also be defined as adiabatic if it is insulated and does not exchange heat with its surroundings or as isolated if the system cannot exchange either energy or matter with its surroundings. A thermodynamic process can be either spontaneous – meaning that once it is started it will proceed to completion without any further stimulation, or nonspontaneous – meaning that energy (in some form) must be supplied to maintain or complete the process. Spontaneous reactions are typically exothermic (heat energy is released upon reaction), and a process that is spontaneous in one direction is non-spontaneous in the opposite direction. In addition to being spontaneous or non-spontaneous, a process is also either reversible or irreversible. A reversible process is one for which, upon an exact reversal of the process, the original states both of the system and of the surroundings are achieved. An irreversible process cannot be restored to the initial conditions simply by reversal of the process, but rather requires external stimulation in the form of work or heat. By this definition, a spontaneous process is also an irreversible process. The first law of thermodynamics deals with the conservation of energy. It states that the total energy of the Universe is constant and energy cannot be created or destroyed, but is rather converted from one form to another or transferred from one system to another. This simple law can be expressed mathematically thus: Usystem þ Usurroundings 1⁄4 0: ðA:1Þ