Entropy and the Second Law of Thermodynamics

E and H describe the energy changes, but tell nothing about the favored direction for a process. To do this, one must take into account the degree of randomness or disorder of a system. The degree of randomness or disorder of a system is measured by a state function called the Entropy (S). Entropy is defined as S = kln(W), where k is the Boltzmann constant (the gas constant R divided by Avogadro's number) and W is the number of thermodynamic substates of equal energy.

The entropy of an ordered state is lower than that of a disordered state of the same system. For example, there are more ways to put a large number of molecules in a random or disorderly arrangement than there are to put them in an orderly arrangement. Thus, the increasing entropy in a system is a thermodynamic driving force.

The second law of thermodynamics states that the entropy of an isolated system will tend to increase to a maximum value. However, this form of the second law is of little use biologically because it applies only to isolated systems (systems that do not exchange energy with their surroundings). Most biological systems, however, are open - they exchange energy and matter with their surroundings. Thus, biological systems undergo changes in energy and entropy in many reactions, and both must determine the direction of thermodynamically favorable processes. The Gibbs Free Energy (G) is a function of state that includes both energy and entropy terms:

G = H-TS, where T is the absolute temperature, H (the enthalpy) measures the energy change at constant pressure, and S (the entropy) measures the randomness of the system. At constant temperature and pressure,

G = H -TS

A decrease in energy (-H) and/or an increase in entropy (+S) tends to make a process favorable. Either a negative H or a positive S tends to make G negative. Thus, the second law can be restated for open systems as follows:

> 1. G must be negative for a process in an open system to be favorable at constant temperature and pressure.

> 2. A positive G indicates a process is not favorable.

See also: Internal Energy (E), Enthalpy