Creationists promote the falsehood that the second law of thermodynamics does not permit entropy to spontaneously decrease, and therefore evolution could not have happened. According to creationists, entropy can only increase, resulting in a "universal decay" of any and all systems. However, the mathematical laws of thermodynamics make it perfectly clear: it is possible for the entropy of a system to spontaneously decrease, providing the over-all entropy of the system's surroundings increases to a greater degree.

Thermodynamics deals in a quantitative manner with the relationship between heat and work. Because of this, its known applications must necessarily be limited to man-made devices and chemical changes for which heat and work parameters can be established. These parameters have been established for a number of biochemical reactions, but this information has not resulted in the general ability to determine the thermodynamics of cell growth in living organisms. Creationists take advantage of this situation by postulating a pseudo science explanation for the obvious flaw in their argument: if all systems can only go in the direction of universal decay, then how can one explain the growth of living things, which is just the opposite of universal decay? Creationist propaganda postulates, with no scientific justification whatever, an "energy conversion mechanism" for living things that "overcomes" the laws of thermodynamics. However, in the case of the evolution of living things, this "energy conversion mechanism" is strangely absent!

The only actual mathematical relationship between entropy and probability is based on the probability of distribution of molecules in a hypothetical "ideal gas." Creationists state that because a flame can not "unburn," its combustion must always result in a 100% increase in entropy. That statement is false, and not supported by the laws of thermodynamics. For example, the Servel gas powered refrigerators operate with a gas flame and no moving parts to produce an entropy decrease in the interior.

Most thermodynamic equations represent a change in the properties of a system when it is changed in some manner. Some examples of change are: (1) A chemical reaction between two interacting systems, as hydrogen and oxygen combining to form water; (2) Absorption of heat by a system, as when heat flows into a house during the summer; (3) Absorption of work by a system, as when air is pumped into a tank; (4) Work done by a system, as when air under pressure runs an air motor. When these kinds of changes take place, there is change in the properties of the system. A mixture of two gases becomes a liquid; the temperature of the house interior rises; the density and pressure of the air changes. Changes in properties of a system are indicated mathematically by the Greek letter capital delta: . The symbol indicates an increase or decrease of the quantity immediately following.

The fundamental definition of the second law of thermodynamics is given by the following equation:

         S = q/T          (1)

Where:   q = heat absorbed by the system
         T = absolute temperature
         S = entropy content of the system
         S = a change in the entropy content S

If the system does work, or has work done upon it, then:

         E = q - w        (2)

Where:   w = work done by the system
         E = energy content of the system
         E = change in energy content

We can substitute equation (1) into equation (2) and obtain:

         E = TS - w       (3) 

The above equations are based on constant temperature conditions. Thermodynamics is not limited to constant temperature conditions, but it is outside the scope of the present application to discuss the effect of temperature variation.

Although equation (2) is valid for any process, equation (3) is valid for reversible processes only. (reversible and irreversible processes will be defined later) In other words, for any process involving a work effect, q = TS only when the process is reversible. However, for a simple heat flow between a system and its surroundings (no work involved), q = TS even when the process is not reversible.

Since equation (3) applies only to reversible processes, special symbols are assigned to the work term w to indicate that the work is performed reversibly. Ordinarily, changes take place under the constant pressure of the atmosphere, and the work term w must include the work against atmospheric pressure (or assisted by the pressure of the atmosphere). This atmospheric work term is designated by PV.

where:   P = atmospheric pressure
         V = change in volume 

The difference between the total reversible work term and the atmospheric pressure work term is designated as a change in G, the Gibbs free energy content:

         w - PV = -G      (5)

The minus sign in front of G indicates that when work is done, the free energy content of the system is reduced.

Combining equations (3) and (5), we obtain:

         E = TS + G - PV   (6)

The sum of the terms E + PV is designated as the enthalpy H. So equation (6) becomes:

         
         G  = H - TS     (7) 

Definitions of reversible and irreversible processes.

As stated previously, the above equations apply to reversible processes. A reversible process is one in which proceeds in such a manner that every step is characterized by a state of balance, in which the process could be reversed by an infinitesimal change in conditions. The concept of reversibility is used as a mathematical tool to develop fundamental thermodynamic relationships, and is very useful in that respect. Nevertheless, no real process is reversible, as that would require infinitesimal temperature differentials and friction losses, and infinite time. Therefore the free energy change G can not actually be completely utilized experimentally. Nevertheless, it can be readily measured experimentally; one procedure is to determine the emf of a voltaic cell under conditions of zero current flow.

All actual processes are therefore "irreversible."

"Irreversible," as used in thermodynamics does NOT mean that the process can not necessarily be reversed by one means or another. It simply means that under the existing conditions it will not spontaneously reverse itself.

Entropy S, internal energy E, and enthalpy H, like pressure, temperature, and volume are "state functions." They depend only on the present condition of the system. Thus the change in entropy in going from state A to state B is always the same, regardless of the path by which the change took place. The change in the entropy of a system in going from state A to state B is the same, regardless of whether or not the process was reversible. However, the overall change of the entropy of a system plus its surroundings will vary in accordance with the degree of irreversibility.

In calculating the change of entropy, it is generally necessary to determine the amount of heat q absorbed (or evolved, if there is a decrease in entropy) when the process takes place reversibly. In some cases the change in entropy accompanying a process will be the same, regardless of whether or not the process is reversible or irreversible. This is the case when there is no work energy that could be transferred as a consequence of the change. An example of this would be the heat transfer when a hot stone is dropped into a bucket of cold water.

A clear distinction must be made between the entropy change of a system and the overall entropy change of a system and its surroundings. In a reversible process the entropy change in a system due to the action of the surroundings is equal and opposite in sign to the entropy change of the surroundings. Therefore the overall net entropy change is zero. In an irreversible process the entropy change in the surroundings is not equal to the entropy change in the system. Therefore the overall net entropy change is greater than zero.

In many cases the entropy change in a system is calculated on a theoretical basis as the entropy change in the surroundings for a reversible process, even though such a process can not actually be carried out experimentally. In other words, the concept of reversibility is a mathematical tool. All real processes are irreversible, although in some cases they may be a very close approximation to reversible processes.

G can be determined by direct measurement of the emf of a voltaic cell. It can also be calculated from absolute entropy values obtained from measurements of heat capacity versus absolute temperature. The mathematics of thermodynamics permits the calculation of the effect of pressure and temperature on values of S, H and G. The theoretical maximum potential for useful work is represented by G. When the value of G calculated from equation (10) is negative, the change can occur, although the use of a catalyst may be necessary to make it happen. Note that it is not always necessary for heat to evolve for a change to occur spontaneously. H can be zero or even less than zero.

When H is zero, equation (10) becomes:

       G = -TS       
  

An example of H equal to zero is the free expansion of a perfect gas. The internal energy of the gas is unchanged on free expansion, and no P-V work is done. In this case there is an entropy change even though there is no energy flow. If the same change takes reversibly, with no heat flow (adiabatic conditions), the work output is equal to G. G is the same in either case, except that in the reversible process it represents the work actually obtained, while in the irreversible free expansion process it represents the work that could have been obtained.