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15.4.1 Criterion for Spontaneous Process

It is a law of nature that things always go in the direction of creating greater chaos— this is the second law of thermodynamics. Any system, except those at temperature equals absolute zero, is always disordered.* The energy required to maintain this disorder, we have found, is called entropy. As mentioned, any system possesses free energy at any instant, this energy being the net energy remaining after the entropy required to maintain the current disorder has been subtracted from the enthalpy (energy content).

When the system goes from state 1 (current state) to state 2, its free energy at the latter state may or may not be the same as the former. If the free energy at state 2

* At absolute zero, all particles practically cease to move and are therefore structured and orderly. © 2003 by A. P. Sincero and G. A. Sincero is the same as that in state 1, the system must be in equilibrium. If the free energy at state 2 is greater than that at state 1, then some outside free energy must have been added to the system. In Table 15.2, this is the case of nitrite as a donor and ammonia as a donor. External free energies of 9.43 kcal/electron-mol and 7.85 kcal/electron-mol, respectively, has been added to the system; these values are indicated by the plus signs. External sources of energy are being required, so these half-cell reactions cannot occur spontaneously; they are said to be endothermic (i.e., requiring external energies to effect the reaction).

On the one hand, when the free energy at state 2 is less than that at state 1, some energy must have been released by the system to the surroundings, thus manifesting in the decrease of free energy. A decrease in free energy is indicated in the table with a negative sign. This energy has been released "voluntarily" by the system without some form of "coercion" from the surroundings. The release is spontaneous, and therefore the reaction is spontaneous.

Note: Thus, this is the criterion for a spontaneous process: When the free energy change is negative, the process is spontaneous.

Judging from Table 15.2, when the electrons that travel through the electron transport system are finally accepted by oxygen, a large amount of energy equal to 18.68 kcal/electron-mol is liberated. This liberated energy is then captured in the bonds of ATP. The same statement holds for the others whose free energy changes have negative signs. Thus, any material in wastewater, edible by organisms, will release energy, resulting in their destruction. The more energy that can be released, the easier it is to be treated using microorganisms.

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