The role of entropy, free energy in assessing thermodynamic spontaneity

Thermodynamic spontaneity—whether a reaction can go—can be measured by changes in either of two parameters: entropy or free energy.

- TAPAN KUMAR MAITRA

These concepts are abstract and can be somewhat difficult to understand.

EntropyIt is represented by the symbol S. For any system, the change in entropy, ΔS, represents a change in the degree of randomness or disorder of the components of the system. For example, the combustion of paper involves an increase in entropy because the carbon, oxygen, and hydrogen atoms of cellulose are much more randomly distributed in space once they are converted to carbon dioxide and water. Entropy also increases as ice melts or as a volatile solvent, such as gasoline, is allowed to evaporate.

Entropy change as a measure of thermodynamic spontaneity. How can the second law of thermodynamics help predict what changes will occur in a cell? There is a very important link between spontaneous events and entropy changes because, whenever a process occurs in nature, the randomness or disorder of the universe (that is, the entropy of the universe) invariably increases. This is one of two alternative ways to state the second law of thermodynamics. According to this formulation, all processes or reactions that occur spontaneously result in an increase in the total entropy of the universe. Or, in other words, the value of ΔSuniverse is positive for every real process or reaction.