Barry Sturman
ABSTRACT: This is the second in a series of articles, prepared at the request of the publisher of this Journal, presenting an introductory outline of chemical thermodynamics and chemical kinetics, with emphasis on those aspects of particular relevance to pyrotechnics. The First Law of Thermodynamics, which was the subject of the first article, cannot explain the direction of change in the physical world. The Second Law of Thermodynamics states that spontaneous change is always associated with an increase in the entropy of the universe. The universe is simply the system of interest plus its surroundings, assumed to be isolated from external influences, while the entropy is a thermodynamic state function. Much of this article is taken up by a discussion of entropy. The relationship between entropy and spontaneous change is clarified when entropy is interpreted as a measure of molecular disorder. The Gibbs Free Energy is a thermodynamic state function that allows the entropy change in the system and its surroundings to be predicted from the thermodynamic properties of the system alone. It provides the basis for predicting the direction of change in chemical systems. Finally, the Third Law of Thermodynamics states that the molar entropy of a pure substance is zero at the absolute zero of temperature. This is developed from Boltzmann’s relationship between entropy and the number of molecular arrangements consistent with the properties of a system. It is shown how the Third Law permits the calculation of absolute values for the molar entropies of pure substances.
Keywords: thermodynamics, entropy, free energy
Ref: JPyro, Issue 12, 2000, pp31-42
(J12_31)
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