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chemical affinity[′kem·i·kəl ə′fin·əd·ē]
a term used to describe the capacity of a substance to react with another substance or to describe the degree of resistance of the resulting compound to decompose into the initial components.
At various times, attempts were made to evaluate chemical affinity in terms of various reaction parameters. In the mid-19th century, the quantity of heat released in the course of a reaction came to be used as the measure of affinity. However, the existence of spontaneous endothermic reactions indicated the restricted applicability of this approach. In 1883, J. van’t Hoff proved, on the basis of the second law of thermodynamics, that the course of a spontaneous reaction is determined by the reaction’s maximum useful work rather than its thermal effect. At the same time, he derived an equation that quantitatively expressed the maximum useful work as a function of the concentrations of the substances taking part in the reaction and the direction of the reaction as a function of the ratios of these concentrations.
At present, attention is being focused not on the maximum work but rather on changes in the Gibbs free energy ΔG for reactions proceeding at constant temperature and pressure or in the Helmholtz free energy ΔA for reactions proceeding at constant temperature and volume. In this case, the concept of chemical affinity is no longer applicable.