Entropy

Abstract

This chapter explores the property called ‘entropy’, which is central to the formulation of the ‘Second Law of thermodynamics’. Entropy is the concept on which almost all applications of thermodynamics in chemistry are based: it explains why some physical transformations and chemical reactions are spontaneous and others are not. A change in entropy is defined in terms of reversible heat transactions. The Carnot cycle is used to prove that entropy is a state function. Meanwhile, the efficiency of a heat engine is the basis of the definition of the thermodynamic temperature scale and one realization of such a scale, the Kelvin scale. The chapter then looks at the Clausius inequality, which is used to show that the entropy of an isolated system increases in a spontaneous change and therefore that the Clausius definition is consistent with the Second Law.