Historical Observations on Laws of Thermodynamics

Abstract

Most textbooks quote four anonymous laws of thermodynamics (zeroth, first, second, and third), but it seems that every author has their own idiosyncratic statements. Why are there so many versions? Why are the laws of thermodynamics not credited with names of their discoverers? We revisit the history of the laws of thermodynamics and consider whether it would be less confusing, to both students and practitioners, if we define separate laws for reversible and irreversible thermodynamics and simply assign names to them. Central to our understanding of chemical thermodynamics are the concepts of “equilibrium” and “state functions”; these require definition before the various laws can be properly formulated. The idea of a state function is implicit in Black’s caloric theory of heat and also suggested by Priestley in phlogiston theory, but it was Gibbs who first represented thermodynamic properties as two-dimensional “surfaces” in 1878; that is, 30 years after the various principles that became the laws of thermodynamics had been discovered. It was even later that Duhem rigorously specified equilibrium states in general, and hence state functions. Here we conjecture that confusion has arisen because there should be two different sets of laws: one for equilibrium thermodynamic processes, and another set for irreversible processes. Then we can identify the laws of equilibrium thermodynamics for changes in enthalpy, energy, and entropy, which can be credited to Hess, Rankine, and Carnot, and corresponding laws of irreversible thermodynamics assigned to Joule, Mayer, and Clausius, respectively.