Julius Thomsen and classical thermochemistry

ArticleinThe British Journal for the History of Science 17(03):255 - 272 · November 1984with4 Reads
DOI: 10.1017/S0007087400021294
Classical thermochemistry is inextricably bound up with the problem of chemical affinity. In 1851, when Julius Thomsen began his career in thermochemistry, the concept of chemical affinity had been in the centre of chemical enquiry for more than a century. In spite of many suggestions, preferably to explain affinity in terms of electrical or gravitational forces, almost nothing was known about the cause and nature of affinity. In this state of puzzling uncertainty some chemists felt it more advantageous to establish an adequate experimental measure of affinity, whatever its nature was. One way of providing affinity with a quantitative description was by means of the heats evolved in chemical processes.
    • "where p 1 and p 2 are the partial pressures of each of the two gases involved in the dissociation of the solid. Thus, in the case of decomposition of ammonium chloride we write: p 1 Á p 2 ¼ constant; similarly, in the case of decomposition of ammonium carbamate we have: p 1 Á p 2 2 ¼ constant.Still, the key idea was Horstmann's suggestion considering that the cause of dissociation was the tendency of entropy to increase, thus denying the Thomsen–Berthelot principle according to which the evolved heat was the cause or determining factor (Dolby 1984; Kragh 1984). That assertion implied that the equilibrium condition was dS = 0. Duhem (1893, p. 104) recognised Horstmann's contributions as the foundation of 'chemical mechanics', which he summarised as follows: 'An absolutely isolated system is in the equilibrium estate if some of the changes that could occur in it do not make its entropy to increase'. "
    [Show abstract] [Hide abstract] ABSTRACT: With this paper, our main aim is to contribute to the realisation of the chemical reactivity concept, tracing the historical evolution of the concept of chemical affinity that eventually supported the concept of chemical equilibrium. We will concentrate on searching for the theoretical grounds of three key chemical equilibrium ideas: 'incomplete reaction', 'reversibility' and 'dynamics'. In addition, the paper aims to promote teachers' philosophical/historical chemical knowledge. The starting point of this historical reconstruction will be the state of the art in the construction of the first affinity tables, based on the concept of elective affinities, during the 18th century. Berthollet reworked this idea, considering that the amount of the substances involved in a reaction was a key factor accounting for the chemical forces. Guldberg and Waage attempted to measure those forces, formulating the first affinity mathematical equations. Afterwards, the first ideas providing a molecular interpretation of the macroscopic properties of equilibrium reactions are presented. Eventually, theoretical chemists integrated previous findings into a new field: thermodynamics. This historical approach may serve as a base for an appropriate sequencing of the teaching and learning of chemical equilibrium. Hence, this paper tries to go beyond the simple development of teachers' conceptions of the nature of chemistry, for it gives suggestions about how teachers may translate such understandings into classroom practice.
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  • [Show abstract] [Hide abstract] ABSTRACT: The first systematic studies on the velocity of chemical reactions (now called reaction rates) were published in the 1850s and 1860s. Inquiring about the course of chemical change, their authors established empirical equations on the basis of their measurement results. But these laws, which represented reaction velocities as proportional to the actual concentration of the reagents, could not be given a physical foundation. The chemists themselves regarded their propositions as mere ad hoc hypotheses. In 1867 Leopold Pfaundler formulated a qualitative theory of chemical processes based on Clausius's version of the kinetic gas theory (and more specifically on his theory of evaporation), and on Saint-Claire Deville's investigations of dissociation processes. Pfaundler's theory was based on farreaching analogies: between evaporation and dissociation; between the gaseous state and the activated state; and between evaporation- and chemical-equilibrium. Four points of Pfaundler's theory must be regarded as essential: (1) the reduction of chemical change to randomly occurring molecular collisions, only showing regularities in great numbers according to the laws of probability; (2) the idea that molecules are in different states of internal and external motion, which determines whether a collision results in a reaction; (3) the view of the reaction step as a transition from internal to external motion and vice versa; and (4) the introduction of a new molecular-kinetic definition of chemical affinity as the maximum of internal motion. With these assumptions, Pfaundler provided the empirical rate equations with a new statistical interpretation and a physical Justification.
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