Heat and affinity

Over the course of over two-thousand years the theory of what specifically heat was and how this related to chemical affinity, transformed from the old four element theories of the Greeks, in which heat was a form of fire, one of the basic elements, to the Arabic alchemical principles (salt, sulphur, mercury), to the modern day conception in which heat is a form of "energy in transit". Between these two points, there were several transitions junctions along the way, in which, during each step, the view of what heat was changed gradually.

Early history
In c. 350 BC, Aristotle, in his Physics, stated that there were four elements out of which all was made: earth, air, water, and fire. Building on this theory, in c. 790 Arabian chemist Jabir ibn-Hayyan (Geber) postulated that metals were formed out of two elements: sulphur, ‘the stone which burns’, which characterized the principle of combustibility, and mercury, which contained the idealized principle of metallic properties. Shortly thereafter, this evolved into the Arabic concept of the three principles: sulphur giving flammability or combustion, mercury giving volatility and its opposite, and salt giving solidity.

In 1524, Swiss chemist Paracelsus adopted Aristotle’s four element theory, but reasoned that they appeared in bodies as Geber’s three principles. Paracelsus saw these principles as fundamental, and justified them by recourse to the description of how wood burns in fire. Mercury included the cohesive principle, so that when it left in smoke the wood fell apart. Smoke represented the volatility (the mercury principle), the heat-giving flames represented flammability (sulphur), and the remnant ash represented solidity (salt).

In 1669, German physician and chemist Johann Becher published his Physica Subterranea, in which, in modification on the ideas of Paracelsus, he argued that the constituents of bodies are air, water, and three types of earth: terra fluida, the mercurial element, which contributes fluidity and volatility; terra lapida, the solidifying element, which produces fusibility or the binding quality; and terra pinguis, the fatty element, which gives material substance its oily and combustible qualities. These three earths correspond with Geber’s three principles. A piece of wood, for example, according to Becher, is composed of ash and terra pinguis; when the wood is burnt, the terra pinguis is released, leaving the ash. In other words, in combustion the fatty earth burns away.

In 1703, a student of Becher’s named Georg Stahl, a newly trained chemist and physician, modified his mentor’s three earth’s theory by renaming terra pinguis as phlogiston from the Ancient Greek phlogios for ‘fiery’. Phlogiston was a name that had been used in the same sense previously by Hapelius (1606), Sennert (1619), and van Helmont (1625). Phlogiston, according to Stahl, is the ‘matter and principle of fire, and not fire itself’ that escapes from burning bodies with a rapid whirling motion, and is contained in all combustible bodies and also in metals, which can be burnt to ‘calces’. Calx is a residual substance, sometimes in the form of a fine powder, that is left when a metal or mineral combusts or is calcinated due to heat. Calx, especially of a metal, is now properly defined as an ‘oxide’, i.e. a chemical compound containing an oxygen atom and other elements. In the phlogiston theory, the calx was the true elemental substance, having lost its phlogiston in the process of combustion. The important point to note in Stahl’s theory is that phlogiston can be restored to the original substance by supplying a replacement phlogiston from any material containing it, such as oil, wax, charcoal, or soot, which was thought to be nearly pure phlogiston. The caloric essentially replaced the phlogiston.

To compound this issue, was Benjamin Thompson’s 1797 noted cannon boring experiment, which showed that by the use of friction created by boring cannons it was possible to convert work into heat. This conflicted Lavoisier’s theory. In other words, how could one produce caloric particles by boring a cannon?

Likewise, in 1799 English chemist Humphry Davy discovery that by rubbing cubes of ice together, in a room colder than the freezing point of water, produces heat and causes melting. In conclusion of this result, he established the following proposition: ‘the phenomena of repulsion are not dependent on a peculiar elastic fluid for their existence, or caloric does not exist.’ He concludes that heat consists of a motion excited among the particles of bodies. ‘To distinguish this motion from others, and to signify the cause of our sensation of heat,’ and of the expansion or expansive pressure produced in matter by heat, ‘the name repulsive motion has been adopted.’ Other inconsistencies, such as James Prescott Joule’s 1843 paddle wheel experiment, which showed that a portion of the work energy that went into turning the paddle wheel was converted into heat that functioned to increase the temperature of the water in a measurable manner, were cropping up during this period such that heat was beginning to be seen as a form of kinetic energy, i.e. energy of matter related to the motions of its constituent particles. In general, kinetic energy can be defined as the vibrational, rotational, or transitional energy possessed by an atom, molecule, or substance and quantified as the work that will be done by the body possessing the energy when it is brought to rest.

Reflections on the Motive Power of Fire
In 1824, however, Sadi Carnot, in his publication Reflections on the Motive Power of Fire, which started the science of thermodynamics, remedied these issues, to a certain extent, when he utilized a tricky combination of Stahl’s theory, that phlogiston can be restored to the original substance, such as by putting the hot working body of a substance in a cylinder in contact with a cold reservoir thereby allowing the system to restore to its original state, and that heat can be transferred from one body to another, such as to pass the caloric particles from a hot body to a cold body, and Lavoisier’s theory, that caloric causes particles to separate from one another, such as in the cylinder of a piston, along with some of the newer kinetic energy theories, e.g. by defining work or motive power in terms of the useful effect that a motor is capable of producing, which can be likened to the elevation of a weight to a certain height, to outline the original version of the second law of thermodynamics, as follows:

:The production of motive power in heat engines is due not to an actual consumption of the caloric, but to its transportation (transfer) from a warm body to a cold body, that is, to its re-establishment (restore) of equilibrium.

In sum, by virtue of these historical foundation precursors of heat theories, namely that the conception of heat evolved from that of fire (c. 350 BC), to sulphur (c. 790), to terra pinguis (1669), to phlogiston (1703), to caloric (1787), to kinetic energy (1797), Carnot essentially argued that engine cycles occur when heat particles are passed cyclically, i.e. first in a forward direction and then in a reverse direction, through the interstices of the working substance, thereby changing the affinities of the constituent particles comprising the working substance, such that expansion and contraction work is produced. However, he reasoned incorrectly, in a footnote, that no change occurs in the substance of the working body, during each cycle. In the correct manner, the change is that of ‘irreversibility’, later formulated in terms of entropy by Clausius beginning in 1850.

Moreover, in this argument, Carnot assumed that heat, like a substance, cannot be diminished in quantity and that it cannot increase. Specifically, he states that in a complete engine cycle ‘that when a body has experienced any changes, and when after a certain number of transformations it returns to its precisely its original state, that is, to that state considered in respect to density, to temperature, to mode of aggregation, let us suppose, I say that this body is found to contain the same quantity of heat that it contained at first, or else that the quantities of heat absorbed or set free in these different transformations are exactly compensated.’ Furthermore, he states that ‘this fact has never been called into question’ and ‘to deny this would overthrow the whole theory of heat to which it serves as a basis.’ This famous sentence, which Clausius spent fifteen years thinking about, marks the start of thermodynamics and signals the slow transition from the older caloric theory to the newer kinetic theory, in which heat is a type of energy in transit.

This transition had a marked effect on the various chemical affinity theories. In short, the older caloric-like theories assumed that when chemical reactions occurred, the particles or species newly put in contact attracted each other by virtue of their mutual affinities, whereby, as a result, they worked to squeeze out the ‘caloric’ particles, from between their pores, thus releasing heat and light in the process. Subsequently, it was reasonably argued, that by measuring the heat released from various reactions one could correlate this to a measure of chemical affinity. Thus, according to this logic, heat release was thought to be a direct measure of affinity. Yet, after 1924, with the transition from the caloric theory to the kinetic theory, this heat measurement theory of chemical affinity needed an overhaul.

In short, following Rudolf Clausius’ 1854 formulation of entropy, attempted measurements of chemical affinity, via calorimetry measurements or according to the electricity and heat produced in half-cells, began to encounter conceptual and theoretical difficulties. Namely, if affinity was indeed related to the heat released or electricity produced by chemical reactions, then a calculation of this measure would need to incorporate, in some way, the bound energy ‘TS’ of the system, i.e. the proportion of internal intermolecular work energy that cannot be measured as external work.

Subsequently, in the beginning of the 19th century, the first major theoretical impediment to the further development of affinity chemistry was the conflict between new science of thermodynamics and older thermal theories of affinity that were gaining ground. Some, during these years, would often mix the two concepts of energy and affinity together. In 1861, for example, Michael Faraday stated that chemical affinity is ‘the force of chemical action between different bodies’ and that it ‘depends entirely upon the energy with which particles of different kinds attract each other.’

On a different front, the new science of thermochemistry was coming into its own. In 1782, French chemist Antoine Lavoisier and French mathematician Pierre-Simon Laplace founded the science of thermochemistry by building the world’s first ice-calorimeter; a device used for measuring the heat of chemical reactions, physical phase changes, and heat capacities. They used it to determine the heat evolved in various chemical changes; calculations which were based on Joseph Black’s prior discovery of latent heat, i.e. the amount of energy in the form of heat that is required for a material to undergo a change of phase, such as liquid to vapor.

Lavoisier and Laplace also showed that the heat evolved in a reaction is equal to the heat absorbed in the reverse reaction and investigated the specific heat and latent heat of a number of substances, and amounts of heat evolved in combustion. In 1777, Lavoisier went further by elaborating on the molecular constitution of bodies contingent upon the interplay of heat and affinities. To explain the phenomenon in which heat causes a volume increase and cold a volume contraction, he states ‘the molecules of bodies do not touch, that there exists between them a distance that heat augments and cold diminishes.’

Moreover, in a manner analogous to the later thermodynamic phrasings used by Helmholtz, Lavoisier defined ‘combined heat’ as the proportion of heat united to a body so that one could not remove it without decomposing it and ‘free heat’ as all the heat that was not engaged in combination. It was not completely free, however, because of the affinity that matter supposedly had for the heat. The constant and determined heat lost in phase changes reflected the heat that passed from the free state to the combined state.

In his 1787 Elements of Chemistry, Lavoisier outlined a connection between affinity, inter-species energetics, caloric, equilibrium, and temperature: ‘although we are far from being able to appreciate all these powers of affinity, or to express their proportional energy by numbers, we are certain, that, however variable they may be when considered in relation to the quantity of caloric with which they are combined, they are all nearly in equilibrium in the usual temperature of the atmosphere.’ This fundamental statement was one of the precursors to Gibbs’ 1876 On the Equilibrium of Heterogeneous Substances.

Similarly, in 1840 Russian chemist Germain Hess formulated the principle that the evolution of heat in a reaction is the same whether the process is accomplished in one-step or in a number of stages. This known as Hess's law. With the advent of the mechanical theory of heat in the early 19th century, Hess’s law came to be viewed as a consequence of the law of conservation of energy.

In thermochemistry, the Thomsen-Berthelot principle was an hypothesis which argued that all chemical changes are accompanied by the production of heat and that processes which occur will be ones in which the most heat is produced. This principle was formulated in slightly different versions by the Danish chemist Julius Thomsen in 1854 and by the French chemist Marcellin Berthelot in 1864. This early postulate in classical thermochemistry became the controversial foundation of a research program that would last three decades.


This principle came to be associated with what was called the thermal theory of affinity, which postulated that the heat evolved in a chemical reaction was the true measure of its affinity. In other words, the thermal theory proposed that all chemical changes are accompanied by the production of heat and that processes which occur spontaneously will be ones in which the most heat is produced.

Later, in 1875, Berthelot outlined his ‘principle of maximum work’, which stated that all chemical reactions will tend to yield the maximum amount of chemical energy in the form of work as the reaction progresses. It was a postulate concerning the relationship between chemical reactions, heat evolution, and the potential work produced there from. Whilst this principle is undoubtedly applicable to the great majority of chemical actions under ordinary conditions, it is subject to numerous exceptions, and cannot therefore be taken as a secure basis for theoretical reasoning in the relation between thermal effect and chemical affinity. The existence of reactions which are reversible on slight alteration of conditions at once invalidates the principle, for if the action proceeding in one direction evolves heat, it must absorb heat when proceeding in the reverse direction.

Berthelot's theory was essentially: ‘every pure chemical reaction is accompanied by evolution of heat; and that this yields the maximum amount of work.’ The effects of irreversibility, however, showed this version to be incorrect. This was rectified, in thermodynamics, by incorporating the concept of entropy. In 1876, in particular, through the thermodynamic works of Willard Gibbs, Hermann von Helmholtz and others to follow, the work principle was found to be a particular case of a more general statement, in that: for all thermodynamic processes between the same initial and final state, the delivery of work is a maximum for a reversible process. The principle of maximum work, essentially, was a precursor to the development of the concept of thermodynamic free energy.

In the newly developing field of electrochemistry, during the first half of the 19th century, using electrolysis, chemical analysis, and stoichiometric investigations, chemists had begun to find exact atomic formulas for different species. Moreover, using the battery to strip electrons off atoms and molecules, they had discovered free radicals, which are atomic or molecular species with unpaired electrons on an otherwise open shell configuration. Unpaired electrons are usually highly reactive, which means that radicals are likely to take part in chemical reactions.


In this direction, the theory of chemical valencies was proposed in 1852 by English chemist Edward Frankland, as discussed, in which he combined the older theories of free radicals and ‘type theory’ with thoughts on chemical affinity to show that certain elements have the tendency to combine with other elements to form compounds containing 3, i.e. in the three atom groups (e.g. NO3, NH3, NI3, etc.) or 5, i.e. in the five atom groups (e.g. NO5, NH4O, PO5, etc.), equivalents of the attached elements. It is in this manner, according to Franklin, that their affinities are best satisfied. Each element was thus assigned with a specific ‘combining power’, which soon afterwards was called quantivalence or valency and valence by American chemists.

In 1855, in an interesting paper, German physician and philosopher Ludwig Büchner published his famous treatise Force and Matter, in which he expounds on the view, similar to Goethe, that matter and force are inseparable and that the chemical affinities between humans are exactly the same as those between atoms and molecules. Matter and force, according to Büchner, are indestructible and can never be separated; when separate, they become abstractions: ‘imagine matter without force, and the minute particles of which a body consists,’ says Büchner, ‘without that system of mutual attraction and repulsion which holds them together and gives form and shape to the body; imagine the molecular forces of cohesion and affinity removed, what then would be the consequence?’

Büchner reasons that the chemical affinities of atoms and molecules are basic manifestations of the anthropomorphic conceptions of love and hate. Specifically, ‘just as men and women attract one another, so oxygen attracts hydrogen, and, in loving union with it, forms water, that mighty omnipresent element, without which no life nor thought would be possible.’ Likewise, ‘potassium and phosphorus entertain such a violent passion for oxygen that even under water they burn, i.e. unite themselves with the beloved object.’

In the 1860s and 70s, the view that heat was type of indestructible fluid-like particle, such as caloric, was losing face. The newer energy theories which were to replace it, however, were still not solidified. Hermann von Helmholtz, for example, in his famous 1862 article ‘On the Conservation of Force’, stated that the older view of the nature of heat was that it was ‘a substance, very fine and imponderable indeed, but indestructible, and unchangeable in quantity.’ The newer view, according to Helmholtz, is that heat is a type of energy that can be transformed, such as into work, a principle discovered predominately by Humphry Davy, Sadi Carnot, and Rudolf Clausius, as is embodied in the first two laws of thermodynamics.
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