Posts Tagged ‘Sadi Carnot’

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JH van ‘t Hoff’s laboratory in Amsterdam

The 1880s were important years for the developing discipline of physical chemistry. The gas laws of Mariotte and Gay-Lussac (Boyle and Charles in the English-speaking world) had reached a high point of refinement in Europe following the work of Thomas Andrews and James Thomson in Belfast, and Johannes van der Waals in Leiden. The neophyte science was now poised to discover the laws of solutions.

The need for this advance was clear. As future Nobel Prize winner Wilhelm Ostwald put it in his Lehrbuch der allgemeinen Chemie (1891), “A knowledge of the laws of solutions is important because almost all the chemical processes which occur in nature, whether in animal or vegetable organisms, or in the nonliving surface of the earth, and also those which are carried out in the laboratory, take place between substances in solution. . . . . Solutions are more important than gases, for the latter seldom react together at ordinary temperatures, whereas solutions present the best conditions for the occurrence of all chemical processes.”

In France, important discoveries concerning the vapor pressures exerted by solutions were already being made by François-Marie Raoult. In Germany, the botanist Wilhelm Pfeffer had developed a rigid semipermeable membrane to study the effect of temperature and concentration on the osmotic pressures of solutions. And in the Netherlands, a talented theoretician by the name of Jacobus Henricus van ‘t Hoff (note the space before the apostrophe) was busy writing up his research on chemical kinetics in a work entitled “Studies in Chemical Dynamics”, which contained all that was previously known as well as a great deal that was entirely new.

Then one day in 1883, while van ‘t Hoff was writing the last chapter of the Studies on the subject of chemical affinity, in which he demonstrates an exact relation between osmotic pressure and the vapor pressures of pure solvent and solvent in solution, a chance encounter with a colleague in an Amsterdam street misdirected his thinking and diverted him onto the wrong conceptual road.

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On page 233 of the Studies in Chemical Dynamics, van ‘t Hoff showed that osmotic pressure (D) has a thermodynamic explanation in the difference of vapor pressures of pure solvent and solvent in solution. Yet having discovered this truth, he promptly abandoned it in favor of an idea which seemed to possess greater aesthetic appeal. It was one of those wrong turns we all take in life, but in van ‘t Hoff’s case it seems particularly wayward.

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Jumping to conclusions

Writing in the Journal of Chemical Education (August 1986), the American Nobel Prize winner George Wald relates how van ‘t Hoff had just left his laboratory when he encountered his fellow professor the Dutch botanist Hugo de Vries, who told him about Wilhelm Pfeffer’s experiments with a semipermeable membrane, and Pfeffer’s discovery that for each degree rise in temperature, the osmotic pressure of a dilute solution goes up by about 1/270.

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Hugo de Vries (1848-1935) and Wilhelm Pfeffer (1845-1920)

In an instant, van ‘t Hoff recognized this to be an approximation of the reciprocal of the absolute temperature at 0°C. As he himself put it:

“That was a ray of light, and led at once to the inescapable conclusion that the osmotic pressure of dilute solutions must vary with temperature entirely as does gas pressure, that is, in accord with Gay-Lussac’s Law [pressure directly proportional to temperature]. There followed at once however a second relationship, which Pfeffer had already drawn close to: the osmotic pressure of dilute solutions is proportional also to concentration, i.e., alongside Gay-Lussac’s Law, that of Boyle applies. Without doubt the famous mathematical expression pv = RT holds for both.”

And thus was born, in a moment of flawed inspiration on an Amsterdam street, the Gaseous Theory of Solutions. It even had a mechanism. Osmotic pressure, according to van ‘t Hoff, was caused by one-sided bombardment of a membrane by molecules of solute and was equal to the pressure that would be exerted if the solute occupied the space by itself in the form of an ideal gas. For van ‘t Hoff, this provided the answer to the age-old mystery of why sugar dissolves in water. The answer was simple – it turns into a gas.

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Compounding the error

The law of osmotic pressure, and the gaseous theory that lay behind it, was published by van ‘t Hoff in 1886. Right from the start it was viewed with skepticism in several quarters, and it is not hard to figure out why. As the above quotation shows, van ‘t Hoff had convinced himself in advance that the law of dilute solutions was formally identical with the ideal gas law, and the theoretical support he supplies in his paper seems predicated to a preordained conclusion and shows little regard for stringency.

In particular, the deduction of the proportionality between osmotic pressure and concentration is analogy rather than proof, since it makes use of hypothetical considerations as to the cause of osmotic pressure. Moreover, mechanism is advocated – an anathema to the model-free spirit of classical thermodynamics.

Before long, van ‘t Hoff would distance himself from claims of solute molecules mimicking ideal gases, thanks to a brilliant piece of reasoning from Wilhelm Ostwald – to which I shall return. But van ‘t Hoff’s equation for the osmotic pressure of dilute solutions

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where Π is the osmotic pressure, kept the association with the ideal gas equation firmly in place. And it was this formal identity that led those influenced by van ‘t Hoff along the wrong track for several years.

One such was the wealthy British aristocrat Lord Berkeley, who developed a passion for experimental science at about this time, and furnished a notable example of how one conceptual error can lead to another.

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Misguided research

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Lord Berkeley (1865-1942)

It was known from existing data that the more concentrated the solution, the more the osmotic pressure deviated from the value calculated with van ‘t Hoff’s equation. The idea circulating at the time was that the refinements of the ideal gas law that had been shown to apply to real gases, could equally well be applied to more concentrated solutions. As Lord Berkeley put it in the introduction to a paper, On some Physical Constants of Saturated Solutions, communicated to the Royal Society in London in May 1904:

“The following work was undertaken with a view to obtaining data for the tentative application of van der Waals’ equation to concentrated solutions. It is evidently probable that if the ordinary gas equation be applicable to dilute solutions, then that of van der Waals, or one of analogous form, should apply to concentrated solutions – that is, to solutions having large osmotic pressures.”

And so it was that Lord Berkeley embarked upon a program of research which lasted for more than two decades and failed to deliver any meaningful results because his work was founded on false premises. It is in the highest measure ironic that van ‘t Hoff, just before he was sidetracked, had found his way to the truth in the Studies, in an equation which rendered in modern notation reads

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where Π is the osmotic pressure and V1 is the partial molal volume of the solvent in the solution. This thermodynamic relationship between osmotic pressure and vapor pressure is independent of any theory or mechanism of osmotic pressure. It is also exact, provided that the vapor exhibits ideal gas behavior and that the solution is incompressible.

If van ‘t Hoff had realized this, Lord Berkeley’s research could have taken another, more fruitful path. But history dictated otherwise, and it would have to wait until the publication in 1933 of Edward Guggenheim’s Modern Thermodynamics by the methods of Willard Gibbs before physical chemists in Europe would gain a broader theoretical understanding of colligative properties – of which the osmotic phenomenon is one.

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Brilliant reasoning

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Wilhelm Ostwald (1853-1932)

But to return to van ‘t Hoff’s change of stance regarding mechanism in osmosis. By 1892 he was no longer advocating his membrane bombardment idea, and in stark contrast was voicing the opinion that the actual mechanism of osmotic pressure was not important. It is likely that his change of mind was brought about by a brilliant piece of thinking by his close colleague Wilhelm Ostwald, published in 1891 in the latter’s Lehrbuch der allgemeinen Chemie. Using a thought experiment worthy of Sadi Carnot, Ostwald shows that osmotic pressure must be independent of the nature of the membrane, thereby rendering mechanism unimportant.

Ostwald’s reasoning is so lucid and compelling that one wonders why it didn’t put an end to speculation on osmotic mechanisms. Here is how Ostwald presented his argument:

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“… it may be stated with certainty that the amount of pressure is independent of the nature of the membrane, provided that the membrane is not permeable by the dissolved substance. To understand this, let it be supposed that two separating partitions, A and B, formed of different membranes, are placed in a cylinder (fig. 17). Let the space between the membranes contain a solution and let there be pure water in the space at the ends of the cylinder. Let the membrane A show a higher pressure, P, and the membrane B show a smaller pressure, p. At the outset, water will pass through both membranes into the inner space until the pressure p is attained, when the passage of water through B will cease, but the passage through  A will continue. As soon as the pressure in the inner space has been thus increased above p, water will be pressed out through B. The pressure can never reach the value P; water must enter continuously through A, while a finite difference of pressures is maintained. If this were realized we should have a machine capable of performing infinite work, which is impossible. A similar demonstration holds good if p>P ; it is, therefore, necessary that P=p; in other words, it follows necessarily that osmotic pressure is independent of the nature of the membrane.”

(English translation by Matthew Pattison Muir)

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Epilogue

For van ‘t Hoff, his work on osmosis culminated in triumph. He was awarded the very first Nobel Prize in Chemistry in 1901 for which the citation reads:

“in recognition of the extraordinary services he has rendered by the discovery of the laws of chemical dynamics and osmotic pressure in solutions”.

But van ‘t Hoff did not have long to enjoy the accolade. “Something seems to have altered my constitution,” he wrote on August 1, 1906, and on March 1, 1911, he died of tuberculosis aged 58.

 

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Mouse-over links to works referred to in this post

Jacobus Henricus van ‘t Hoff Studies in Chemical Dynamics

Wilhelm Ostwald Lehrbuch der allgemeinen Chemie (1891) [English Version – see page 103]

Lord Berkeley On some Physical Constants of Saturated Solutions

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P Mander June 2015

Sadi Carnot (1796-1832) One of the great original thinkers.

Sadi Carnot (1796-1832) One of the great original thinkers.

The milestone memoir Réflexions sur la Puissance Motrice du Feu (Reflections on the Motive Power of Fire), published by French engineer Sadi Carnot in 1824, marks the starting point of thermodynamics as a theory-based science. In this work, Carnot developed his powerful ideas with the aid of the caloric theory, which viewed heat as an effect caused by an all-pervading, invisible fluid called caloric. An important tenet of the theory was that caloric was considered to be conserved in all thermal processes.

The fact that Carnot published no other work during his short life led later theoreticians, notably Rudolf Clausius and James Clerk Maxwell, into the error of assuming that Carnot never questioned the validity of the caloric theory. But after Carnot’s death in 1834, a bundle of his papers was found whose contents reveal that he had not only questioned the caloric hypothesis, but had reached the point where he felt compelled to abandon it in favour of its eventual successor, the dynamical theory.

This cannot have been an easy decision for Carnot, since the rejection of the caloric theory in favour of the dynamical theory robbed him of the very principle he had employed in Réflexions sur la Puissance Motrice du Feu to reach his groundbreaking conclusions regarding the motive power of heat.

In this post, I will examine the dilemma that Carnot faced in contemplating this conceptual shift.

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When Carnot began thinking about how steam engines turned heat into work, the theory of machines that produced work through mechanical force supplied by men or animals, the wind or a waterfall was already well understood.

In his memoir, Carnot begins by saying that “a similar theory is evidently needed for heat-engines”, and commences his analysis by noting that the production of motive force in steam engines is always accompanied by the transportation of caloric from a body at elevated temperature to another whose temperature is lower.

It should be noted that in the early 19th century when Carnot was writing his memoir, steam engines had such low thermal efficiency that any consumption of caloric in its passage between the hot and cold reservoirs would barely have been perceptible. So it was quite reasonable for Carnot to think that caloric was conserved by a steam engine in motion.

But how did a steam engine produce motive power? With the aim in mind of ‘a similar theory’ to the mechanical engine, Carnot likened the passage of caloric to the passage of water from a higher reservoir to a lower reservoir as it drives a water wheel – there is no loss of water, and motive power depends on the quantity of water transported and the height of the waterfall.

In an equivalent way, Carnot saw the motive power of a steam engine arising from the quantity of caloric transported, and the ‘height of its fall‘, by which he meant the difference in temperature between the hot and cold bodies. And as with the water wheel analogy, the process involved no loss of caloric.

With this model-based principle in place, Carnot was ready to start finding answers to important questions.

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The first question Carnot poses is one that had been long asked by engineers: Is the motive power of heat invariable or does it vary with the working substance employed to realize it?

To furnish an answer, Carnot conducts a thought experiment involving an imaginary steam engine undergoing a cyclical sequence of four entirely reversible isothermal and adiabatic operations, the hot and cold reservoirs being maintained at given temperatures – a stroke of conceptual genius which we know today as the Carnot cycle.

In this steam engine, Carnot imagines the transfer of a quantity of caloric from a body A to a colder body B, both maintained at a given temperature. Assuming no loss of motive power of caloric, he shows that the motive power produced in a single Carnot cycle can be used to return the same quantity of caloric from body B to body A by operating the cycle in the reverse direction. He then concludes: “An indefinite number of alternative operations of this sort could be carried on without in the end having either produced motive power or transferred caloric from one body to the other.”

He then puts the question “Can there exist a working substance that makes better use of the heat than the steam employed in the cycle just described?” and uses the following argumentation to supply the answer:

If it were possible by any method whatever to make the caloric produce a quantity of motive power greater than we have made it produce by our first series of operations, it would suffice to divert a portion of this power in order by the method just indicated to make the caloric of the body B return to the body A from the refrigerator to the furnace, and thus be ready to commence again an operation precisely similar to the former, and so on: this would be not only perpetual motion, but an unlimited creation of motive power without the consumption either of caloric or of any agent whatever. Such a creation is entirely contrary to ideas now accepted, to the laws of mechanics and of sound physics. It is inadmissible.

To put it in modern parlance, in the first series of operations using steam as the working substance, a quantity C of caloric is transported from body A to body B, producing a quantity M of motive power. Then operating the cycle in reverse, the same quantity M of motive power is used to transport the same quantity C of caloric from body B back to body A, thus restoring the initial conditions. The net result is that no caloric is transferred and no motive power produced.

In the second series of operations using a working substance that is imagined to be more effective than steam, an identical quantity C of caloric is transported from body A to body B, but this time it produces a quantity M+m of motive power. Then operating the previously described cycle (using steam) in reverse, the quantity M of motive power is used, as before, to transport the quantity C of caloric from body B back to body A, thus restoring the initial conditions. The net result is that no caloric is transferred, but a quantity m of motive power is produced – an operation which could be endlessly repeated, producing an unlimited quantity of motive power from nothing. This violates the principle and disproves the initial assertion. Carnot thus concludes: “the maximum of motive power resulting from the employment of steam is also the maximum of motive power realizable by any means whatever“.

Carnot has proved that the  motive power of heat is independent of the working substance employed to realize it.

He then asks a second question – whether the motive power of heat is unbounded or subject to an assignable limit – and goes on to prove by further thought experiment that its quantity is fixed solely by the temperatures of the bodies between which the transfer of caloric is effected.

The important point to note is that Carnot arrives at these powerful results by applying the principle that  caloric is a conserved quantity in thermal processes.

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But even as his memoir was going to print (he published it at his own expense), it appears that doubts about the caloric theory were already forming in Carnot’s mind. A sentence in the manuscript which describes the theory as “beyond doubt” was changed at proof stage into less certain language: “the theory … does not appear to be of unquestionable solidity. New experiments alone can decide the question.

The kind of experiments Carnot had in mind are revealed in the bundle of papers found after his death. The 23 loose sheets, containing questions, speculations, fragments of essays, and proposed experiments almost identical to those Joule was later to conduct, chart Carnot’s increasing belief that heat and work are equivalent. He lists Rumford’s experiments on the boring of cannon and the friction of wheels on their spindles among the experimental facts undermining the caloric theory.

One of the sheets contains the following paragraph, in which Carnot identifies heat and work as interconvertible forms of a conserved quantity, and effectively states the first law of thermodynamics in relation to cyclical thermodynamic processes (ΔU = 0, Q – W = 0) :
Heat is simply motive power, or rather motion which has changed its form. It is a movement among the particles of bodies. Wherever there is destruction of motive power, there is at the same time production of heat in quantity exactly proportional to the quantity of motive power destroyed. Reciprocally, wherever there is destruction of heat, there is production of motive power.

On one of the last sheets, Carnot writes:
When a hypothesis no longer suffices to explain phenomena, it should be abandoned. This is the case with the hypothesis which regards caloric as matter, as a subtle fluid.

But as Carnot becomes increasingly convinced that heat and motive power are interconvertible, he is at the same time caught in a dilemma. Because whereas the caloric theory enabled him to prove an assertion by disproving the contrary assertion, the dynamical theory fails to do so.

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As Carnot contemplated the interconvertability of heat and work, he would no doubt have re-run the reversible steam engine thought experiment on the basis of the dynamical theory:

In the first series of operations using steam as the working substance, a quantity H of heat is taken from body A, a quantity M of motive power is produced, and a quantity H-M of heat is transferred to body B. Then operating the cycle in reverse, the quantity H-M of heat is taken from body B, a quantity M of motive power is consumed, and a quantity H of heat is transferred to body A, thus restoring the initial conditions. The net result is that no heat is transferred and no motive power produced.

In the second series of operations using a working substance that is imagined to be more effective than steam, an identical quantity H of heat is taken from body A, but this time a quantity M+m of motive power is produced. Taking h to be the quantity of heat equivalent to the quantity m of motive power, a quantity H-M-h of heat is transferred to body B. Then operating the previously described cycle (using steam) in reverse, the quantity H-M of heat is taken as before from body B, a quantity M of motive power is consumed, and a quantity H of heat is transferred to body A, thus restoring the initial conditions.

The net result is that a quantity h of heat has been taken from the colder body B and a quantity m of motive power has been produced, an operation which could be endlessly repeated. But since we have defined the equivalence relation h=m, there is no violation of principle and the dynamical theory therefore fails to disprove the absurd result of the limitless production of motive power solely by consuming the heat of a body.

This is the core of the dilemma Carnot faced.
He doubted the caloric theory, but it proved his assertions.
He favoured the dynamical theory, but it did not prove his assertions.

One of the loose sheets found after Carnot’s death reveals this difficulty in a passage where he convinces himself that heat can be converted into motive power and vice versa. But while he sees many advantages in this hypothesis, he notes “it would be difficult to explain why, in the development of motive power by heat, a cold body is necessary; why motion cannot be produced by consuming the heat of a body.

The explanation, as we now know, is provided by the second law of thermodynamics. Although Carnot had no time in his short life to grapple with this dilemma, it is not fanciful to suggest that he would have found the solution had he lived. More than twenty years before Clausius, Sadi Carnot had already effectively stated the first law, and as the last quote shows, he only needed to turn his question into an assertion to find the essential statement of the second law.

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In textbooks dealing with thermodynamics, the story is often recounted that Carnot’s groundbreaking memoir Réflexions sur la Puissance Motrice du Feu (Reflections on the Motive Power of Fire) went unnoticed by the scientific world when it was published, and lay forgotten for 24 years until Lord Kelvin seized on its significance and brought Carnot’s seminal work – in which lay the seeds of the second law of thermodynamics – into the spotlight of publicity.

This popular account, although broadly accurate, is incomplete in one important respect. It neglects the vital role played by Clapeyron.

Benoît Émile Clapeyron (1799-1864)

Benoît Émile Clapeyron (1799-1864)

Sadi Carnot (1796-1832) and Émile Clapeyron (1799-1864) were contemporaries. Both were engineers by training, and both had studied at the École Polytechnique in Paris. When Carnot published his memoir (at his own expense) in 1824, Clapeyron had already left Paris and was teaching at an engineering school in St. Petersburg, Russia. He returned to Paris just two years before Carnot’s untimely death during a cholera epidemic in 1832. By that time, Sadi Carnot’s little book had long since disappeared from the booksellers’ shelves. But Clapeyron had evidently obtained a copy, and had clearly recognised the importance of his compatriot’s work. Clapeyron’s own Mémoire sur la Puissance Motrice de la Chaleur (Memoir on the Motive Power of Heat), published in Journal de l’École Polytechnique in 1834, contains a full restatement of Carnot’s theoretical principles – albeit in a more analytical form.

While Carnot’s privately published booklet quickly faded into obscurity, Clapeyron’s memoir published in an academic journal did not. It was read in scientific circles and remained available in scientific libraries, and so kept Carnot’s powerful ideas alive. In 1837, Clapeyron’s memoir was translated into English and formed one of the papers presented in Volume I of Taylor’s Scientific Memoirs. This was how Lord Kelvin became aware of Carnot’s work – we know this because the following footnote appears in the historic paper On an Absolute Thermometric Scale which Lord Kelvin published in Philosophical Magazine in October 1848:

“Carnot’s Theory of the Motive Power of Heat – Published in 1824 in a work entitled Réflexions sur la Puissance Motrice du Feu, by M. S. Carnot. Having never met with the original work, it is only through a paper by M. Clapeyron, on the same subject, published in the Journal de l’École Polytechnique, Vol. XIV, 1834, and translated in the first volume of Taylor’s Scientific Memoirs, that the Author has become acquainted with Carnot’s Theory.”

William Thomson, later Lord Kelvin (1824-1907)

William Thomson, later Lord Kelvin (1824-1907)

At the end of 1848, Lord Kelvin succeeded in obtaining a copy of Carnot’s original work, and the following year published a lengthy paper entitled An account of Carnot’s Theory of the Motive Power of Heat in the Transactions of the Edinburgh Royal Society, XVI, 1849.

Curiously, just as Lord Kelvin came to discover Carnot’s masterwork through the agency of Émile Clapeyron’s memoir, so did another key figure in the development of thermodynamics – Rudolf Clausius.

In his first and most famous paper On the Motive Power of Heat, and on the Laws which can be deduced from it for the Theory of Heat published in Annalen der Physik  in 1850, Clausius cites Carnot’s memoir on the opening page and adds this footnote:

Réflexions sur la Puissance Motrice du Feu, par S. Carnot. Paris, 1824. I have not been able to obtain a copy of this work, and am acquainted with it only through the work of Clapeyron and Thomson [Lord Kelvin]”

Rudolf Clausius (1822-1888)

Rudolf Clausius (1822-1888)

Clapeyron’s memoir had been translated into German in 1843 and published in the same journal in which Clausius’ 1850 paper appeared – Annalen der Physik – so it likely to be this translation to which Clausius refers. It is doubtful whether Carnot had much inkling of how crucially important his ideas would be to the development of thermodynamics. But there is no doubt that Clapeyron immediately saw value in them. In the introduction to his memoir, Clapeyron describes Carnot’s ideas as ‘both fertile and beyond question‘ and ‘worthy of the attention of theoreticians‘.

He was right. The two foremost theoreticians of the day certainly found Carnot’s fertile ideas worthy of attention. And we have Émile Clapeyron to thank for enabling Lord Kelvin and Rudolf Clausius to discover them.