Müller I. A history of thermodynamics. The doctrine of energy and entropy (1185104), страница 37
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We shall consider a similar situation belowwhen we deal with the ammonia synthesis.It was Helmholtz who pointed out Berthelot’s misunderstanding aboutthe decisive role of the heat of reaction in 188236 and – we know it, but hedid not – he had been anticipated by Gibbs. Most scientists, however,learned about the delicate balance between ǻu and ǻs from Helmholtz andthat is why the free energy F = U – T·S is known as the Helmholtz freeenergy in English speaking countries.Le Châtelier (1850–1936), the most eminent chemist at the turn of thecentury, did not indulge in speculation. He simply reported what he hadobserved when, in 1888, he pronounced the principle of least constraint orsimply le Châtelier’s principle: Every change of one of the factors of anequilibrium [e.g. pressure or temperature] brings about a rearrangement of34It was Berthelot who coined the words endothermic and exothermic.In a calorimetric bomb the reactions proceed at constant volume.
Therefore the heat ofreaction is equal to ǻu; for a reaction that happens at constant pressure the heat of reactionis ǻh = ǻu + pǻv, because some of the energy change is converted into work.36 H. Helmholtz; “Die Thermodynamik chemischer Vorgänge” [Thermodynamics ofchemical processes] Sitzungsberichte der preussischen Akademie der Wissenschaften.Berlin (1882 ).351565 Chemical Potentialsthe system – actually of its constituents – in such a direction as to decreasethe original change. For instance: An endothermic reaction – one with apositive heat of reaction – proceeds further at increased temperature, so thatthe temperature in the end does not rise quite as far as it would have donewithout the reaction. Similarly: A volume-increasing reaction is helpedalong by a pressure decrease, so that the eventual pressure drop is smallerthan without the reaction.Le Châtelier, when he translated Gibbs’s work into French in 1899, musthave had mixed emotions when he saw that his principle had been provedby Gibbs ten years before he himself stated it.
However, there was aconsolation: Gibbs’s proof was valid only for ideal gas mixtures, whereas leChâtelier’s statement claims general validity.Ostwald, the German translator of Gibbs, had said that he undertook thetask because he believed in hidden treasures in Gibbs’s work. He was right,and le Châtelier and later Haber and Bergius were chemists who uncoveredand lifted the treasures. Of course, Roozeboom had been another one, seeabove.Fritz Haber (1868–1934)Fritz Haber was a chemist who knew Gibbs’s work well enough to makeuse of it in the production of ammonia NH3 from the nitrogen N 2of the air.
The overall stoichiometric formula readsH 2 13 N 2 o 23 NH 3and the heat and entropy of reaction are37∆hRkJ30.8 moland ∆sRJ59.5 molK.Thus at normal temperature TR and pressure pR the energy, or enthalpy,drops and it is therefore conducive to the formation of ammonia. Theentropy drops also and that fact is bad for ammonia. But the energetic termkJdominates, since 'J4 64 ' U 4 .
13.1 molholds, so that the Gibbs freeenergy favours ammonia, and that is what counts, or it should be; seeFig. 5.7 which shows Gibbs free energies as a function of the extent ofreaction. For the reference state TR = 298 K and pR = 1atm the minimum ofG lies very close to 100% ammonia.37Chemists – at least the an-organic types – like to use molar quantitiesrelated to the mass-specific quantitiesthe molar mass andMrDPDPoaD by aDaD M D , where / Dis the relative molecular mass.aDwhich are/ TDgmolisFritz Haber (1868–1934)157Fig.
5.7. Gibbs free energies for the ammonia synthesis as functions of the extent of reactionAnd yet, nothing happens when hydrogen and nitrogen are mixed. Noammonia is formed and the mixture is perfectly stable, or rather metastable, since the strong chemical bonds between the atoms in H2 and N2must first be severed or weakened before ammonia can be formed.For that purpose Haber used perforated iron sheets whose surfacecatalyses the dissociations H2 ĺ 2H and N2 ĺ 2N at high temperature, say500°C. Unfortunately, such a high temperature emphasizes the negativevalue of 'U 4 so that the minimum of the Gibbs free energy lies on the sideof hydrogen and nitrogen, cf.
Fig 5.7, and once again, nothing happens. Butthen Haber knew what to do: The stoichiometric equation shows that thereaction, if it proceeds all the way, cuts the number of molecules by halfand, since all constituents are ideal gases, the volume is halved as well.Therefore, by le Châtelier’s principle and Gibbs’s formulae, a high pressureshould assist the reaction. Haber put the mixture under 200 atm andachieved a good output of ammonia,38 cf. Fig. 5.7.Ammonia can easily be converted into nitrates which the world cravesfor the production of fertilizers and explosives. Before Haber the mainsource of nitrates were the guano fields on the west coast of South America,over which Chile, Peru and Bolivia fought the guano war.
Chile won andBolivia lost her access to the sea.The Haber-Bosch synthesis was developed in 1908, just in time for thefirst world war. It was clear that in case of war Germany would be cut offfrom guano imports by a British naval blockade and therefore a hugeammonia plant was built in Saxony. Its output supplied the German armythroughout the four years of war easily. The country ran out of men andfood and morale, but never of explosives.
Haber received the Nobel prize in38The process is known as the Haber-Bosch synthesis after Haber, of course, and Karl Bosch(1874–1940), who suggested a good strong material for the pressure vessel. The apparatusis exhibited on the campus of the University of Karlsruhe, where it rusts away on anunkempt lot.1585 Chemical Potentials1918 and, after the war he was made director of the Kaiser WilhelmInstitute for physical chemistry.39Haber’s patriotism led him to propagate and direct the use of chlorineand mustard gas as a means of warfare on the western front.
Chlorine wasfirst. On April 22, 1915 it was used at Ypres against Canadian troops. Thetroops fled and the result was an unprecedented five-mile gap in the front.The strategic effect, however, was nil, since the German general staff hadnot really believed that the project would work, and was not prepared for anoffensive.40Only a little later Haber became a tragic figure. He was Jewish and, whenHitler came to power, he was stripped of all posts and driven into exile. Hewas not alone in that, of course, but while others were made welcome inBritain by an international initiative of scientists led by Ernest Rutherford,Haber was not, because of his poison gas activity.
He left for Italy but dieden route.Haber continued in what he saw as his patrioticduty after the disastrous war. He attempted toisolate gold from sea water in the hope to helpGermany repay the huge war indemnitydemanded under the Versailles peace treaty.In this effort Haber failed.
However, he couldhave saved himself the effort because in the endthe indemnity was never paid.Fig. 5.8. Fritz HaberThe impact of chemistry on war and warfare confirmed itself in thesecond world war. That war was largely fought by mobile troops withmechanized transport, and by tanks and airplanes, and the biggest logistical problemwas the supply of fuel. Germany has no natural mineral oil but a lot ofcoal, – both brown coal and pit coal.
And again, just in time, it becamepossible to convert both types of coal into benzine. That was the invention39It is a sign of the schizophrenia of German politics between the two world wars that theKaiser Wilhelm Institute retained its name in the time of the Weimar republic and duringthe national-socialist rule, although the monarchy was thoroughly discredited. It tookanother world war to shake the name loose.
The institute was renamed Max PlanckInstitute in 1946, cf. M. Planck: “Physikalische Abhandlungen und Vortraege” [Papersand lectures on physics] Vieweg, Braunschweig (1958). Foreword by M.von Laue.40 According to I. Asimov: “Biographies.” loc.citSocio-thermodynamics159of Friedrich Karl Rudolf Bergius (1884–1949),41 who had studied catalytichigh-pressure chemistry under Nernst and Haber.
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