14 PICKERISG THE PRINCIPLES OF THERMOCHEMISTRY. 111.-The Principles of Thermochemistry. By SPENCER UkIFREVlLLE PICKEMNG M.A. THE connection between a chemical action and the heat evolved therein is a question of vital importance to all chemists and physicists yet the fundamental principles on which it depends are at present in an eminently unstable and unsatisfactory condition. On the one hand it is acknowledged that the atoms possess a certain amount of potential energy or affinity which becomes partially or entirely saturated when they combine together and which in its saturation must according to the principles of physical science, evolve an exactly equivalent amount of kinetic energy or in the calorimeter heat,* while on the other hand the heat evolved in a reaction is not yet recognised as a measure of the affinities saturated, and there are thousands of instances known in which the saturation of these affinities apparently results in a paradoxical absorption of heat.I n 1853 Thomsen (Pogg. Ann. 92 34 and Ber. 6 425) stated that “ every simple or complex action of a purely chemical character is associated with a production of heat.” Naumann in 1869 (Lieb. Ann. 151 158) stated that “with few exceptions capable of being otherwise explained those chemical changes which must take place with an absorption of heat are so to speak indirect changes. That is t o say they occur simultaneously and are dependent upon other changes which are themselves accompanied by a production of heat.” f Unless we assume the existence of some form of kinetic or potential energy which has hitherto escaped our observation an aseumption which we are scarcely justified in making even when all other possible explanations fail PICKERIKG THE PRINCIPLES OF THERMOCHEMISTRY.15 While Berthelot in 1879 (Me‘c. Chim. 2 421) thus enunciated his more emphatic and comprehensive Principe de travail maximum : “ Every chemical change which is completed without the intervention of external energy will always tend towards the production of that body or that system of bodies developing the maximum quantity of heat,” adding a.s a corollary that “ every chemical reaction capable of occurring without the previous expenditure of work and without the intervention of external energy necessarily occurs if it develop heat.” No one can study the mass of thermochemical data which exist without concluding that such statements cannot be very far from the truth a t the same time we must agree with L.Meyer (Modern, Theories 429) in considering that Berthelot’s explanation of many endothermic reactions is decidedly forced ; for to explain reactions occui~ing in solution he has to refer them to the heat of formation of the anhydrous molecules thus ignoring the chemical nature of solu-tion of which however he is a firm supporter ; nor does he attempt my explanation of the heat absorbed when many strong solutions are diluted a chemical action again according to his views.* But when we examine the wording of Berthelot’s principle more closely we see that the expression “ tends towards ” destroys the whole value of the statement and affords R loophole for explaining (?) away any discordant facts whereas if “ produces ” be substituted for “ tends towards the production of” the statement must be met with an absolute denial hydrogen and oxygen do not combine a t O”, though they would develop heat in so doing; water decomposes at 2000“ absorbing heat ; and endothermic compounds are often when formed quite stable.On still wider grounds however we must reject any principles, such as those of Rerthelot and Thomsen the gist of which lies in a distinction between physical and “ purely chemical ” action a distinc-tion which never has been possible and which is every day becoming less possible. The whole of the thermal results in any action must be explained on one and the same principle.* Meyer’s objection to Berthelot’s principle on the ground that it is applicable to bodies at the absolute zero only is based I think on an unfair interpretation of what Berthelot meant by heat being an external energy ; similarly his objection on the score of the heat of neutralisation bearing evidence opposed to that of the “ nvidity ” of acids is entirely due to his misconception of the quantities measured in neutralisation ; the heat of neutralisation affords as a matter of fact stronger evidence than any yet obtained of the direct proportionality of tho heat evolved to the affiity saturated (Trans. 1887,593). For a criticism of some of the explana-tions of endothermic actions given by Berthelot see Rathke Ueber die Prinripien der Thermochemie Halle 1881 1 6 PICKE RING THE PRINCIPLES OF THERMOCHEMISTRY.In seeking for such a principle we must start with obtaining it clear conception of the facts to which this principle must apply. On the one hand it is a fact that substances will not combine until a certain temperature be reached even though their combination at lower temperatures would be accompanied by an evolution of heat ; while on the other hand it is also a fact that when the temperature of any body is raised to a certain point i t decomposes whether its decomposition be attended with an absorption or an evolution of heat, this being probably true even of the elementary molecules themselves, Thus the conversion of the potential energy of affinity into kinetic energy (heat) occurs only above a certain definite temperature, different for each different body and this kinetic energy becomes converted back again into affinity at some higher temperature ; but we know far too little about atomic motions to attempt my explana-tion of how these temperatures are conditional in each special case.We must content ourselves with the fact that combination occurs between certain limits of temperature only. The phenomena which we have to explain are therefore confined to those accompanying actions which do actually occur and the only principles which we can attempt to enumerate are those which deter-mine whether a certain reaction will occur provided the temperature be between the limits at which i t is possible and to determining which out of several possible reactions will occur.The accepted principles of dissociation the chemical nature of solution and the teaching of the thermal results of neutralisation, will I believe afford grounds sufficient for the foundation of such a principle. Inasmuch as combination is the result of the saturation of affinity, and the saturation of this affinity must always liberate a correspond-ing amount of heat it is evident that every act of contbinadion must cause a n evolution of heat and that in any reaction where heat is absorbed this absorption must be due to some accompanying decomposition. But as decomposition cannot be the direct result of affinity it must be due to the fact that some of the substances present are at the temperature of +he reaction above that temperature at which they begin to dissociate.This dissociation moreover cannot be that of any of the products for even if the products were entirely dissociated back again into the reagents this would be tantamount to no com-bination of the reagents having occurred and hence the minimum value of the heat evolved would be nil and not a negative quantity.* * Heat would be absorbed if the compound dissociated into substances simpler than the reagents themselves ; if for instance KC1 combines with water to form a hydrate which when formed dissociates partially into K and C1 &c. ; but such a caae is I think impossible. If the more saturated hydrate dissociated into K an PICEERING THE PRINCIPLES OF THERMOCHEMISTRY. l’i Any dissociation occurring must therefore be that of the reagents.Hence ‘‘ in every endothermic reaction one or more of the reagents must be in a partially dissociated condition.” Moreover when combination occurs it will occur independently o j whether it involves subsequent absorption of heat through the removal of the products qf dissociation and the consequent necessity for the occurrence of further dissociation. It is obvious that the affinity of the reacting substances and not the subsequent consequences of their reaction can alone determine whether they shall react or not. It also follows as a matter of necessity I think that in any compl~x system of atoms where two or more arrangements are possible and where the various products remain within the sphere of action capable of further interaction where such inieraction is possible (that is where the tem-perature is within the limits above mentioned) those products the formation of which is attended with the greatest evolution of heat wit1 be formed to the ezclusion of the others.Thus in illustration of my meaning if potassium be brought into the presence of excess of hydrochloric and hydrobromic acids in aqueous solution the two actions-K + HC1 = KC1 + H and K + HBr = KBr + H, are both known to be possible and it is also known that the KCl and KBr formed remain in the solution capable of reacting with any excess of HBr and HCl respectively ; in such a case my proposition states the potassium will be converted entirely into chloride or bromide according as the gross heat of formabion of one or other of these salts is the greater.* Before enquiring as to how far the principles are supported by known facts and how endot,hermic reactions may be explained in accordance with them we must point out that those endothermic reactions which are brought about by the so-called concurrence of an0 ther exothermic reaction require in reality no special explanation, the endo- and exo-thermic reactions forming in reality but one action.Thus copper will not dissolve in weak sulphnric acid the action being endothermic but zinc does dissolve the action being exothermic ; yet when alloyed with zinc copper dissolves in the acid the action being C1 &fortiori the less saturated anhydrous salt would do so also and the dissocia-tion products of the hydrate would therefore not be simpler than the reagent.* Some of both salts would no doubt be formed in the first instance but the one which was formed with the least evolution of heat would be eventually entirely converted into the other this reaction being ex hyp. one of those which does occur, and which having once taken place could not be reversed without an absorption of heat. VOL. LV. 18 PTCKERINO THE PRINCIPLES OF THERMOCHEMISTRY. accompanied with an evolution of heat. But this is not really a case of the copper dissolving but of a compound of copper-brass-dis-solving and ,since brass is capable of dissolving with an evolution of heat it is but in accordance with general observation to find that it does so. As true solids do not react with each other the cases to be examined are those in which gases or liquids figure.The reactions indicated below are the chief reactions in which gases are concerned and which are endothermic at 18". HZO 0,aq. c12,o. X O . N,O. N,02. Nz0,O. N20,2H20( = (NH,NO,). N2,2H20 = (NHdNO,). C2,% C,N,H. S 02aq S. H,I. I co27c* This list affords a striking argument in favour of the above prin-ciples for not one of these reactions is capable of taking phce at ordinary temperatures. One or two of them occur directly at higher temperatures (C,& ; C2,H2j but at these temperatures they are probably exothermic at any rate our imperfect knowledge of the heat capacity of the bodies concerned is not sufficient to enable us to affirm that they are not so. In the case of GO + C = 2C0 the reaction which takes place at 600" is no doubt endothermic at that tempera,ture and affords one of the simplest illustrations of the satisfaction of affinity producing endothermic results indirectly owing to the dissociation of the reagent.Thus at 600" carbon dioxide is partially dissociated that is the stable condition of a mass of that gas is oC02 + (1 - x)CO + (1 - x ) + 0 2 and if either of these three siibstances be removed the amount of dissociation will be increased or diminished till this stable condition be reproduced. Carbon being capable of combining with the free oxygen at this temperature, evolving 28,000 cal. in so doing removes this oxygen and necessi-tates the liberation of a fresh supply by the further dissociation of the dioxide and so on till this latter has been entirely decomposed a decomposition which absorbs 68,000 cal.leaving the algebraic sum of the actions at -40,000 cal.* * Referring to this action Rathke states that he considers it possible that th PICRERING THE PRINCIPLES OF THERMOCHENISTRY. 19 No real difficulties arise till we come to consider cases in which liquids are chiefly concerned and the three most difficult classes of such cases are the following :-(1.) The endothermic results on dissolving solids in liquids. (2.) The endothermic results on diluting strong solutions. (3.) The endothermic results attending double decomposition between substances in solution. Neither the immediate nor the mediate source of the absorption of heat which occurs when many salts are dissolved in water (to take a concrete instance of a solid and solvent) have ever been elucidated.It is j u s t as insufficient to say that it is due to the fusion of the solid as to say that this is a physical action and therefore requires no explanation. As a matter of fact the heat absorbed cannot possibly be accounted for by the fusion of the solid. The absorption amounts in many cases to some ~0,000 cal. per equivalent of salt.,* and this represents bnt a portion possibly but st half o€ the total value of the endothermic action for it is always counterbalanced t o a greater or less extent by the heat evolved in the formation o€ the hydrates of which the solution is composed. The heat of fusion of very few salts has been determined and to calculate its heat of fusion at the ordinary temperatures at which it is dissolved it is neces.mry to know not only its heat of fusion at the temperature at which fusion naturally occurs but also its heat capacity (specific heat) in the liquid and solid state.Potassium and sodium nitrates are the only anhydrous salts for which we have these data but judging by a comparison of these salts with ot'her bodies, their heat of fusion is not abnormally small yet it amounts to only-- (5300 + 3849 =) -1451 cal. for NRNO at 18", -(4800 + 2678 =) -2122 , for KNO ,, quantities wholly insufficient t o explain the heat absorbed in the dissolu-tion of the salts at 18" which is -5000 and -8M3 cal. respectively. Moreover we have a still more fatal objection in the fact that the heat absorbed i n dissolving a salt increases rapidly as the temperature is lowered,+ whereas the heat absorbed in.its fusion diminishes with a fall of temperature.presence of the second reagent may of itself inducedissociation of the first one. This amounts to the inadmissible conception of the satisfaction of a6nity producing directly a further supply of affinity that ie an endothermic reaction occurring in which affinity is the only agent. The whole virtue of dissociation in producing ei&khermic results consists in the presence of a third body (the product of the dissociation) capable of reacting with the other reagent ; till some of this third body is present no such reaction can take place. * I refer for the sake of simplicity to anhydrous salts only. t This increase cannot be due to a diminution of heat evolved in the formation of 0 20 PICKERING THE PRINCIPLES OF THERMOCHEXISTRY.But when a salt is dissolved in excess of water it becomes far more disintegrated than when it is merely fused. When fused the mole-cules are within the sphere of each other's attraction and are indeed, I believe combined to form molecular aggregates but this cannot be the case when they are separated from each other by several hundred molecules of the solvent ; they are then as much beyond the sphere of each other's attraction as if they were in the gaseous condition ; indeed insisting as I think we munt do on the continuity of the liquid and gaseous conditions,-a continuity which has so often been urged against the hydrate theory of solution but which as a matter of fact is of vital importance to that theory,-we must acknowledge that the condition of La substance dissolved in excess of water is identical with that of its vapour at the same temperature and to separate the molecules from each ohher to the same extent whether the condition be the dissolved or the ordinary gaseous condition must absorb the same amount of heat.Thus we have not only the heat of fusion of the solid but also its heat of volatilisation as a source of absorption of heat and the sum of these two quantities would cer-tainly be amply sufficient to account for the absorption noticed on dissolution," being in those cases where data are known ten or twenty times greater than the heat of fusion alone; and moreover it is a quantity which increases as the tempemture falls precisely what is noticed in the heat absorbed in dissolution.Thus the heat of fusion of Br2 is -388 cal. at 18" and diminishes by 4.8 cal. for every fall of 1" ; tlhe heat of volatilisation is - 7562 cal. increasing 11.3 cal. for a fall of 1". The sum of these two is -8032 cal. increasing by 6.5 cal. for a fall of 1". In the case of water-Heat of fusion at 18" = -(1580 +8*64(t - 18)). Heat of volat. a t 18" = -(lo798 - 12.05(t - 18)). Sum a t 18" . . . . . . . . = -(12378 -3*41(t - 18)). Having thus traced to its source the absorption of heat which occurs during dissolution the next question is what forces exist sufficient to bring about such endothermic results ? As previously stated they must be due to the dissociation of the reagents,-the salt the hydrates these hydrates are lesa dissociated at lower than a t higher tempera-tures and the heat of complete formation of any particular hydrate is probably influenced but very little by temperature a t any rate this is so with the formation of solid hydrates as I have shown ('l'rans.1887 335). * The heat of volatilisation in such a case mould be 580 cal. less than it is when the substance is vaporised in the usual way since this amount of heat is absorbed in the external work of expansion in the latter case PICKERISG THE PRINCIPLES OF THERMOCIBGMISTRY. 2 I or the water. Dissociation of the salt may occur to a certain extent in the case of hydrated salts but it is scarcely worth enquiring further into its influence since it will not help us to explain other cases; it must therefore be the dissociation of the water which acts as the primary cause.On views which I and others have for some years been pressing all liquids and solids must be regarded as consisting of compounds or aggregates of t4he fundamental molecules these aggregates just like the hydrates in a solution being more or less dissociated and being reduced t o less complex ones as the temperature rises ; and the recog-nition of these aggregates can alone give a satisfactory explanation of the physical properties of matter in its three conditions. Thus in true gases such aggregates as the vapoitr-density tells us, do not exist and from gases we learn that the heat capacity of each atom is a constant quantity.* With solids as with imperfect gases, the heat capacity though very nearly the sum of the atomic heats is not exactly so and with solids increases slightly with the tempera-ture ; this is exactly what would occur if the solids consisted of dis-sociating aggregates tlie heat absorbed by the dissociation renders the apparent heat capacity greater than it is with gases and as the aggregates dissociating become less complex and theleefore more firmly united as the temperature is higher more heat mill be absorbed in their decomposition and hence the apparent heat capacity of the solid will increase.Rut this variation in the heat capacity of solids will not be very great since the stability of solid particles is unfavourable to dissociation ; when however we come to liquids, where the particles are less restrained in their motions dissociation will take place to a much greater extent and we find as a consequence, that the heat absorbed in this dissociation is so great that it not only renders the apparent heat capacity of a liquid much greater than that of the corresponding solid or gas but that it makes it increase so rapidly with a rise of temperature and produces such irregularities in the increase that no approach to any so-called law can be observed here.As the boiling point is appruached the absorption of heat generally becomes much greater and under ordinary atmospheric conditions an almost sudden absorption (heat of vaporisation) occurs at this point when the simplest possible aggregates are resolved into their fundamental molecules.The irregularities observed in the expansion and other physical properties of many liquids can also be explained only on the supposition of a #discontinuous action which is wholly inconsistent with the idea of the fundamental molecule being * But not the same in the two ca8es. Wit,h gases 2.4 and with solids 6'4 is the heat capacity of each atom 22 PICKERINO THE PRINCIPLES OF THERMOCHEMISTRT. the acting unit of a liquid.* The continuity of the liquid and gaseous conditions is strictly adhered to according t o this view and the heat of volatilisation of a liquid a t any temperature (less the heat absorbed in producing the expansion) is simply the heat of decomposition of the liquid aggregates existing a t that temperature into the funda-mental molecules.Every physical fact relative to water tends to show that its compo-sition in the liquid condition is pre-eminently complex atnd its heat of volatilisation tells us that a t 18" (for instance) the heat of formation of the water aggregates is as much as 10,000 cal. approximately.+ It is argued that the so-called determinations of the molecular weights of solids and liquids by measuring the extent to which they lower the freezing point of some solvent (Rxoul t's method) proves that these molecular weights are of a very simple character. But these results which are inconsistent with so many other facts receive an easy explanation on my views as to the nature of dissolved substances. Raoult's method applies only to dilute solutions and in these dilute solutions the substance is really in the gaseous condition and we are determining the molecular weight not of the solid or liquid but of the gas.$ Now according to the kinetic theory of gases which in this respect applies equally to all fluids a mass of water consisting of' agqregstes having the average composition of zH,O at a temperature of 18" is made in reality of aggregxtes some a t a temperature above lS" some a t a temperature below 18" those aggregates which are at the higher temperature will be dissociated into less complex aggregates than * All that is said of the heat capacity may indeed be said of the expansion of substances.Perfect gases where no dissociation occurs expand regularly ; in dis-sociating gasps the expansion increases rapidly ; iu solids where but little dissocia-tion is possible the expansion is comparatively constant whereas in liquids where dissociation may occur to a large extent the expansion increases rapidly and often irregularly.I t may be suggested that the peculiarities in the expansiou are the causes of those in the heat capacity but this still leaves us with the equally d i 5 ~ u l t problem of explaining the former. I t is far more probable that they are both con-sequences of dissociation. t Taking the iiienn heat capacity of 18 grams of water between 18' and 100" as 18.1 and that of steam at constant volume as 6'65 and the heat of volatilisation of 18 grams of water at 100" as 9650 cal. we get. [9650 + 82(18*1 - C.65)=] 10,589 cal. as the heat of volatilisation at 18" of which 580 cal.are absorbed in producing the accompanying expansion. The correctness of this value hou ever is doubtful as the heat capacity of steam below 130"has not been determined ; but an error of even several thousand caioiqies would not affect the present argument. $ Ramsay (Trans. 1888,623) found the method applicable to nitrogen tetroxide when dissolved in only 18 mols. of acetic acid ; but in this case the vaporisation of the tetroxide due to its dilution would be materially increased by the tern-perature of the determination being only 10' below its normal boiling point PICKERING THE PRINCIPLES OF THERRIOCHEMISTRY. 23 sH20 say mH,O mlH,O &c. whereas those at the lower temperature will be more complex say zH,O zlH20 ; at this particular temperature, therefore the stable condition of a mass of water is such that there is a certain number of x m and zHzO aggregates presenh and if any of these be removed from the sphere of action more dissociation or com-bination will take place till this stable conditioii be restored, I n fact the water at this temperature is continually giving off fundamental molecules (that is has a vapour-tension) a certain number of these fundamental molecules must be present in the mass of the water and these molecules possess a potential energy equivalent to 10,000 cal.greater than that of the average particles constituting this mass and these particles will be able to effect a combination with evolution of heat which in the case of the average particles would involve the absorption of some 10,000 C R ~ .and therefore be impossible. One fundamental water molecule coming in contact with a salt molecule would thus be capable of combining with it if the heat of volatilisation of the salt molecules was liot greater than 10,000 cal. : the simultaneous arrival of two such water molecules would combine with the salt if its heat of volatilisatiou were double this quantity; but it is not necessary even to have recourse t o this simultaneous arrival even in such cases €or the combination of the salt molecule with one molecule of water only would not remove the former entirely from the sphere of attraction of its fellows it would not be completely volatilised and would not require to be supplied with as much as its full heat of volatilisation.The free water molecules being thus removed by their combination with the salt from the sphere of action other water aggregates must, according to the laws of dissociation split up to supply the vacancies, and this action is arccompanied by an absorption of - 10,000 for every 18 grams thus dissociated. But if this absorption of heat were not subsequently counterbalanced we should have proved far too much. I n the cases which have been investigated it has been found that each salt molecule combines ultimately with over 100H20; the heat absorbed in liberating this 100HzO would be 1,000,000 cal. not t o mention the heat absorbed i n the volatilisation of the salt, and it is quite impossible to imagine that the heat of combination of the water and salt molecules is so great as to nearly and often indeed more than counterbalance such an absorption." But here the teaching of the heat of neutralisation comes to our aid ; we learn from it as I have shown (Trans.1888 872) that the affinity which binds the dis-solved molecules to those of the solvent does not affect that by which * The heat developed in the mere combination of each water molecule with a salt molecule is probably between 200 and 5000 cal 24 PIOKIIRINO THE PRINCIPLES OF TIIERDIOCHEMISTRT. the solvent molecules are united with each other ; in dilute solutions, the water molecules are just as much combined with each other as they are in a mass of pure water the hydrates present are compounds of the salt with the aggregates or polymers of H,O ; and thus when a solid is dissolving as the proportion of the water molecules in the hydrate increases these then recombine with each other and in doing so will of course evolve the same amount of heat that their produc-tion from the water aggregates absorbed.The net results obtained, therefore when dissolution is complete will simply be the algebraic sum of two quantities (1) the heat evolved in the combination of the salt and water molecules (2) the heat absorbed in volatilising the salt molecule; and according as the former or latter of these is the greater so will the heat of dissolution be positive or negative ; but the motive power if I may use such a term which produces these results is the energy contained in the free water molecules. When a salt is dissolved by admixture with ice the heat absorbed is greater by the heat of fusion of the ice than in the case of water, but the same explanation will be sufficient in this case also.Ice near its melting point is certainly in a state of incipient fusion ; and some of the particles of liquid water present in it are certainly dissociated, even as far as the fundamental molecules as is proved by the con-siderable vapour-pressure of ice at this temperature. We therefore, have the same motive power as in the case of water. It may be predicted however that no such action would occur if the ice were perfectly dry as it is at a temperature some degrees below zero and it is certain that no such action could take place if the temperature were below that of the formation of the so-called cryohydrate.The endothermic results noticed in marly cases when strong solutions are diluted are but the extension of the action primarily occurring when the salt is dissolved. As the dilution is increased the hydrates become higher and less dissociated evolving heat while the salt becomes more entirely volatilised absorbing heat and the sign of the thermal change depends on the relative value of these two actions. Some years ago (Chem. News 64 217) I was led to believe in the existence of t w o such opposite actions from the mere study of the curves representing the heat of dilution as given by Thomsen. h o t h e r endothermio reaction also undoubtedly occurs in many cases-the dissociation of a salt into its free acid and base. This I think is a purely mechanioal action operating in the following manner.The salt in question when liquid is somewhat unstable and partially dissociated at the existing temperature even when no water at all is present and the extent of this is limited by the chances which oocur of the dissociated components meeting each other when in a suitable condition and recombining and these chances o PICRERING THE PRINCIPLES OF THERMOCHEMISTRY. 25 meeting are diminished a hundredfold when we increase a hundred times the space over which the substance extends by diluting it. The amount of dissociation occurring would thus be directly pro-portional t o the volume of the liquid and Thomsen’s results with acid sulphates (Thermochem. 3 Platte VI) tend to support t h i s view.* On such a view dilution could never start but only increase disso-ciation and we have no grounds for supposing i t to be otherwise.The third important class of enthodermic reactions to be explained is that in which double decomposition occurs between two dissolved substances. Double decomposition between two salts presents us with an instance of most frequent occurrence and to investipte this we may go to the root of the matter by ascertaining how and on what principles a base divides itself between excess of two different acids. According to the deduction drawn above from theoretical considera-tions the acid which evolves the most heat on neutralisation will take the whole of the base and consequently if both acids have the same heat of neutralisation as is generally the case when excess of water is present they will each take the same amount of the base this re-ferring only to cases where the salts formed are stable and remain in solution and it being assumed of course that the acids are present in equivalent proportions.When the acids are not present in equivalent proportions the base will divide itself between tbem in proportion to the number of equivalents of each present ; the division being regulated simply by the chances of impact. This is nothing but the law enunciated by Berthollet in 1803 and discarded at the present time as being altogether insufficient. B u t I think that it will yet be found sufficient while the more elaborate theories of recent days will fail. In cases where one of the salts formed is partially dissociated, the stable salt will be formed to the exclusion of the dissociated one when the solvent water is very large; but if the water is not in large excess there will be st limit to the dissociation and some of the less stable salt will be formed.For the salt being dissociated means that a t the given temperature its condition of stability is xAB + (1-%)(A + B) (A and B being the acid and base which form it) ; the free base B coming in contact with and com-bining with the stronger acid A ie thus removed from the sphere of action and more of the salt AB dissociates to give a further supply * There are of course other actions oocurring besides the dissociation of the salt the dilution proceeds ; we should not therefore expect the action to be repre-sented by a straight line but by lines which more nearly approach straightness than they do in cases where these other actions are the only ones occurring as in the case of diluting stable salts.Such are the characters of the curves in the two cases 26 PICKERING THE PRINCIPLES OF TEERXOCHEMISTRY. of free base and this action must continue as long as any dissociation at all takes place. But the proportion of the free weak acid present increasing apoint will be reached when every molecule of AB will find itself within the sphere of action of so many moleciiles of this acid that there would always be one of these present in a suitable condition to recombine with the base B the instant it dissociates from its former acid molecule. Practically there would be no longer any dissociation under these circumstances.By separating the salt AB farther from the free acid an increase in the proportion of water present would increase the limits of dissociation and therefore also the proportion of stable salt formed. It is obvious also that this latter would be increased by adding more of the stronger acid and diminished by more of the weaker one. All the determinations which have been made of the division of base between two acids by thermal methods depend on the comparison of the action of an acid on the base with that of snlphuric acid on the same base. Supposing in the first place that the heat of neutralisation of H2SOa per H displaced is the same as that of HCl and HNO,(a supposition which I shall shortly justify) then when 4NaOH is mixed with 4HCl and 2H2SOr the system formed according to the princi-ples of division of the base which I have laid down would be 2NaC1 and 2HC1 NazSOc and H,S04,-an equal division.But as a matter of fact whatever the explanation of the fact may be (and the explana-tion will be given below) Na2S04 and H,SO react on each other and form the acid salt 2NaHS04 thus leaving no free sulphuric acid, and an alteration in the division of the base would therefore become necessary in order to supply the place of the free sulphuric acid thus removed. In fact sulphuric acid acts as a monobasic acid only in this reaction and consequently we should compare 4HC1 with 4H,S04 in which case the base would divide itself equally between the two acids whereas when we compare 4HC1 with 2 H z S 0 4 the base would divide itself in the proportion of two equivalents to the former ai;d one to the latter.Now these theoretical deductions are entirely supported by the ascertained facts of the case. HN03 HC1 HBr and HI acids of which the heat of neutralisation is the same and which form stable salts divide the base between them equally the relative numbers obtained by Thomsen being 100 100 89 and 79 a@ these numbers are certainly as nearly equal as could be expected seeing t,hat Thorn-sen’s method contains many obvious sources of inaccuracy for he regards certain small quantities as being negligible he assumes certain actions to be represented by true hyperbolae which (judging from my results on the heat of dilution) they are not and the water which he The latter action is therefore of paramount importance PICKERIXG THE PRINCIPLES OF THERMOCHENISTRY.27 used was only 100H20 per each equivalent a quantity far too small to admit of t’he thermal results of dissolution being complete.* With iH2S04 and iH2S04 in accordance with my deductions we find that one-third only of the base is taken the numbers found being 49 and 45 respectively,? while with the other acids investigated the numbers were as follows :-Trichloracetic acid 36 Orthophosphoric acid . 25 + Oxalic acid 24 Monocbloracetic acid 9 Hydrofluoric acid 5 & Tartaric acid 5 + Citric acid 5 Acetic acid 3 4 Boracic acid (,B,Os) 1 + Silicic acid 0 Hydrocyanic acid 0 Of these all except phosphoric and oxalic acid form salts dissociated by water.and hence in accordance again with my deductions we find that they take either none of the base or but a small proportion of i t ; the dissociation of the trichloracetates is very small comparatively, and hence the acid gives an exceptionally high value. With oxalic acid, the quadrantoxalate is probably formed and hence 2C2H204 instead of QC2H204 should be compared with HN03 in which case the value for the avidity would be 96 that is nearly 100 as with other strong acids. With phosphoric acid only the results seem rather anomalous, but it must be reuiembered that in addition to other sources of uncer-tainty these numbers were not obtained by direct comparison with nitric acid but with sulphuric acid where the results may be compli-cated by the formation of acid and double salts.Ostwald (J. pr. Chenz. [ 2],19,473) made some determinations of the so-called “avidity” of acids when used in normal and decinormal solutions which showed that with the weaker acids the avidity was * For objections raised on other grounds Bee Hagemann Eilaige kritische Bermerkzcngen zur Aviditatsf ormell. t I.e. when HNO and +H2SO4 is mixed with zNaOH for every 100NaOH taken by the former acid the latter takes only 49 or one-third of the whole. $ The fluorides are certainly dissociated in solution as is shown by their alkaline reaction and their action on glass. The abnormally large heat of neutralisation of hydrofluoric acid is probably due to the 8ame causes as those which I have suggested (Trans. 1885 598) to meet the case of sulphuric acid this would involve the recognition of the acid having a more complicated constitution t,han HE” many facts inchding its thermal reactioiis render this very probable.Berlin 1887 28 PICRERING THE PRINCIPLES OF THEHMOCHEMISTRY. far greater when dilute than when strong. These results apparently in opposition to my deductions above are easily explained. The comparison was made by determining their relative action on calcium oxalate and the more the calcium salt formed was dissociated the more acid would there be left to continue the action on the oxalate. Directly opposite results would certainly be obtained by other methods. We assumed in the above that the heatt of neutralisation of srilphuric acid was the same per H atom displaced as that of the other acids, namely 13,800 cal.As a matter of fact, the experimental value is 2 x 15,690 cal." for the displacement of the two atoms of hydrogen ; but as we have seen i n this case the two atoms of hydrogen are not displaced but only the first one and the displacement of this evolves only 14,754 ; but even this number is greater than i t should be under perfect conditions for on the one hand it would be reduced by several hundred cal. if the dilution were infinite while on the other the acid salt formed is partially decomposed into the normal salt and free acid, a clecomposition which evolves heat and the removal of these two sources of evolution of heat would no doubt reduce the heat of neutralisation to the normal 13,800 cal.One more difficulty which lies at the root of the matter remains to be explained namely why is the acid sulphate formed in preference to the normal sulphste since the formation of the latter corresponds to the greater evolution of heat? Or in other words why does the normal salt combine with free acid to produce the acid sulphate with absorption of heat ? The explanation given by Berthelot (11 642) is not I believe, far from the truth. He points out that although the reaction between the sulphate and acid in weak solutions is endothermic yet between the anhydrous substances it would be accompanied by an evolution of heat. But it is not necessary to go back so far as the anhydrous substances to find an exothermic reaction and as these anhydrous substances do not exist in the solutions by so doing we do not in my opinion obtain any real explanation.The reaction how-ever will be exot,hermic when it takes place between the sulphate and any of the lower hydrates of the acid some molecules of which must certainly be present owing to dissociation even in comparatively dilute solutions. Thus although the reaction-NaqSO,2OOHZO + HzSOa200H,O = 2(NaHS04200HzO) gives -1870 cal. yet NazS04200H20 + H2S049H20 = 2 (NaHSOa104.5H20) * The number refers to H2SO4,200Hz0 with 400H20 it would be about 300 -1. smaller PICRERING THE PRINCIPLES OF THERMOCHEMISTRT. 29 gives (- 1870 + 2150 - 150 =) +130 cal. and with still lower hydrates of the acid and of the normal sulphate the reaction would become rapidly more exothermic. This reaction being known to occur when the substances are taken in this degree of hydration, would necessarily occur here and the removal of these lower hydrates of sulphuric acid from the sphere of action would necessitate a fresh dissociation of the higher ones to supply their places; hence the absorption of heat observed.The action is but an illustration of the principles here enunciated,-% possible action must always take place if it develops heat whatever absorption of heat its occurrence subse-quently involves owing t o the partial dissociation of the reagents. It is scarcely necessary however to point out that this reaction is never complete at any rate in weak solutions for it is limited by the reverse action the acid sulphate formed being dissociated back into free acid and neutral salt by the action of excess of water.” And an increase in the amount of water present will not only increase this reverse action but will also diminish the chances of the combination of the acid and normal salt by diminishing the proportion of the lower hydrates of the former in the liquid.It will thus be seen that the whole notion of the distribution of a base between two acids being det.ermined by certain constants peculiar to the acids termed their “ affinity,” or “ avidity,” becomes unnecessary and incorrect. And the manner in which this distribution occurs, instead of being irreconcilable with the results of the heat of neutralisation as L. Meyer maintains is determined solely by it and the known laws of dissociation. It is indeed incomprehensible how these ideas of “ avidity ” could have been accepted almost without question as has been the case, unless it be that the interesting mathematical exercise relating to affinity indulged in by Guldberg and Waage (ztudes mr Zes afinite’s Chimique Christiania 1867) was sufficient (as is generally the case when x and y is introduced into chemistry) to ensure the blind acceptance of t h a t which common sense would reject.What can be the meaning of an acid having a property which is a “ constant,” and which yet varies continuously with a variation in the proportion of the solvent present. In every cage which has been investigated the water bas a most marked influence on the division of the base. Thus Thomsen’s results (I 131) which he interpretad as showing that the water has no in-fluence on the avidity give-# This reaction would be exothermic with any hydrate of the acid aulphate which contains sufficient water to form on disso&tion a hydrate of the acid higher than about H,S04,9H,0 30 PICKERINCI THE PRINCIPLES OF THERMOCHEMISTRY, Avidity of H2S04 when 150 H20 for each double equivalent is present When 200 H,O is present 7 300 7 = 52* = 46 - 49 -while Berthelot who combats the idea of this “avidity,” gives the following values for the reaction of-i$K2SOa on HNOy in total of 2 litres of water -1810 cal.7 99 9 4 9 -1780 ,, 77 7 9 9 9 8 97 -1600 ,, 7 9 9 9 20 9 7 -1500 ,, And Ostwald (Zoc. sup. cit.) gives amongst others the following numbers :-In I n Relatire avidity of N solution.N/10 solution. Formic acid 2-33 12.9 Acetic acid. . 1.05 7-35 Monochloracetic acid . 4.6 21.3 Citric acid. . 2.75 14.4 Yet these numbers have been accepted as being constants in each case.? It must even be doubted whether the concordance between the vdues obtained by different methods is sufficient to warrant us in con-cluding that the numbers obtained really represent the division of the base which has taken place in the particular solutions investigated ; thus the values for-Sulphuric acid vary between nearly 100.0 and 46.0 Formic acid vary between 12.9 , 2.6 Acetic acid vary between 7.4 , 1.2 Monochloracetic acid vary between 22.0 , 5.1 Trichloracetic acid vary between 89.9 , 36.0 Oxrtlic acid vary between 43.0 , 22.6 Isobutyric acid vary between .5.8 , 0.9 Citric acid vary between . 14.4 , 3.1 and so on; numbers which no one who was not bent on proving a pet theory, If the proportion of water was sufficient to dissociate all the acid sulphate formed the +H,SO would either take all the base or exactly half of it according as its heat of neutralisation (to form the normal salt) in this state of dilution still remained greater than that of the nitric acid 13,800 cal. or which ie quite possible, was reduced so as to be equal to it. t Perhaps I am somewhat unfair in my criticism of Ostwald’s opinions he cer-tainly admits that the water present as well as the temperature influence the values for the affinity to a very great extent ; but on the other hand the whole ides of the existence of such a thing aa a conutant of atfinity is dependent on ite non-variation PICKERINO THE PRINCIPLES OF THERMOCHEMISTRY.3 1 irrespective of fact would ever have regarded as being identical in the respective cases.* When a base (NaOH) is mixed with two acids (HCl and HBr) of which the heat of neutralisation is the mme the enormous number of molecules actually taken in any experiment is sufficient reason for practically equivalent amounts of the two salts (NaC1 and NaBr) being formed but it is not so apparent why these same salts should be formed in equivalent proportions as we know they are when we mix one of them with the other acid NaCl with HBr for instance, unless dissociation occurs to such an extent that the fundamental molecules themselves are broken up into their constituent atoms, which would then combine with the other atoms of opposite character according to the frequency of collision that is in equivalent propor.tions. This resolution into atoms cannot I think be maintained but the necessity for it is obviated if more than two hydrates of each substance saturated to different extents be present. For the lower and less saturated hydrates of the one salt might react with the higher hydrates of the opposite acid so as to produce an evolution of heat while the same would occur with the lower hydrates of the other salt acting on the first acid and thus we should get ever-occurring opposite reactions admitting of the known interchange of radicles and soon rekulting in a condition of equilibrium.From the principle that the sum of the kinetic and potential energy of any system is an unalterable quantity and that affinity is energy it follows that the heat evolved in any reaction is the difference between the total energy of the system before and after the reaction and hence it seems at first sight that we should be able t o calculate the total energy in any substance and consequently the heat evolved in any reaction from a knowledge of the heat necessary to raise the reagents and compound from the absolute zero t o the temperature of the reaction. The principle of this method is indeed applicable with absolute certainty to the determination of the diference in the heat evolved in any reaction at two different known temperatures (Person’s principle the non-application of which would mean that energy could be created and destroyed see Trans.1887, 329) but it fails when we attempt to apply it to any determination of the actual amounts measured by extending our arguments as far as the absolute zero for the following reasons. * The most reliable thermal method (though even this would not be absolutely certain) would be t,o make a series of determinations with acids neutralised to different extents previously with the base and to plot out the results in a diagram and thus find the proportional neutralisation requisite to form solutions which on being mixed would develop no heat. Thomsen’s results quoted above were deduced from some determinations of this kind made by him 32 PICKERISG THE PRINCIPLES OF THERMOCHEMISTRY.(1.) The heat capacity of substances at ordinary temperatures affords no clue as to what their heat capacity would be at such low temperatures indeed we have good reason to suppose that great and comparatively sudden changes would be experienced in this heat capacity before the absolute zero were reached (see L. Meyer 87). (2.) Because we do not know where this absolute zero may be situated the generally accepted temperature of -273" is simply what the zero would be if a gas remained a gas and contracted regu-larly when cooled to this point; both of which suppositions we know to be incorrect in questions on thermodynamics -273" gives correct results simply because it is applied only to cases where perfect gases are in question and its use is simply equivalent to the shatement that gases expand 2+3 of the volume at 0" C.for each degree." Person's attempt (Awn. Chim. Phys. 21 295 ; 27 250) to find the absolute zero from other data treated in a precisely similar manner, led to discordant results. Assuming that ice and water could remain as such at the absolute zero and that they had then the same heat capacity as at known temperatures he found that the absolute zero should be -160" instead of -273" and if he had applied his principle (as it should be applicable if true) to the case of water and steam he would have found the still less acceptable result +850" C.f for his absolute zero. It is useless to base any theory on the supposition of facts being otherwise than we know them to be. (3.) Because it by no means follows that at the absolute zero of temperature potential energy of affinity as well as kinetic energy would be non-existent.Facts indeed favour the contrary view. Affinity can cease to exist as such that is become converted into heat only by being saturated by the combination of the sub-stances endowed with it. No such saturation can take place on cooling a perfect gas since a perfect gas is a substance in which the fundamental molecules never come within the sphere of each others i t The confirmation of -273" by Joule's evolution of Carnot's function (Scientifi Papers ii 290) is not independent as it is based on the coefficient of expansion of gases. Raoul Pictet's (Compt. rend. 88,855) calculation of the melting points of the elements based on -273" being the absolute zero gives more acceptable confir-mation but it depends on an hypothesis to start with and the variation of the constant obtained between the somewhat wide limits of 4 and 5 would allow of considerable latitude in the zero point taken.t The absolute zero is according to Person's argument the temperature at which the heat of fusion of ice is tail and similarly it should be that a t which the heat of volatilisation of wat,er is also nil. It appears to me that this latter tem-perature must be identical with the critical temperature of the liquid in question, but to calculate it properly we should have to take the actual (unknown) heat capacities a t these temperatures and not those a t other lower temperatures as ie the case abore ISOMERIC SULPHONIC ACIDS OF P-NAPHTHTLAMINE.33 attraction (in proof of which we find that when perfect gases combine to foPm a perfectly gaseous compound the heat of their combination a t constant volume is the same at all temperatures) with solids, the constancy in the heat given out in cooling a t most 6.4cal. per atom shows that the greater part of this is in all probability due to the fall of temperature only and that very little of it is due to combination ; this leaves but the heat evdved in the passage of the substance from the perfectly gaseous to the solid condition less that evolved by the mere fall of temperature to account for t'he total affinity possessed by the perfect gas an& this would I think fall far short of the amount of affinity known to. be possessed by many gases, for it could rarely if ever amount to. 10,000 cal. per moleede." The heat of neutralisation gives us again much information on Chis point for it shows that the agnity which serves to unite the similar molecules of a solvent with each other and which could aloiie become saturated by a fall of temperature is not the only €ree afiinity possessed by tbe molecixles fop it is independent of bhnt affinity owing to which these molecules combine with those of a salt and effect its dissolution; in other words there is other affinity besides that which could become satisfied by cooling the liqmid. For these reasons the so-called absolnte zero can give us no aid in calculating the heat evolved in a chemical reaction and we must be content to wait f o r the present till some other meana of doing so be discovered