5i6 



NATURE 



{April i, 18S0 



CHEMICAL EQUILIBRIUM 



THROUGHOUT the history of chemistry two lines of 

 advance may be traced. At one time chemists 

 have endeavoured to answer the question, what does this 

 substance do ? At another time they have inquired, of 

 what is this substance composed ? 



Function and composition have been, and continue to 

 be, the two great guides in the development of chemical 

 science. 



Chemistry has always had her kinetical as well as her 

 statical problems. 



And in recent times, as dynamical reasonings have been 

 more and more applied to chemical phenomena, we find 

 the broad distinction still prevailing. 



" We can already distinguish two lines along which 

 dynamical science is working its way to undermine at 

 least the outworks of chemistry. ... Of these two lines 

 of advance one is conducted by the help of the hypothesis 

 that bodies consist of molecules in motion, and it seeks to 

 determine the structure of the molecules and the nature 

 of their motion from the phenomena of portions of matter 

 of sensible size. The other line of advance, that of 

 thermodynamics, makes no hypothesis about the ultimate 

 structure of bodies, but deduces relations among observed 

 phenomena by means of two general principles- — the con- 

 servation of energy, and its tendency towards diffusion " 

 (Clerk- Maxwell, Science Conferences, South Kensington, 

 1876). 



In a paper published in Nature (vol. xx. p. 530), I 

 endeavoured to give a short sketch of the work of Guld- 

 berg and Waage on the influence of mass on chemical 

 action. The theory of these naturalists is largely based 

 on the hypothesis of the molecular structure of bodies 

 and is developed by the application of dynamical reason- 

 ing to experimentally determined facts. The theory is 

 a most successful attempt to explain the nature of the 

 motion of certain molecular systems " from the pheno- 

 mena of portions of matter of sensible size." 



Guldberg and Waage deduce the conditions of equili- 

 brium of many representative chemical systems ; but they 

 do this by simplifying the phenomena, by considering only 

 the force of affinity, and by overlooking the action of all 

 " secondary forces." They show how chemical equi- 

 librium is modified by changes in the value of the coeffi- 

 cients of affinity, and by changes in the masses of the 

 reacting bodies. They regard each chemical change as 

 proceeding through two or more phases, and as eventually 

 returning to its original phase, and thus completing itself, 

 unless prevented by the action of extraneous forces. 



The work of the Norwegian Professors is confirmatory 

 of the kinetic theory of chemical action, that theory, 

 namely, which regards molecular decompositions and 

 recompositions as continuously proceeding even in appa- 

 rently stable chemical systems. 



A most important paper by Prof. Willard Gibbs, of 

 Yale College, bearing on the thermodynamical problem 

 of the equilibrium of chemical systems, appeared some 

 time ago in the Transactions of the Academy of Sciences 

 of Connecticut (vol. iii.). This paper was summarised 

 and rendered intelligible to the chemist by the late Prof. 

 Clerk Maxwell in one of those marvellously condensed 

 and suggestive sketches which he, perhaps better than 

 any other naturalist of modern times, knew how to draw. 

 (he. cit.) 



Prof. Gibbs deduces from the principles of the conser- 

 vation and dissipation of energy, a general expression for 

 the stability of any phase of matter with regard to any- 

 other phase. 



If K represent the stability of a given phase A with 

 respect to any other phase B, then the phase A will tend 

 to pass into the phase B if K is negative ; but if K be 

 zero or positive, the phase A is absolutely stable. 



K varies with the component masses, volume, and 



entropy (called the magnitudes of the system by Clerk 

 Maxwell), and with the temperature, pressure, and the 

 potentials of the component substances (called the intensi- 

 ties of the system) : " the potential of any component 

 substance is the intensity with which the body tends to 

 expel that substance from its mass." 



The phase A may be stable in itself, and, nevertheless, 

 "may have its stability destroyed by contact with the 

 smallest portion of matter in certain other phases." 



No absolutely unstable phase can exist for any finite 

 time, but such a phase may form an intermediate stage 

 between other relatively stable phases. Indeed, " the 

 region of absolutely unstable phases is in contact with 

 that of absolutely stable phases at the critical point. 

 Hence, though it may be possible by preventing the body 

 from coming in contact with certain substances to bring 

 it into a phase far beyond the limits of absolute stability, 

 this process cannot be indefinitely continued, for before 

 the substance can enter a new region of stability, it must 

 pass out of the region of relative stability into one of 

 absolute instability, when it will at once break up into a 

 system of stable phases^' (Clerk Maxwell, loc. cit.). 



That certain phases of heterogeneous substances were 

 unstable has, of course, been long known to chemists— 

 although such phases have been almost entirely dis- 

 regarded in chemical investigations — but we are now 

 taught that not only is the existence of such phases 

 recognised by the great principles of the conservation and 

 dissipation of energy, but that the conditions of their 

 existence, and of their relations to stable phases of the 

 same mass of matter, can be deduced from these 

 principles. 



Chemists have long groped after some definite con- 

 necting link which should bind their more empirical 

 generalisations with the great principles of energy which 

 are so far-reaching in their application to physical 

 science ; the genius of a mathematician seems at last to 

 have revealed the bond. 



As examples of what might be called strained equili- 

 brium, that is, of systems carried into phases much 

 beyond the limits of absolute stability, and of the sudden 

 overthrow of the equilibrium by small exciting causes, 

 Prof. Clerk Maxwell notices the case of water, freed 

 from air and surrounded by a liquid of high boiling 

 point, remaining in the liquid state at a temperature 

 much above the boiling point corresponding to the pres- 

 sure, but exploding instantly it comes in contact with any 

 gas ; he also cites the equilibrium of a 37 per cent, solu- 

 tion of calcium chloride when cooled below -37°, as 

 described by Guthrie in his researches on cryohydrates. 



Many other similar cases might be noted. In my own 

 work I have recently met with certain phenomena which 

 may, I believe, be explained by the general principle now 

 under consideration. 



In studying the effects of mass, time, &c, on the 

 decomposition which may be formulated — 

 x BiCL + x' HC1 + x" H.,0 = (x-n) BiOCl + 11 BiCl 3 



+ {x- + 2 (x-n)} I1C1 + {.r" - (x-n)} H 2 0,' 

 I noticed that if such a quantity of water be cautiously 

 poured on to the surface of a solution of bismuthous 

 chloride in hydrochloric acid, as just suffices to produce a 

 trace of solid bismuthyl chloride (BiOCl) at the surface of 

 contact of the two liquids, and if the liquids be then mixed, 

 the amount of bismuthous chloride which has undergone 

 decomposition after a given time— provided the time be 

 short— is much more than if the water be added to the 

 bismuth solution with constant stirring. 



Indeed, I found that it was possible to arrange two 

 systems, each containing the same quantity of BiCI-,, HO, 

 and H 2 0, so that one of these should remain clear, i.e., 

 without formation of BiOCl, whilst in the other a consider- 

 able amount of decomposition should occur. 

 1 Chem. Soc. Journal, Proc 1879. P- 3"- 





