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BOOK II.

CHEMICAL KINETICS.

CHAPTER I.

DISSOCIATION.

160. WE must now proceed to consider some of the phenomena which are connoted by the term chemical kinetics. In the introduction to book I. I said that this term is to be used as including the facts and principles which on the whole relate to chemical action, as opposed to those which are for the most part connected with chemical composition. We have however seen how impossible it is to carry out this division in any but the broadest way. We have seen that the same facts may be, indeed must be viewed, now from the statical, now from the kinetical stand-point. But while this is true, it is, I think, established by the history of chemistry that progress has been made at some periods more by seeking knowledge of the composition than of the reactions of compounds, while at other times inquiries into the functions rather than the composition of different kinds of matter have been most productive.

A chemical change must be looked at from two points of view; the relations between the reacting bodies, and the relations between the forces concerned in the change, must be studied.

161. The reactions of elements and compounds when produced in contact with other substances furnish problems for

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the solution of which both statical and kinetical methods are required. Such reactions have been considered from the point of view of the composition of the changing system, and some of them have been also regarded from the stand-point of thermal chemistry. Emphasis was laid on the necessity of considering not only the composition of the whole changing system, but also the conditions under which the change proceeds, e.g. the relative masses of the reacting substances, the rate of evolution of the 'nascent' products, the temperature, &c.

162. The fifth section of the second chapter of book I. is occupied with questions which cannot be even approximately answered except by the help of essentially kinetical conceptions. We there learned to recognise the existence of substances intermediate between true compounds,-the properties of which are for the most part conditioned by the nature, number, and mutual interactions of the atoms which compose their molecules,—and true mixtures,—the properties of which are the means of those of their constituents. But the consideration of molecular compounds' led us to regard the chemical system of which such compounds form a part as continually undergoing change, and as held in equilibrium, as a whole, only by the mutual actions and reactions of its parts. In that section (par. 101) facts were stated which seemed to shew that molecules of different degrees of complexity may be simultaneously present in a gas. We found, for instance, (par. 101) that the gas obtained by heating phosphorus pentachloride contains molecules of PCI, and Cl, in addition to molecules of PCI,; and that as temperature rises the former kinds of molecules increase in number, while the latter decrease, until at a little above 300° no PCl, molecules remain. The most simple and probable explanation of the numbers representing the vapour density of acetic acid at different temperatures and pressures is, that at temperatures below 120° (pressure being normal) the gas consists for the most part of molecules having the composition C,H,O,, but that at 250° or

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1 Book I. chap. 11. Section 1.

2 Book I. chap. IV. pars. 122-124.

so (pressure being unchanged) the formula CHO, represents the composition of by far the greater number of molecules which compose the gas. The explanation which was given of these and similar facts regarding the relations of the densities of gases to temperature and pressure was based on the kinetic theory of gases (see par. 101, pp. 207-8). If that explanation is accepted, it follows, I think, that the condition of a gas in which dissociation is proceeding must be such that the equilibrium momentarily established between molecules of different atomic composition is being continually disturbed, and fresh conditions of equilibrium are being established, which in turn last only for a very short time1.

In this gradual change of one kind of molecules into another kind brought about by the action of heat we have one of the distinctive features of dissociation, or thermolysis, the phenomena of which it is the object of the present chapter to examine2.

163. The term dissociation is generally used to indicate the resolution of more into less complex molecules, by the action of heat; the amount of resolution increasing as the temperature increases, and decreasing when the temperature decreases. For each substance there is a temperature at which the whole, or almost the whole, is converted into two or more new

1 It has been asserted (Deville and Troost, Compt. rend. 64. 237: 91. 54; Berthelot, do. 91. 77) that the observed changes in the densities of various gases, e.g. N2O4, 12, &c., at different temperatures are to be regarded as indicative of variations in the coefficients of expansion of these gases. If this is so, then the relations between the volumes of those gases and the temperature are very curious. The values of the coefficients of expansion of those gases which are generally supposed to dissociate when heated, must, on this view, be regarded as increasing rapidly with increase of temperature until a maximum is attained, and then again decreasing to a constant value. In connection with this subject it ought to be remembered that Gay-Lussac's coefficient of expansion (0'00365) is only approximate, that indeed Boyle's law is only approximately true, but that at the same time the relation between the volume and the temperature has been carefully examined for several gases, and it has been shewn that even at very high temperatures the coefficients of expansion of these gases are constant. [The gases in question are H, O, N, S, Te, Hg, CO2, HCl, AsО; see V. Meyer, Ber. 13. 2022.]

2 In connection with this subject generally see article 'Dissociation' in Neues Handwörterbuch der Chemie.

bodies'. If the temperature is now allowed to fall, the products of the original change gradually recombine until the initial state of the system is again attained. When on the other hand, a substance undergoes decomposition by the action of heat there is a certain temperature-interval within which the change, if started, goes to completion; moreover the initial state is not regained by allowing the products of the change to cool in contact with each other.

When a substance dissociates at least one of the products must be gaseous under the conditions of the experiment.

164. The process of dissociation presents some analogies with that of evaporation. In both there is a gradual change brought about by the action of heat. As the rate of evaporation is conditioned by the pressure exerted on the liquid by the vapour, so the rate of dissociation is conditioned by the sum of the partial pressures of the gaseous products of the action 2.

For any temperature there is a certain pressure, such that when this is reached the process stops, but it may be again started either by decreasing the pressure or by increasing the temperature. This pressure has been called the dissociationpressure, or better the equilibrium-pressure. Dissociation

may therefore be stopped by allowing one or more of the gaseous products to accumulate in contact with the dissociating body; on the other hand dissociation may be caused to proceed rapidly by removing one or more of the gaseous products as quickly as it is produced.

Thus Wurtz found that the gas obtained by vapourising phosphorus pentachloride into an atmosphere of phosphorus trichloride was composed for the most part of molecules of PCI,; although when the same compound was vapourised at the same temperature into air the molecules of PC1, were for

1 The numbers given in par. 101 for the densities of various compounds sufficiently represent the gradual progress of dissociation-phenomena. A formula is given on p. 204 (note) by means of which the amount of dissociation of gaseous compounds may be calculated from observations of the densities at different temperatures.

2 See especially Deville, Compt. rend. 56. 195; and Leçons sur la dissocia

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the most part dissociated into PCI, and Cl. So also Pebal shewed that ammonium chloride may be permanently decomposed by vapourising it in an arrangement which permits of the rapid removal of the less dense product of dissociation, although when the same compound is vapourised into an enclosed space, and the products (ammonia and hydrochloric acid) are allowed to cool in contact with each other, the whole of the original substance is regained in the solid form.

165. When solid calcium carbonate is heated in a closed space lime and carbonic anhydride are formed, but on cooling calcium carbonate is reproduced.

Debray has shewn that, at a red heat, the direction of this change is dependent only on the pressure. For each temperature there is a maximum pressure exerted by the gaseous carbon dioxide at which the direct change, xCaCO ̧ = CaO + CO, + (x − 1) CaCO,, stops; at 860° this equilibrium-pressure = 81 m.m. of mercury and at 1000° it is equal to 520 m.m. If a system consisting of CaO, CaCO,, and CO, is slowly cooled, the whole of the carbon dioxide is absorbed by the lime, but if the temperature is rapidly lowered some of the dioxide remains uncombined with the lime. If the temperature is slowly lowered a certain amount of carbon dioxide is absorbed, the pressure is changed, and the equilibrium is overthrown. But a new condition of equilibrium is attained, to be again destroyed by absorption of more carbon dioxide following on a further lowering of temperature. Finally stable equilibrium is attained when the whole of the dioxide has combined with the lime. If however the temperature is caused to decrease rapidly, the normal absorption of the dioxide corresponding to each change of temperature cannot be completed, and the cooled system is composed of chalk, lime, and carbon dioxide*.

166. Dissociation is thus essentially a reversible process; it is accompanied by absorption of a definite quantity

1 Compt. rend. 76. 601.

3 Compt. rend. 64. 603.

2 Annalen 123. 199.
Pogg. Ann. 149. 222.

"A physical process is said to be reversible, when the material system can

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