NATURE 



285. 



THURSDAY, JULY 29, 1880 



CHEMICAL DYNAMICS 

 Essai dc Mc'canigue Chimiqtie fojidee siir la Thcrmochcmie. 

 Par M. Berthelot. Two Vols. (Paris : Dimod, Editeur, 

 1879-) 



THE problems which the chemist attempts to scire 

 may be broadly divided into two groups. In studying 

 the heterogeneous distribution of molecules, the chemist 

 finds that new relations of molecules, in other words, new 

 substances, are produced ; he must study the composition 

 and properties of these substances. He also finds that 

 these changes involve a consideration of the relative 

 positions of the changing body and of other bodies, that 

 is to say, he recognises the action of force. He must 

 endeavour to determine the laws of action of this force. 

 The study of the properties and composition of substances 

 has received more attention than that of the general laws 

 of chemical force ; both methods of investigation must, 

 however, be applied to chemical phenomena, before these 

 can be fully explained. 



The general properties of a compound may be regarded 

 as depending chiefly on the composition of that com. 

 pound, or chiefly on the function or " power of doing " 

 of the compound. There have always been schools of 

 chemistry which paid most attention to composition, as 

 there have been schools which made function pre-eminent. 

 Modern chemistry is attempting to connect both in a 

 general scheme of classification ; whilst at the same time 

 she endeavours to learn the conditions under which 

 chemical force is exerted, and hopes thus to elucidate 

 general laws. 



The atomic theory, which is one great outcome of the 

 study of the composition and function of chemical sub- 

 stances, has of late years been merged in the wider 

 molecular theory of matter. 



This theory, assuming the existence of a grained 

 structure in matter, proceeds to deduce, by dynamical 

 reasoning, the amount of motion existing among these 

 little parts of which matter is built up. The laws of 

 Boyle and Charles are among the primary results of this 

 deduction. But under certain conditions gases do not 

 exactly obey these laws ; hence the theory asserts that 

 under certain conditions the molecules exert mutual 

 action. 



Another deduction from the theory is the statement 

 usually known as Avogadro's law — " Equal volumes of 

 gases at the same temperature and pressure contain equal 

 numbers of molecules." This statement provides the 

 chemist with a means for determining molecular weights. 

 But the chemist in applying Avogadro's law is obliged to 

 admit that in many reactions the parts of molecules really 

 part company. He attempts to picture to himself this 

 molecular splitting. 



Let the molecule A consist of two parts, a and /', the 

 molecule B of two parts, c and d; let these parts be in 

 motion. Under certain conditions the stress between a 

 and c and the stress between b and d may be greater than 

 that between a and h, and c and d; the original molecules 

 are decomposed, and new molecules, C and D, are 

 formed. The stress between a and c considered from the 

 Vol. XXII. — No. 561 



point of view of a or c alone is a force exerted by a on c 

 or by c on a. This force is the force of chemical affinity. 



The result of the action of this force is a new configura- 

 tion of the system A B ; the energy of the new system, 

 C D, will be different from that of the original system. 



Chemical action, thus regarded, is a re-arrangement of 

 parts of molecules imder the influence of the force called 

 affinity. Chemical energy is thus regarded as potential 

 energy. 



Now a chemical action between A and B will take 

 place under certain definite conditions only, hence 

 although the absolute value of the affinity of A for B may 

 be a constant, the course of the change and the entire 

 result of the change will nevertheless be largely dependent 

 on physical conditions. No force may be exerted except 

 at high temperatures ; the change of momentum of A 

 will depend on its position relative to B ; the relative 

 positions at which this change occurs may only be gained 

 at high temperatures. The force exerted may be small ; 

 still if a chemical change occur at all, there must be an 

 action between the parts of A and the parts of B. 



Now let this mutual action begin, let no energy be 

 added to the system from without, but let the system as a 

 whole lose energy; the energy so lost may be measured in 

 the form of heat. But more than one re-arrangement of 

 the parts of two molecules may frequently be possible ; 

 which will be produced ? A system is in equilibrium when 

 its entropy (using the term in the Clausian sense) has 

 reached a maximum. Hence that system whose entropy 

 is the greatest of the entropies of the possible systems 

 will be produced. 



This is substantially Berthelot's " law of maximum 

 worlj," a law which lies at the foundation of his system 

 of thermal chemistry. But a system not marked by 

 possessing the largest amount of entropy of all the possible 

 systems, may nevertheless be the most stable under the 

 experimental conditions ; the stability will depend on 

 pressure, temperature, relative masses, &c. Hence in 

 order to determine the actual result of a chemical action, 

 the conditions of stability, in other words, the relations to 

 temperature, pressure, &c., of the various possible pro- 

 ducts of the reaction must be known. The necessity of 

 this knowledge is insisted on by Berthelot. To determine, 

 therefore, the product of a given chemical action one 

 must measure the quantities of heat evolved in the 

 passage of the system from the standard state to each 

 of the possible new states, and one must know the con- 

 ditions of existence and stability of each of these states 

 This problem therefore presents both a chemical and a 

 physical question for solution. The solution of the 

 chemical question is much aided by a knowledge of the 

 laws of atom-linking ; but these cannot be here con- 

 sidered. 



A measurement of the heat evolved in a chemical 

 change evidently enables us to find the difference between 

 the energy of the original and final chemical systems ; the 

 total heat change being independent of intermediate 

 states through which the system may pass. So if work 

 is done on a chemical system whereby it is caused to 

 assume a new configuration, this work measures the 

 energy transferred from the initial to the final system ; 

 in this case heat will be absorbed during the chemical 

 change. 



