310 REPORTS ON THE STATE OF SCIENCE, ETC. 



and therefore a third and final stage must ensue in wliich each newly formed 

 molecule loses a definite number of its molecular quanta, and passes into that 

 phase which is in equilibrium with the surroundings. 



The second stage of the reaction, to which alone the ordinary equation 

 applies, may be exemplified by the combination of hydrogen and chlorine. When 

 these gases are mixed they have no influence on one another, since they both 

 exist in non-reactive phases, but on the supply of the necessary number of its 

 molecular quanta a molecule, say of chlorine, is converted into its reactive 

 phase. Its molecular field is now in such a condition that it can form an 

 additive complex, C12.H2. Since atomic energy quanta can be lost by the 

 change Clz.Hj -^^HCl + HCl. this change takes place, and two freshly synthesised 

 molecules of liydrogen chloride are formed. 



The three stages of a reaction can be expressed in a general way by three 

 equations : 



1. A + E ergs = Ai 



2. A^ = B' + F ergs 



3. B" = B + G ergs. 



In the first stage the molecule A absorbs the amount of energy E ergs, and 

 is converted into the reactive phase A'. In the second stage this activated 

 molecule reacts to give the new and freshly synthesised molecule or molecules B' 

 with evolution of the energy F ergs. In the final stage these newly synthesised 

 molecules pass into their normal and non-reactive phase B with evolution of the 

 energy G ergs. The observed molecular heat of reaction in the change from 

 A to B is given by F + G — E multiplied by the Avogadro constant, and is positive 

 or negative — that is to say, the reaction is exothermic or endothermic, according 

 to whether F-f G is greater or less than E. 



It is evident that the energy E is equal to one or more molecular quanta 

 characteristic of the molecule A, and that the energy G is equal to a whole 

 number of molecular quanta characteristic of the molecule B. The energy F 

 must also be equal to one molecular quantum characteristic of the molecule B, 

 since, if the molecule A' loses any energy whatever, it will lose its reactivity 

 and no reaction will occur. The recognition of these three stages of a chemical 

 reaction and the quantitative expression of the energy change associated with 

 each is of fundamental importance. 



The three methods of supplying the necessary increment of energy E to 

 produce the necessary phase change for activation of a molecule have already 

 been discussed in detail. They consist in exposing the molecule either to infra- 

 red rays of frequency equal to the atomic, intramolecular, or molecular frequencies 

 of the molecule, or to light of frequency equal to that characteristic of the normal 

 and non-reactive phase of the molecule, and in submitting the molecule to the 

 influence of a solvent or catalyst. If the necessary increment of energy, E, is 

 small, it may readily be supplied by the first or the third method. This is the 

 more common case, and it explains why so many reactions are induced by rise 

 in temperature and why so many take place in solution. When reactions take 

 place in solution the first stage is apparently absent, but it must be remembered 

 that in such cases the first stage occurs at the moment the solution is formed. 



In order to obviate any criticism at this point, based on ionic reactions, it may 

 be stated that ionisation is a property of a particular phase of a molecule, which 

 is formed in solution. Nothing antagonistic to the phase theory is to be found 

 in the fact that one of the properties of a particular phase is its resolution into 

 ions. Mention may also be made of the undoubted reactivity of the non-ionised 

 molecules which are present in the ionic equilibrium. 



Again, some difficulty may be caused by a restricted use of the term ' sol- 

 vent.' The molecules, which by virtue of their residual affinity cause the phase 

 change on the part of the reactant molecule, are effective whether an actual 

 solution is formed or not. The phenomenon is, in fact, one of catalysis in which 

 the activating molecule acts as catalyst. A catalyst is, therefore, a substance 

 the molecules of which by virtue of their residual aflSnity form complexes with 

 the reactant molecules, and activate them by the supply of one or more molecular 

 quanta to each. In heterogeneous catalysis the action of the catalyst must be- 



