Mr Rideal, On the latent heats of vaporisation 291 



On the Latent Heats of Vaporisation. By Eric Keightley 

 Rideal, M.A., Trinity Hall. 



[i^rnf? 28 February 1921.] 



A number of attempts have been made to associate the latent 

 heats of evaporation (L) with the natural periods of atomic vibra- 

 tion (v) as calculated by the methods of Lindemann*, Einstein, 

 BernouiUi and Nernst or determined experimentally by Rubens. 

 There is, however, no relationship between the infra red vibration 

 frequencies of the elements and their latent heats of evaporation. 

 On the assumption, however, that intermolecular, chemical and 

 physical actions take place with energy transfer in quanta and not 

 continuously, it should be possible to derive the latent heat of 

 evaporation of an element from some natural vibration frequency 

 or spectral hne. Although this spectral hue need not necessarily 

 be in the infra red portion of the spectrum. It is especially desirable 

 to test this hvpothesis in the case of the latent heats of evapora- 

 tion since this is a typical physical process and a conformity to 

 theo'rv would confirm the supposed identity of physical and chemi- 

 cal forces and at the same time from the thermodynamic equations 

 connecting the latent heats of evaporation and the vapour pressure 

 together with the molecular kinetic effusion equations of Herzt, 

 MarceUn and Langmuir it would be possible to calculate the mde- 

 termined integration constant of the vapour pressure formula and 

 thence the so-called Nernst chemical constants. 



According to the quantum theory applied to the energy change 

 involved in evaporation considered as a chemical process the latent 

 heat of evaporation per gm. mol. should be given by the expression 



L = Nh (^products ~ ''reactants)j 



where h is Planck's constant, Vp. the vibration frequency of the 

 products the metal vapour and tv. of the reactants, the solid or 

 liquid evaporating. The evaporation of a metal may be imagined 

 to take place by two different processes. We may remove a com- 

 plete atom from the metal to the vapour state, then the activating 

 frequency of the metal atom in the vapour state will correspond 

 to some very small energy transfer, since experiment has indicated 

 that practically all vapour atoms coming m contact with the solid 

 metal stick, i.e. as far as reactivity with the solid metal is concerned 

 practically all the atoms are active, thus v products will correspond 

 to a radiation far in the infra red and is probably not far removed 



* Physical Chemistry, McLewia, vol. m. 

 t Ann. Physik, 17, 177 (1882). 



