PHYSICAL CHEMISTRY 537 



PHYSICAL CHEMISTBT. By W. E. Garner, M.Sc, University 

 College, London. 



'Thermochemistry. — Notable advances have been made in recent 

 years in the determination of the heats of formation of chemical 

 compounds from the elements in the gaseous and monatomic 

 state. Ultimately it will be possible to refer all heats of forma- 

 tion to this new datum-line, and to evaluate the heats of forma- 

 tion of the individual chemical linkings of any given chemical 

 compound. 



In the case of a few simple gaseous molecules, at compara- 

 tively high temperatures, it is possible to determine the heat 

 of formation of the chemical linking, directly from the degree of 

 dissociation. The heats of dissociation of the halogens and 

 hydrogen have been obtained in this manner fairly accurately 

 (Pier, Zeit. Elektrochem., 1908, 62, 417 ; Bodenstein, Zeit. Elek- 

 trochem., 1 910, 16, 961 and 1916,22, 327 ; and Langmuir,/.^. C.S., 

 191 5, 37, 417). The extension of our knowledge of crystal 

 structure, ionisation potentials, and spectroscopy has made 

 possible other methods of estimation of the energy of formation 

 of chemical linkages (see Lewis, Annual Reports, Chem. Sac, 

 1 921). It is thus possible to deduce values for the heat of 

 dissociation and ionisation of hydrogen, oxygen, and nitrogen 

 molecules from the ionisation and resonance potentials of these 

 gases. Q = 23 X E, where E is the corresponding potential 

 (. . .). Franck, Knipping, and Kriiger find that the heat of 

 dissociation of hydrogen determined in this manner is 81,300 

 calories, which is in agreement with the value obtained by 

 Langmuir. Diatomic gases, such as hydrochloric acid, are more 

 complex in their behaviour on ionisation ; Knipping {Zeit. fUr 

 Phys., 192 1, 7, 328) following a method of formulation due to 

 Haber, shows that hydriodic acid can undergo the following 



'^^"^^^ '■ „, . Qh. H. I, Dh + D, 



HI < — -f, -i < "" ' "■ H,I. 



A A 



X Ih 



H+, I- < £^ H+, I, e. 



where Qhi = the heat of formation of HI. from H, and I, 

 where Dh and Dj = the work of dissociation of hydrogen and 



iodine respectively, 

 Ih = the heat of combination of the hydrogen 



ion with an electron, 

 Ej = the heat of formation of an iodine ion 

 from the atom, 

 and X = the heat of formation of hydriodic acid 

 from hydrogen and iodine ions. 



