PROCEEDINGS 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. V, pp. 1-37. January i, 1903. 



A NEW LAW IN THERMOCHEMISTRY. 



By Frank Wigglesworth Clarke, 

 Chief Chemist U. S. Geological Survey. 



It is well understood by all who have closely considered the 

 subject, that the so-called " heats of formation " of chemical com- 

 pounds are not true physical constants, but complexes of very 

 diverse factors. Take, for example, the heat of formation of 

 hydriodic acid ; which, when calculated from gaseous iodine, 

 has according to Thomsen, the value of — 605 small calories. 

 This value, however, is the algebraic sum of at least three 

 quantities, one plus and two minus, as the following equation 

 shows : 



2HI — H2 — I2 = — 1210 calories. 



That is, heat is absorbed in decomposing the molecules of 

 h3^drogen and iodine, and liberated by the subsequent union of 

 the separated atoms ; the sum of the three terms being negative. 

 The entire series of changes is endothermic ; but the final step, 

 the combination of hydrogen with iodine, must evidently be an 

 exothermic operation. 



A still more complex illustration is furnished by the formation 

 of arsenic trichloride, AsClj. Here solid arsenic unites with 

 gaseous chlorine, to form a liquid compound ; and the thermal 

 value of the transformation is affected by changes of physical 

 state. Furthermore, heat is lost in dissociating the tetratomic 

 molecule of arsenic and the diatomic molecule of chlorine, and 

 evolved by the combination of these elements with each other ; 



Proc. Wash. Acad. Sci., January 1903 (i) 



