Mixter — Thermochemistry and the Periodic Law. 519 



Art. XLIII. — Thermochemistry and the Periodic Law. 

 Heats of Combination Linear Functions of the Atomic 

 Weights of Related Elements ; by W. G. Mixter. 



[Contributions from the Sheffield Chemical Laboratory of Yale University.] 



Different writers have alluded to a periodicity in the 

 quantity of the heat of formation of compounds. Mendeleeff* 

 gives Laurie's diagram for expressing the periodic variation in 

 the heat of formation of chlorides and states (1) that " Thom- 

 sen . . . saw a correlation in the calorific equivalents of analogous 

 elements, although he did not remark their periodic variation ; 

 (2) that the uniformity of many thermochemical deductions 

 must gain considerably by the application of the periodic law, 

 which evidently repeats itself in calorimetric data, and if these 

 data frequently lead to true forecasts, this is due to the perio- 

 dicity of the thermal, as well as of many other properties, as 

 Laurie remarked ; and (3) that the heat of formation of the 

 oxides is also subject to a periodic dependence which differs 

 from that of the heat of formation of the chlorides, in that the 

 greatest quantity corresponds with the bivalent metals of the 

 alkaline earths (magnesium, calcium, strontium and barium) 

 and not with the univalent metals of the alkalies, as is the case 

 with chlorine, bromine and iodine." There has been, however, 

 no study of thermochemistry in the light of the periodic law. 

 Such a study reveals more than Mendeleeff suggests and shows 

 in many instances that the heat of formation is a linear func- 

 tion of the atomic weight. We shall see the application of 

 this law to elements of a series in Mendeleeff's classification 

 and to the members of a group of closely related elements, 

 such as the halogens and the alkali-earth metals. Also that 

 the variation from linear in other cases is quite uniform. 



The thermochemistry of the different series will first be 

 considered and then the thermochemistry of the groups. 

 Table I is a convenient arrangement of the elements to use in 

 referring to the periodic classification. 



Thermochemistry of the Series. 



The thermochemistry of oxygen and chlorine is more com- 

 plete than that of any other element, that is to say, the heats of 

 formation of more oxides and chlorides are known than of any 

 other class of compounds, hydrocarbons excepted. Hence only 

 oxides and chlorides are included in the diagram, fig. 1. It 

 should be understood that the total heats of formation of the 

 higher oxides and chlorides are not represented but only the 



•* Principles of Chemistry, II, 30. 



Am. Jour. Sci.— Fourth Series, Vol. XXXVII, No. 222.— June. 1914. 

 36 



