524 Mixter — Thermochemistry and the Periodic Law. 



Table II. 



Potassium Rubidium Caesium 



R + Aq 46-4 R 4*7-2 R 48*5 Cal. R 



48-1 T 48-2 Bek. 5T0 Bek. 

 45-2 Johannis 



2R + O 86-8 R 83-5 R 82"7 R 



97-1 Bek. 94-9 Bek. 100-0 Bek. 



R + O + H 102-8 deF. 102-0 deF. 101-3 deF. 



R + CI 105-2 deF. & T 105-9 deF. 109-9 deF. 



2R + S + 40 343-9 deF. & T 344-7 deF. 349-8 deF. 



Rengade appears to be the only one who has determined the 

 heats of solution of all three of the metals, namely, potassium, 

 rubidium and caesium. Hence his results only are incorporated 

 in Table III for these values. For a similar reason deFor- 

 crand's are taken for the chlorides and sulphates since we have 

 only his experiments on the heats of formation of the rubidium 

 and caesium compounds. The values for KC1 and K 2 S0 4 are 

 derived from K + O -f H = 102-8 and Thomsen's data for 

 HC1 and H 2 S0 4 , the heats of neutralization and solution. As 

 already suggested, there is some uncertainty in the thermo- 

 chemistry of the alkali metals. It is best, however, to use the 

 results of one investigator for analogous compounds as his 

 errors are likely to be constant. 



The numbers in brackets in Table III are linear values cal- 

 culated from the atomic weights of the three metals and the 

 heats of formation of the potassium and caesium compounds. 







Table III. 









Potassium 



Rubidium 



Linear values 



Caesium 



2R + Aq 



92-8 



94-5 



[ 94-8] 



96-9 Cal. 



2R -f 20 + 2H 



205-6 



204-0 



[204-1] 



202-6 



2R+ 



86-8 



83-5 



[ 84-8] 



82-7 



2R + 2C1 



211-4 



211-9 



[215-6] 



219-7 



2R + S + 40 



343-9 



344-7 



[346-7] 



349-8 



The quantities of heat evolved when the three metals are 

 dissolved in water are linear functions of their atomic weights. 

 The same holds for the heats of formation of the hydroxides, 

 while the observed result for rubidium chloride is 2 per cent 

 below the linear value, and rubidium sulphate is 0*6 per cent 

 low. The diagram fig. 2, where double atomic weights are 

 made abscissas and heats of formation ordinates, shows the 

 thermochemistry of the first group. The heat effects of the 

 subgroup of potassium, rubidium and caesium are linear, while 

 those of the other members diminish irregularly with increas- 

 ing atomic weights. 



