134 Mixter — Heat of Combination of Acidic Oxides. 



In experiment 3 the considerable quantity of unburned sub- 

 stance is due to the fact that the mixture was in contact with 

 the cold bomb and was not in an inner silver cup. 



Chromium Trioxide. 



The chromium trioxide was free from sulphuric acid and 

 sufficiently pure for the purpose. It was fused, allowed to 

 cool in a desiccator and then coarsely pulverized. A little ses- 

 quioxide remained after a combustion. The amount of tri- 

 oxide equivalent to it was deducted from the trioxide taken. 

 As the heat of oxidation of the sesquioxide to the trioxide is 

 small the error due to reduction is insignificant. The results 

 following show the heat effect of the combination of chromium 

 trioxide with sodium oxide. 



Experiments. 



1 



Chromium trioxide taken 4-000 4-782 grams 



" " reduced 0'097 0-176 " 



" " combined 3-903 4-606 " 



Carbon . 0*515 0*523 " 



Sodium peroxide . 10*6 16- " 



Water equivalent of system 3408* 3504- " 



Temperature interval 2-514° 2*595° 



Heat observed 8567 c 9093 c 



" of oxidation of carbon —5676 — 5805 c 



" " " " iron for ignition —62 — 62 c 



" absorbed by oxygen given off +187 +324 c 



3016 3550 c 



For 1 gram of chromium trioxide ._ 773 77l c 



The mean of the two results is 772° for 1 gram and for a 

 gram molecule of chromium trioxide it is 77000°. 



The heat of formation of the oxides of chromium is derived 

 from the above data, thus : 



3Na 2 2 -f Cr = Na 2 Cr0 4 + 2Na 2 + 158800° 



3NaO + 30 = 3NaO + 58200° 



Na o + Cr + 30 = Ka.0i*0 4 + 217000 C 



NaO + CrO, = NaCrO, + ._ 77000 ( 



Cr + 30 = CrO, + _ _.. 140000 c 



