Mixter — Formation of the Oxides of Cobalt and Nickel. 201 



8. Heat of reaction of manganese dioxide with sodium, 

 peroxide. — Manganese dioxide was prepared by heating man- 

 ganese nitrate until the residue ceased to lose weight at 400°. 

 A weighed amount of the dioxide left on ignition very nearly 

 the calculated quantity of Mn.,0,. 



Combustions of the following mixtures were made : 



't> 



Manganese dioxide 8-000 lO'OOO grams 



Sulphur 1-000 1-000 " 



Sodium peroxide 19- 21- " 



Result for 1 gram of MnO, 368° 323"= 



The mean is 345° and for a gram molecule of manganese 

 dioxide it is 30-0°, adding 19-4° for the heat of Na.O+O gives 

 for MnO„+0+]SIa„0=Na,MnO,+49-4°. 



For the heat e£fect of Mn + 20 we have 169-0° - 49-4° = 

 119-6°. As the experiments do not show that Mn and MnO^ 

 are completely oxidized in the fusion to MnO^ the results are 

 to he regarded as approximations. The errors cannot be large 

 since Le Chateliers* found that Mn + 20 = MnO, (crystalline) 

 +126-0° or 6-4° more than given above for amorphous MnO^ ; 

 and the heat of formation of an amorphous compound is 

 always less than that of the crystalline form. Manganese 

 trioxide decomposes at common temperature into the dioxide 

 and oxygen. This spontaneous change indicates that the 

 reaction MnO^+O is endothermic, at least it is not exothermic. 

 The heat effect of MnO„0, JSTa.O = 49-4° results chiefly from 

 the reaction JSTa^O+MnOg. 



*C. E.,exxii, 81. 



