HEAT OF FORMATION OF OXALATES. 



365 



We find, moreover, the heat of formation of metallic oxides in 

 the tables (p. 130). 



These are the values relative to the more common metallic 

 oxalates : 



Zn + = ZnO 

 Pb + = PbO 

 Cu + = CuO 

 Hg + = HgO 



Ag 2 + = Ag 2 



+ 86-4 

 + 53-4 

 + 38-0 

 + 31-0 

 + 7-0 



By the method of double decomposition the heat liberated 

 by the union of the metallic oxides with oxalic acid was 

 measured ; or l 



H 2 C 2 4 (diluted) + ZnO (precipitated) = ZnC 2 4 + H 2 

 H 2 C 2 4 + PbO = PbC 2 4 + H 2 



H 2 C 2 4 + CuO = CuC 2 4 + H 2 



H 2 C 2 4 +HgO = HgC 2 4 + H 2 



H 2 C 2 4 +Ag 2 = Ag 2 C 2 4 + H 2 



These data having been obtained, it is only necessary to add 

 together the heats of formation of the oxalic acid, the metallic 

 oxide and that of their reciprocal combination, and then deduct 

 the heat of formation of water, H 2 (69 CaL), in order to 

 find the heat of formation of the metallic oxalate from its 

 elements. 



+ 25-0 

 + 25-6 

 + 18-4 

 + 14-0 

 + 25-8 



Acid (solid) 

 Zinc salt 

 Lead salt 

 Copper salt 

 Mercuric salt 

 Silver salt 



H 2 + C 2 + 4 = H 2 C 2 4 



Zn + C 2 + 4 = ZnC 2 4 



Pb + C 2 + 4 = PbC 2 4 



Cu + CL + 4 = CuC 2 4 



.. ,, . Hg + d; + 4 = HgC 2 4 



, Ag 2 + C 2 + 4 = Ag 2 C 2 4 



+ 197-0 

 + 237-1 

 + 204-7 

 + 182-1 

 + 170-7 

 + 158-5 



3. If we note the heat of formation of 2 eq. of carbonic acid 

 from carbon (diamond) and oxygen, or 



2(0 + O a ) = 2C0 2 liberates + 188-0, 



it is easy to calculate the heat brought into play when an 

 oxalate is decomposed into gaseous carbonic acid and free 

 metal, the reaction being referred to the ordinary temperature 



H 2 C 2 4 (solid) = H 2 + 2C0 2 

 ZnC 2 O 4 = Zn (solid) + 2C0 2 

 PbC 2 4 = Pb (solid) + 2C0 2 

 CuC 2 4 = Cu (solid) + 2C0 2 

 HgC 2 4 = Hg (liquid) + 2C0 2 

 Ag 2 C 2 4 = Ag 2 (solid) + 2C0 2 



- 9-0 



- 49-1 



- 16-7 

 + 5-9 

 + 17-3 

 + 29-5 



4. We see from this that zinc and lead oxalates cannot be 

 decomposed into carbonic acid and metal with a liberation of 



1 The calculation is made here on the supposition that the precipitated 

 oxalates are anhydrous, or rather, that the heat liberated is essentially the 

 same for the anhydrous and precipitated salts ; which, in fact, has been 

 proved to be the case for the salts of mercury and silver. 



