136 PHENOMENA, ATOMS, AND MOLECULES 



where the parentheses represent pressures in atmospheres. In the flame 

 gases the water vapor is 17.4 per cent dissociated and the hydrogen is 

 27.4 per cent dissociated. 



For the combustion of acetylene with oxygen we may consider the 

 reactions : 



C2H2 + ^/2 O2 = 2 CO2 + H2O + 302,000 calories I 



C2H2 + V2 O2 = 2 CO + HoO + 166,000 calories II 



C2H2 + O2 = 2 CO + H2 + 102,600 calories III 



In a flame having the proportion of oxygen given in the first reaction 

 the flame gases would contain a relatively large amount of oxygen owing to 

 the dissociation of both the carbon dioxide and the water vapor. For oxy- 

 acetylene welding such strongly oxidizing flames are avoided and the 

 oxygen ratio is cut down nearly as low as that in reaction III. 



Let us calculate the temperature and flame composition for this case. 

 The reaction does not go to completion as indicated above in reaction III, 

 but should be written 



C2H2 + O2 = 2 CO + WoHo 4- wiH (12) 



where W2 + V2W1 = i (13) 



The heat of the reaction, 102,600— (i — n^) 98,000 calories, must 

 equal the heat content of the reaction products 2Q3 -\- n^Qo 4" wiQi. From 

 the resulting equation we find 



108,600 — 19.5 T — 0.00145 T^ , . 



'^i "^ ~o i z. — ^ :r^T^ (14) 



48,500 + 1-72 T — 0.000225 T^ ^ ■^ 



The partial pressures of H and H2 are 



y, - ni ^ ni , . 



^' 2 + ni + n2 3 + 72% '^ ^^ 



and P2= 'l 7; ""' (16) 



3 + V2 wi 



so that the equilibrium constant is 



p2 (i-V2ni) (3 + V2W1) ^ ^^ 



At any assumed temperature we can calculate Wi and thus get i^ as a 

 function of T. We also get i^ as a function of T from Table I. The inter- 

 section of the two curves gives T = 3270° K. ; Wi = 0.5676; W2 = 0.7162. 

 Thus we conclude that when acetylene burns with an equal volume of 

 oxygen the maximum temperature may rise to 3270° K. and at this tem- 

 perature the composition of the flame gas is carbon monoxide 60.9 per cent. 



