Note 24 — Chapter IX · COMPOUNDS OF CARBON WITH OXYGEN AND NITROGEN

The first product of combustion of charcoal is always carbonic anhydride, and not carbonic oxide. This is seen from the fact that with a shallow layer of charcoal (less than a decimetre if the charcoal be closely packed) carbonic oxide is not formed at all. It is not even produced with a deep layer of charcoal if the temperature is not above 500°, and the current of air or oxygen is very slow. With a rapid current of air the charcoal becomes red-hot, and the temperature rises, and then carbonic oxide appears (Lang 1888). Ernst (1891) found that below 995° carbonic oxide is always accompanied by CO2, and that the formation of CO2 begins about 400°. Naumann and Pistor determined that the reaction of carbonic anhydride with carbon commences at about 550°, and that between water and carbon at about 500°. At the latter temperature carbonic anhydride is formed, and only with a rise of temperature is carbonic oxide formed (Lang) from the action of the carbonic anhydride on the carbon, and from the reaction CO2 + H2 = CO + H2O. Rathke (1881) showed that at no temperature whatever is the reaction as expressed by the equation CO2 + C = 2CO2, complete; a part of the carbonic anhydride remains, and Lang determined that at about 1,000° not less than 3 p.c. of the carbonic anhydride remains untransformed into carbonic oxide, even after the action has been continued for several hours. The endothermal reactions, C + 2H2O = CO2 +2H2, and CO + H2O = CO2 + H2, are just as incomplete. This is made clear if we note that on the one hand the above-mentioned reactions are all reversible, and therefore bounded by a limit; and, on the other hand, that at about 500° oxygen begins to combine with hydrogen and carbon, and also that the lower limits of dissociation of water, carbonic anhydride, and carbonic oxide lie near one another between 500° and 1,200°. For water and carbonic oxide the lower limit of the commencement of dissociation is unknown, but judging from the published data (according to Le Chatelier, 1888) that of carbonic anhydride may be taken as about 1,050°. Even at about 200° half the carbonic anhydride dissociates if the pressure be small, about 0·001 atmosphere. At the atmospheric pressure, not more than 0·05 p.c. of the carbonic anhydride decomposes. The reason of the influence of pressure is here evidently that the splitting up of carbonic anhydride into carbonic oxide and oxygen is accompanied by an increase in volume (as in the case of the dissociation of nitric peroxide. See Chapter VI., Note 46). As in stoves and lamps, and also with explosive substances, the temperature is not higher than 2,000° to 2,500°, it is evident that although the partial pressure of carbonic anhydride is small, still its dissociation cannot here be considerable, and probably does not exceed 5 p.c.