The Principles of Chemistry, Volume I

by Dmitry Ivanovich Mendeleyev

[17 bis] The degree or relative magnitude of the dissociation of CO₂ varies with the temperature and pressure—that is, it increases with the temperature and as the pressure decreases. Deville found that at a pressure of 1 atmosphere in the flame of carbonic oxide burning in oxygen, about 40 per cent. of the CO₂, is decomposed when the temperature is about 3,000°, and at 1,500° less than 1 per cent. (Krafts); whilst under a pressure of 10 atmospheres about 34 per cent. is decomposed at 3,300° (Mallard and Le Chatelier). It follows therefore that, under very small pressures, the dissociation of CO₂ will be considerable even at comparatively moderate temperatures, but at the temperature of ordinary furnaces (about 1,000°) even under the small partial pressure of the carbonic acid, there are only small traces of decomposition which may be neglected in a practical estimation of the combustion of fuels. We may here cite the molecular specific heat of CO₂ (i.e. the amount of heat required to raise 44 units of weight of CO₂ 1°), according to the determinations and calculations of Mallard and Le Chatelier, for a constant volume C_v = 6·26 + 0·0037t; for a constant pressure C_p = C_v + 2 (see Chapter XIV., Note [7]), i.e. the specific heat of CO₂ increases rapidly with a rise of temperature: for example, at 0° (per 1 part by weight), it is, at a constant pressure = 0·188, at 1,000° = 0·272, at 2,000°, about 0·356. A perfectly distinct rise of the specific heat (for example, at 2,000°, 0·409), is given by a comparison of observations made by the above-mentioned investigators and by Berthelot and Vieille (Kournakoff). The cause of this must be looked for in dissociation. T. M. Cheltzoff, however, considers upon the basis of his researches upon explosives that it must be admitted that a maximum is reached at a certain temperature (about 2,500°), beyond which the specific heat begins to fall.

[18] Percarbonic acid, H₂CO₄ (= H₂CO₃ + O) is supposed by A. Bach (1893) to be formed from carbonic acid in the action of light upon plants, (in the same manner as, according to the above scheme, sulphuric acid from sulphurous) with the formation of carbon, which remains in the form of hydrates of carbon: 3H₂CO₃ = 2H₂CO₄ + CH₂O. This substance CH₂O expresses the composition of formic aldehyde which, according to Baeyer, by polymerisation and further changes, gives other hydrates of carbon and forms the first product which is formed in plants from CO₂. And Berthelot (1872) had already, at the time of the discovery of persulphuric (Chapter XX.) and pernitric (Chapter VI., Note 26) acids pointed out the formation of the unstable percarbonic anhydride, CO₃. Thus, notwithstanding the hypothetical nature of the above equation, it may be admitted all the more as it explains the comparative abundance of peroxide of hydrogen (Schöne, Chapter [IV].) in the air, and this also at the period of the most energetic growth of plants (in July), because percarbonic acid should like all peroxides easily give H₂O₂. Besides which Bach (1894) showed that, in the first place, traces of formic aldehyde and oxidising agents (CO₃ or H₂O₂) are formed under the simultaneous action of CO₂ and sunlight upon a solution containing a salt of uranium (which is oxidised), and diethylaniline (which reacts with CH₂O), and secondly, that by subjecting BaO₂, shaken up in water, to the action of a stream of CO₂ in the cold, extracting (also in the cold) with ether, and then adding an alcoholic solution of NaHO, crystalline plates of a sodium salt may be obtained, which with water evolve oxygen and leave sodium carbonate; they are therefore probably the per-salt. All these facts are of great interest and deserve further verification and elaboration.

[18 bis] If CO₂ is the anhydride of a bibasic acid, and carboxyl corresponds with it, replacing the hydrogen of hydrocarbons, and giving them the character of comparatively feeble acids, then SO₃ is the anhydride of an energetic bibasic acid, and sulphoxyl, SO₂(OH), corresponds with it, being capable of replacing the hydrogen of hydrocarbons, and forming comparatively energetic sulphur oxyacids (sulphonic acids); for instance, C₆H₅(COOH), benzoic acid, and C₆H₅(SO₂OH), benzenesulphonic acid, are derived from C₆H₆. As the exchange of H for methyl, CH₃, is equivalent to the addition of CH₂, the exchange of carboxyl, COOH, is equivalent to the addition of CO₂; so the exchange of H for sulphoxyl is equivalent to the addition of SO₃. The latter proceeds directly, for instance: C₆H₆ + SO₃ = C₆H₅(SO₂OH).

As, according to the determinations of Thomsen, the heat of combustion of the vapours of acids RCO₂ is known where R is a hydrocarbon, and the heat of combustion of the hydrocarbons R themselves, it may be seen that the formation of acids, RCO₂, from R + CO₂, is always accompanied by a small absorption or development of heat. We give the heats of combustion in thousands of calories, referred to the molecular weights of the substances:—

R =	H2	CH4	C2H6	C6H6
	68·4	212	370	777
RCO2 =	69·4	225	387	766

Thus H₂, corresponds with formic acid, CH₂O₂; benzene, C₆H₆, with benzoic acid, C₇H₆O₂. The data for the latter are taken from Stohmann, and refer to the solid condition. For formic acid Stohmann gives the heat of combustion as 59,000 calories in a liquid state, but in a state of vapour, 64·6 thousand units, which is much less than according to Thomsen.

[19]

Fig. 63.—Gas-producer for the formation of carbon monoxide for heating purposes..