Decomposition of Carbon Dioxide

Carbon-dioxide gas is very stable, and requires a high temperature for its decomposition. The thermal dissociation of this gas was first investigated by St. Claire-Deville, who passed a rapid stream of it through a porcelain tube heated to about 1300° C., and collected the issuing gas over potash solution. In this way the carbon dioxide was proved to be dissociated into carbon monoxide and oxygen to the extent of about 0.2 per cent. Le Chatelier investigated the equilibrium represented by the equation

2CO₂ ⇔ 2CO + O₂

by igniting an explosive mixture of 2 volumes of carbon monoxide and 1 volume of oxygen which issued rapidly from a gas-burner, and causing the hottest part of the flame, whose temperature was estimated to be 3000° C., to impinge upon a pierced silver tube through which a stream of water passed. The gases were thus drawn from the flame through the hole in the tube, and were collected and analysed. In this way the dissociation of carbon dioxide was estimated to amount to 40 per cent, at 3000° C.

From this result, and assuming the gas pressure to be 1 atmosphere, Trevor and Kortwright have calculated the percentages of dissociation of carbon dioxide at different temperatures and pressures to be as follow, the heat of dissociation Q_t, also being derived from the temperature:

Temp.°C.	0.001 Atm	0.01 Atm	0.1 Atm	1 Atm	10 Atm	100 Atm.	Qt
1000	0.11	0.05	0.024	0.011	0.005	0.0024	-62,662 cals.
1500	9.5	4.6	2.2	10	0.5	0.2	-56,125 cals.
2000	57.7	34.7	18.3	90	4.3	2.0	-46,767 cals.
2500	87.0	69.6	46.0	25.7	13.0	6.3	-34,640 cals.
3000	93.9	83.4	62.7	[40.0]	21.6	10.8	-19,742 cals.
3500	95.4	87.0	69.7	46.1	25.7	13.0	0
4000	95.1	86.4	66.6	45.0	24.9	12.6	+18,343 cals.

It will be observed that the extent of dissociation reaches a maximum at about 3500° C. at all pressures, and that at this temperature the heat of dissociation, which has been increasing from a negative value with rising temperature, passes through the zero-point and becomes positive. The figures in this table are calculated from a single experimental value, that of the dissociation at 3000° C. under 1 atm.; nevertheless the few other existing experimental values support them. The following results for the dissociation of carbon dioxide below 1300° C. have more recently been obtained by Nernst and v. Wartenberg, Langmuir, and Loewenstein:

Temperature ° C.	Per Cent, (dissociation).	Observer.
1027	0.00414	N. and W.
1122	0.0142	La.
1127	0.01-0.02	N. and W.
1170	0.025	La.
1205	0.029-0.035	N. and W.
1225	0.0471	La.
1277	0.04	Loe.
1292	0.064	La.

They agree accurately with values calculated from a formula.

The dissociation of carbon dioxide in the carbon monoxide-oxygen flame has been investigated by Haber and Rossignol, by the method of Deville, and these authors find that the value of the equilibrium constant K in the equation

K = [CO₂]/[CO]×[O₂]^½

(where the brackets denote partial pressures) is about 4, which corresponds to a dissociation of about 37 per cent., the temperature of the flame being 2600°-2670° C.

The dissociation of carbon dioxide at very high temperatures has been determined by Bjerrum, by an explosive method, to be as follows:

T. ° abs	1500°	2640°	2879°	2945°	3116°
Dissociation per cent	0.04	21.0	51.7	64.7	76.1

Carbon dioxide is decomposed into carbon monoxide and oxygen by means of electric sparks, a fact first observed by Henry; whilst Buff and Hofmann found that the decomposition is more rapid if steel electrodes are used, since these combine with the liberated oxygen. Even then, however, the decomposition is not complete, but it rises to 65 per cent, when the pressure is reduced to 1 mm. The silent electric discharge, ultra-violet light, and radium rays also decompose carbon dioxide.

Carbon dioxide is decomposed by purely chemical means by heating sufficiently electropositive metals in the gas. Aluminium, for instance, reduces the gas according to the reaction:

2Al + 3CO₂ = Al₂O₃ + 3CO,

and a piece of burning magnesium ribbon continues to bum in the gas with separation of carbon, thus:

CO₂ + 2Mg = 2MgO + C,

whilst potassium burns brilliantly in a stream of carbon dioxide with formation of carbonate and separation of carbon, thus:

3CO₂ + 4K = 2K₂CO₃ + C.