Page:Text-book of Electrochemistry.djvu/309

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��DEVELOPMENT OF HEAT.

��CHAP.

��why, in the explosion of a mixture of hydrogen and oxygen, the temperature does not rise so high as would be expected from the calculation.

As an example of a similar, but more thoroughly studied, displacement of the equilibrium by temperature and pressure, we may consider the decomposition of carbon dioxide into carbon "monoxide and oxygen, which takes place according to—

2C02^2CO + 02,

with evolution of 136,000 cal. The volume change is the same as in the dissociation of water vapour, and the heat change does not differ very appreciably from that found for water vapour, 1 16,000 cal. The two equilibria must therefore be similar in character, since the dissociations at a correspond- ing point (with respect to temperature and pressure) are of the same oi-der of magnitude (at 2000°, and 1 atmo. pressure cixrbon dioxide is dissociated to the extent of 5 per cent., water to a slightly greater extent).

Le Chatelier has calculated that, of 100 molecules of carbon dioxide, the following number is dissociated at the te.mperature and pressure given : —

��Pressure In atmos. ' 1000^.

��O-OOl 1.

10 . 100.

��IftOO^

��2000°.

18*

10* 6* 2-5* l-2»

��2600°.

�3000^

�3500°.

�SB*

�86*

�36*

�37*

�18*

��4000^

��Since the heat of dissociation of water vapour is lower (in the ratio 12 : 14) than that of carbon dioxide, the

��* As ill the calculatioDS for the temperature 2000^, an error has ovidently been made in the original paper, the numbers indicated by an asterisk • are taken from the curve given by Le Chatelier {Zeit physikal. Chem., 1888, 2, 785) instead of from the table.

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