Page:Text-book of Electrochemistry.djvu/308

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log IS.- 31- j.gg ^ 2-3025 ' 293 V Ti A = if + 101-3 (^^-=^).

In the neighbourhood of 20° log K increases for every degree by -oqq- = 0*346, since Ch.^ may be regarded as

constant, and log K increases three times as quickly aa log Ch, consequently log G becomes greater by 0*1153 for each degree ; (7h therefore increases in the proportion 1 : 1*3 per degree, and reaches a tenfold value by raising the tempera- ture by 7*7°. At 100° the quantity of hydrogen has risen to 1'15 X 10"**, and at the critical temperature (365°) to 123 X 10"® gram-molecules per litre.

So long as water is present in the liquid condition there is only an exceedingly small dissociation into hydrogen and oxygen. From the above formula it would appear also that log K may never reach a higher value than M + 101*3 = 15*95, however high the temperature be raised, i.e. according to the theory, even at the highest temperature the dissociation cannot go beyond a certain limiting value. In this, however, it is assumed that no change of volume occurs, otherwise the pressure relationships would have to be taken account of. Now, since in the dissociation of water into hydrogen and oxygen two molecules give rise to three, i.e, the volume increases (provided that the pressure is constant, and that all the substances are present in the gas state), the decomposition must increase when the volume becomes greater. If the pressure be kept constant, the volume steadily increases with rising temperature. Consequently the gaseous dissociation of water vapour (at constant pressure) increases with the temperature, and the increase in the degree of dissociation is unlimited. It has been experimentally found (Deville) that above 2000° water vapour is appreciably dissociated (7). This dissociation at the high temperature is the reason

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