Page:Text-book of Electrochemistry.djvu/307

 292 DEVELOPMENT OF HEAT. chap.

one hand, the velocity of reaction is generally very greatly increased with rising temperature. As an example of this we may cite the formation of water from a mixture of hydrogen and oxygen, which hardly proceeds at all at the ordinary temperature, but which takes place with explosive violence above 580^ On the other hand, a change of tem- perature causes a displacement of the equilibrium which is established between the components of every chemical system. Again, we may take water and a mixture of hydrogen and oxygen as an example. Theory (see p. 256) requires that at 20° a litre of water contains 0*65 x 10'*' gram-molecules of hydrogen, and half as many gram-mole- cules of oxygen. This quantity of mixed hydrogen and oxygen cannot be detected by chemical methods, but from electrical observations, such as those of Helmholtz (6*), it can be calculated. Now, there must be an equilibrium between the water and the mixture of gases dissolved in it —

2H2 +02^ 2H2O.

If we denote the concentrations of the three substances hy Ch, Coy and Ch^^, the following equations should be valid

(see pp. 85 and 94) : —

l»g ^ = ^wi-^Bif, - To) + ^'

where /i is the quantity of heat which is absorbed when two mols of hydrogen and one mol of oxygen combine to form liquid water (- 136,800 cal.).

The value of K applies to the temperature 2\. At Tq, log K = M. Now, at 20°, To = 293 ; C^o = 55-5 ( = xag o) ; On = 0-65 X 10-^; and Co = 033 x lO'^^. From this we obtain —

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