Page:Text-book of Electrochemistry.djvu/121

 io6 VELOCITY OF REACTION. chap.

where fi is the heat which is absorbed when ordinary sugar is transformed into the variety which can be attacked. Since -B, expressed in calories, is equal to 2, we calculate for cane sugar fi = 25,640 cal. per gram-molecule. The corresponding number for ethyl acetate is 11,160 cal. per gram-molecule. According to this hypothesis, the velocity of the reaction should be directly proportional to the concentration of the molecules which can be attacked.

This view is supported by experiments on the rate of solution, p, of zinc in dilute (0'1-normal) hydrochloric acid. This velocity is hardly altered by change of temperature, as is shown by the results of Ericson-Aur^n (8) contained in the following table : —

��Temperataie.

�H

�o

��In this case the velocity of reaction is, within the experimental error, independent of the temperature. This can be explained by assuming that the ordinary zinc mole- cules can be attacked, or are "active,"' or that the heat of transformation of the inactive into active molecules is zero. The first assumption seems the more probable.

Rothmund's results (9) on the influence of pressure on the velocity of reaction are in good agreement with the hypothesis.

It must be observed that at higher concentrations of the acid the temperature has a very great influence on the speed of solution of zinc, as Guldberg and Waage (10) found. This may be due to the formation of a layer of concentrated zinc salt solution round the metal, which protects it from further action if not removed by agitation. The agitation is

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