Page:Outlines of Physical Chemistry - 1899.djvu/193

 The numbers in the last column are derived from those in the second column by subtraction of 0*58 Oal. for each molecular volume which has disappeared in the formation of the organic substance. For methane, the reaction C + 2H 2 = CH 4 gives rise to a condensation of one molecular volume. The heat of formation under constant volume is therefore 21-75-0-58 = 21-17 Cal.

Passing now to the analysis of the thermal effects accompanying the formation of the hydrocarbons and denoting by :

d, the quantity of heat necessary to bring about the disintegration of one gram-atom of carbon ;

v u the heat evolved when two atoms of carbon unite with a single bond ;

v 2, the same with a double bond ;

v 3, the same with a triple bond ;

q, the quantity of heat evolved when one atom of hydrogen separates from the hydrogen molecule and unites with an atom of carbon ; we can express these effects in the following manner :

(C, H 4 ) = - d + 4? = 21-17 Cal.

(C 2, H 2 ) = - 2d H- v 3 + 2q = - 4777 „ (C 2, H 4 ) = - 2d + v 2 + 4tq = - 3-29 „ (C 2, H 6 ) = - 2d + v x + 6? = 27-40 „

The value of d is known. We have, therefore, a system

of four equations containing four unknowns, leading to the

solutions :

These are the figures by the aid of which Thomsen has calculated the heat of formation of a certain number of other hydrocarbons, and his theoretical results have been confirmed by experiment.

The calculated heat of formation of benzene accord- ing to Kekute's formula is —55-08 Cal., and for any other formula with nine single bonds —12-48 Cal. The

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