Page:Outlines of Physical Chemistry - 1899.djvu/187

 HEAT OF FORMATION

��In this r6sum6 of thermo-chemistry we do not desire to give too many numerical data. The following tables give the thermo-chemical numbers for a few of the most impor- tant series of substances.

The heats of combustion and formation are given for the temperature 16°, under constant pressure and starting from carbon in the form of diamond.

��Htdrocabbons

��• Kftme

�Formula

�Heat of Formation

�i Heat of Combustion

� �Gas

�Liquid

�Methane.

� �213-5 Cal.

�Ethane.

� �372-3 „

�Propane. Butane, normal

� �528-4 „ i 687-2 „

�Hexane, normal

�i

�j 991-2 „

�Di-isopropane.

�CfiHn

� �| 998-9 „

�Ethylene. Propylene Isobutylene

�C-jH,

� �3411 „

499-3 „

1 650-2 „

�Amylene (ordinary).

� �| 811-3 „

�Acetylene

�CgHg

� �■ 315-7 „ is, according to Berthelot, equal to (94-8 x 2) + (69 x 8) = Cal., therefore the difference 28*8 Cal. represents the heat of formation of this hydrocarbon.

We may notice that in the case of the saturated hydro- carbons a difference of CH 2 in the molecular constitution corresponds to an almost constant difference of 158 Cal. in the heat of combustion. A similar regularity is noticed in the case of other homologous series.

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