Introduction to physical chemistry / by James Walker.

  • Walker, James, Sir, 1863-1935.
Date:
1919
Catalogue details

Licence: In copyright

Credit: Introduction to physical chemistry / by James Walker. Source: Wellcome Collection.

    161/462 (page 139)
    example we may take propionic acid, C3H602, whose heat of combustion has been found by direct experiment to be 386,500 cal. If we sub- tract 2H20 from the molecular formula, we are left with the residue C3H2, the elements of which have the heat of combustion 3 x 94,300 cal. + 68,300 cal. = 351,200 cal. It is evident that the approximation is here by no means close, the error in this case being about 10 per cent. A better result may usually be obtained by subtracting the oxygen, not with the corre- sponding quantity of hydrogen, but with the corresponding quantity of carbon, and then estimating the heat of combustion of the elements in the residue. In the above example we subtract C02 from the formula C3H602, and have C2H6 left as residue. This gives the heat of combustion 2 x 94,300 cal. + 3 x 68,300 cal. = 393,500 cal., a much better approximation to the experimental value. As another instance of the two methods of calculation, we may take cane sugar, C12H220lr By Welter’s rule we subtract 11H20 from the molecule, and for the residue C12 get the heat of combustion 12 x 94,300=1,131,600 cal. By the other method we subtract 5'5C02 in order to dispose of the 11 atoms of oxygen, and obtain the residue 6-5C and 11H2. The heat of combustion of these quantities of the elements is 6-5 x 94,300 + 11 x 68,300 = 1,364,200 cal. The value experimentally found is 1,354,000 cal., a number much closer to the second calculated value than to the first. Some hydrocarbons have a greater heat of combustion than that of the carbon and hydrogen contained in them. The heat of combustion of acetylene, for example, is 310,000 cal.; the heat of combustion of the two atoms of carbon and the two atoms of hydrogen contained in its molecule being 2 x 94,300 cal. + 68,300 cal. = 256,900 cal. This corresponds to a heat of formation of -53,100 cal., i.e. this amount of heat is absorbed on formation of acetylene from its elements. We have here, then, an example of an endothermic compound formed from its elements with heat absorption, in contradistinction to the bulk of compounds, which are exothermic, i.e. are formed from their ele- ments with evolution of heat. Other common examples of endothermic compounds are carbon disulphide, which is formed with a heat absorption of 28,700 cal., and gaseous hydriodic acid, which is formed ■with a heat absorption of 6100 cal. Endothermic compounds like these are comparatively unstable, and give out heat on their decom- position. Hydriodic acid gas, for instance, is decomposed by gentle heating; carbon disulphide can be split up into its elements by mechanical shock; and carbon and hydrogen may be regenerated from acetylene by the passage of electric sparks through the gas. The substance hydrazoic acid, or azoimide, N3H, has a large negative heat of formation, which is no doubt closely associated with its extremely explosive properties.