Online Encyclopedia

CH4

Online Encyclopedia
Originally appearing in Volume V26, Page 808 of the 1911 Encyclopedia Britannica.
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CH4 +40 = CO2+2 H2O +213800. Now we know the heats of formation of carbon dioxide (from diamond) and of liquid water to be 94300 cal. and 68300 cal. respectively. The above equation may consequently be written, if x is the heat of formation of methane, —x+0 =—94300-(2 X 68300) +213800 X =17000 cal. This heat of formation, like that of most hydrocarbons, is comparatively small: the heat of formation of saturated hydrocarbons is always positive, but the heat of formation of unsaturated hydrocarbons is frequently negative. Fot example, ethylene, C2H4, is formed with absorption of 16200 cal., acetylene, C2H2, with absorption of 59100 cal., and liquid benzene, C6H6, with absorption of 9100 cal. Since the heat of combustion of a hydrocarbon is equal to the heat of combustion of the carbon and hydrogen it contains minus its heat of formation, those hydrocarbons with positive heat of formation generate less heat on burning than the elements from which they were formed, whilst those with a negative heat of formation generate more. Thus the heat generated by the combustion of acetylene, C2H2, is 316000 cal., whereas the heat of combustion of the carbon and hydrogen composing it is only 256900 cal., the difference being equal to the negative heat of formation of the acetylene. For substances consisting of carbon, hydrogen and oxygen, a rule was early devised for the purpose of roughly calculating their heat of combustion (J. J. Welter's rule). The oxygen contained in the compound was deducted, together with the equivalent amount of hydrogen, and the heat of combustion of the compound was then taken to be equal to the heats of combustion of the elements in the residue. That the rule is not very accurate may be seen from the following example. Cane-sugar has the formula C12Hz2On. According to Welter's rule, we deduct 11 0 with the equivalent amount of hydrogen, namely, 22 H, and are left with the residue 12 C, the heat of combustion of which is 1131600 cal. The observed heat of combustion of sugar is, however, 1354000, so that the error of the rule is here 20 per cent. A much better approximation to the heat of combustion of such substances is obtained by deducting the oxygen together with the amount of carbon necessary to form CO2, and then ascertaining the amount of heat produced by the residual carbon and hydrogen. In the above case we should deduct with 11 0 the equivalent amount of carbon 5.5 C, thus obtaining the residue 6.5 C and 22 H. These when burnt would yield (6.5 X94300) +(II X683oo) =1364250 cal., an amount which is less than I per cent. different from the observed heat of combustion of sugar. Neither of the above rules can be applied to carbon compounds containing nitrogen. § 8. Heat of Neutralization.—It has already been stated that the heats of neutralization of acids and bases in aqueous solution are additively composed of two terms, one being constant for a given base, the other constant for a given acid. In addition to this, the further regularity has been observed that when the powerful nonobasic acids are neutralized by the powerful monacid bases, the heat of neutralization is in all cases the same. The following table gives the heats of neutralization of the commoner strong monobasic acids with soda: Hydrochloric acid . HCl . Hydrobromic acid . HBr Hydriodic acid . . HI . Nitric acid . HNO3 Chloric acid . HCIO3 Bromic acid . HBrO3 Within the error of experiment these numbers are identical. It was at one time thought that the greater the heat of neutralization of an acid with a given base, the greater was the strength of the acid. It is now known, however, that when weak acids or bases are used, the heat of neutralization may be either greater or less than the normal value for powerful acids and bases, so that there is no proportionality, or even parallelism, between the strengths of acids and their heats of neutralization (see SOLUTIONS). § 9. Heat of Solution.—When substances readily combine with water to form hydrates, the heat of solution in water is usually positive; when, on the other hand, they do not readily form hydrates, or when they are already hydrated, the heat of solution is usually negative. The following examples show the effect of hydration on heat of solution in a large quantity of water: Heat of Solution. Heat of Hydration. I. Sodium carbonate Na2CO3 +564o cal. Na2CO3, H2O e +2250 .. +3390 cal. Na2CO3, 2H20 . +20 „ +5620 „ Na2CO3, 101-I20 . -1616o „ +21800 II. Sodium sulphate Na2SO4 . +46o cal. Na2SO4, H2O -1900 „ +2360 cal. Na2SO4, IoH2O . -1876o „ +19200 „of chemical equilibrium by the application of the second law. For an account of work in this direction see CHEMICAL ACTION.
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