Combustion, heat various compounds

Later, the energy aspects of combustion were studied, beginning with determination of the heat of combustion of various compounds and the pressures and temperatures which develop in explosions, and ending with the application of chemical thermodynamics to questions of equilibrium, dissociation and completeness of combustion in flames (1820-1900). 

The knowledge of isomerization equilibria is particularly important in hydrocarbon chemistry, and this is reflected in the body of very accurate measurements of heats of combustion of hydrocarbons which has become available since 1939, The spate of results has been such that determinations of the heats of combustion of various compounds have been replicated in several laboratories, and it is of interest to compare their results. The heats of combustion of several r/oparaffins, for example, have I >cen deter-... 

The heat of combustion of hexane, C6H14(1), is —4163 kj-mol Use this fact and the values for the enthalpies of combustion of various organic compounds found in Table 6.3 to compare benzene, ethanol, hexane, and octane as possible fuels by doing the following, (a) Calculate the heat produced per gram of each of these substances, (b) From standard reference sources, find the density of each liquid and determine the heat produced per liter of liquid. 

Stuart has made very interesting calculations with a number of disubstitution products of benzene he starts from the various possible interactions of the substituents and calculates the internal molecular potential of the whole molecule this is then compared with the deviations of the heats of combustion of the compounds from the additivity rule, with the result that considerable repulsion potentials are shown by the ortho-compounds, which practically disappear in the meta- and para-derivatiyes. 

The D values may be easy or difficult to measure, and they can be estimated by various techniques, but there is no question as to what they mean. With E values the matter is not so simple. For methane, the total energy of conversion from CH4 to C + 4 H (at 0 K) is 393 kcal mol (1644 kJ mol ). " Consequently, E for the C—H bond in methane is 98 kcal mol (411 kJ mol ) at OK. The more usual practice, though, is not to measure the heat of atomization (i.e., the energy necessary to convert a compound to its atoms) directly but to calculate it from the heat of combustion. Such a calculation is shown in Figure 1.11. 

---