Combustion thermochemistry

The application of non-combustion thermochemistry to organic compounds is chiefly of interest in two respects in the determination of small differences in heats of formation of similar compounds with large heats of combustion, and in the determination of heats of formation of compounds such as halogen compounds and organo-metallic compounds, where combustion calorimetry is attended by special difficulties. 

No one has ever claimed that hydrogen thermochemistry will replace combustion thermochemistry but, as Kistiakowsky pointed out in his first paper on the subject (Kistiakowsky et al., 1935), reaction thermochemistry, despite its lack of generality, is a powerful tool because it can be expected to yield more accurate enthalpies of formation than combustion thermochemistry for some molecules under some circumstances. Reaction thermochemistry, especially hydrogen... 

Recent advances in computational chemistry have made it possible to calculate enthalpies of formation from quantum mechanical first principles for rather large unsaturated molecules, some of which are outside the practical range of combustion thermochemistry. Quantum mechanical calculations of molecular thermochemical properties are, of necessity, approximate. Composite quantum mechanical procedures may employ approximations at each of several computational steps and may have an empirical factor to correct for the cumulative error. Approximate methods are useful only insofar as the error due to the various approximations is known within narrow limits. Error due to approximation is determined by comparison with a known value, but the question of the accuracy of the known value immediately arises because the uncertainty of the comparison is determined by the combined uncertainty of the approximate quantum mechanical result and the standard to which it is compared. 

Determining AfH°29S (hydrocarbons) by combustion, despite its seeming simplicity, is not an easy job. Combustion thermochemistry requires meticulous control of experimental conditions (Steele et al., 2002). The difficulty of this task and its expense in time and money mean that researchers in need of AfH°29% data are likely to find vast gaps in the thermochemical record. At present, chemists are synthesizing new compounds far more rapidly than thermochemical properties are being measured, hence the reliance in many contemporary studies and in industrial laboratories on computational methods. The simplest of these... 

All petroleum energy products, as distinct and dissimilar as they can be, are subjected to the process of flame combustion. It is helpful at this point to bring to mind some definitions and general laws of thermochemistry. 

Rossini F D (ed) tBSd Experimental Thermochemistry vo I (New York Interscience) Skinner H A (ed) 1962 Experimental Thermochemistry yo II (New York Interscience) Sunner S and Mansson M (eds) 1979 Combustion Calorimetry (Oxford Pergamon)... 

Almost all of the directly measured thermochemical data for the sulfoxides, sulfones, sulfites and sulfates are due to the work of Busfield and Mackle and their coworkers at the University of Leeds and The Queens University, Belfast1-14. This work involved measurement of enthalpies of combustion, fusion and vaporization. It is the basis of the subsequent compilations of Benson and coworkers15, Cox and Pilcher16 and Pedley, Naylor and Kirby11. The data given by the latter are used as the basic data set in the present work. Corrections and omissions are noted in the next section. Data on additional compounds were sought by searching the IUPAC Bulletin of Thermochemistry and Thermodynamics for the years 1980 198318, and by searches of Chemical Abstracts. 

For values of heats of combustion of large numbers of organic compounds hydrocarbons and others, see Cox, J.D. Pilcher, G. Thermochemistry of Organic and Organometallic Compounds, Academic Press NY, 1970 Domalski, E.S. J. Phys. Chem. Ref. Data, 1972,1, 221. For large numbers of heats of formation values (from which heats of combustion are easily calculated) see Stull, D.R. Westrum Jr., E.F. Sinke, G.C. The Chemical Thermodynamics of Organic Compounds, Wiley NY, 1969. 

Mean heat capacities for the combustion gases are readily available in handbooks and texts on heat and material balances. The following values are taken from K. A. Kobe, Thermochemistry of Petrochemicals, reprint No. 44, Pet. Ref. 1958 converted to SI units, J/mol°C, reference temperature 0°C. 

Detailed structural calculations have been carried out for this system. This is because the neutral isomer, C2HsO, which is implicated in the thermochemistry of ethanol, is of interest in pollution control, atmospheric chemistry, and combustion. Also, there is new information available from photoionization experiments with which to compare theoretical calculations. For details of these comparisons, see Curtiss et al.73 In the earlier theoretical studies of Nobes et al.,74 calculations were performed at the MP2 and MP3 levels with basis sets of double plus polarization (6-13G ) with electron correlation. These studies revealed four stable minima for the system protonated acetaldehyde, CHj-C H-OH <-> CH3-CH=0+H the methoxymethyl cation, CH3OCH2 protonated oxirane, (CH2)2OH+ and vinylox-... 

In fluorine thermochemistry, two key heat values frequently occur. They are the dissociation energy of difluorine, required for evaluation of fluorine bond energies and the heat of formation of hydrogen fluoride, a product in hydrolysis, hydrogenation, fluorine combustion, or neutralization reactions. These values have been difficult to measure and have changed considerably over the years. A recommended set of values has been reported in recent CODATA bulletins (60) which are collected in Table I together with older values and corrections to update them. 

Skinner, H. A. Combustion calorimetry of organometallic compounds. In experimental thermochemistry, (editor, 2nd edit, in course of publication) Advances in Organometallic Chemistry 2, 49 (1964)... 

Much of the discussion of oxygen flame calorimetry presented in section 7.3 is directly applicable to fluorine flame calorimetry. As in the case of bomb calorimetry, however, the special properties of fluorine combustion systems and problems associated with handling fluorine require a somewhat different experimental method [109,115,116]. Thus, for example, a metal burner should be used. Also, the fact that the mixing of many gases with F2 may lead to spontaneous ignition hinders the use of a premixed flame. Fluorine combustion calorimetry has been used to study the thermochemistry of important reactions, such as... 

Equation 7.31 was derived from a least squares fit to the data given in W.N. Hubbard, D. W. Scott, G. Waddington. Standard States Corrections for Combustions inaBomb at Constant Volume. In Experimental Thermochemistry, vol. 1 F. D. Rossini, Ed. Interscience New York, 1956 p. 93. 

Thermochemistry has been closely associated with techniques such as reaction-solution and combustion calorimetry, although it has long been recognized that thermochemical information can also be obtained from noncalorimetric methods, such as equilibrium and kinetics experiments. 

Melius. C. F., Thermochemistry of hydrocarbon intermediates in combustion Application of the BAC-MP4 method, private communication (1990). 

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