Flame calorimetry

When hydrocarbons or other organic compounds are burnt in a flame calorimeter it is often necessary to pre-mix the gas with oxygen to increase flame stability and obtain complete reaction. However, if the optimum proportion of oxygen is exceeded the flame temperature becomes too high and thermal decomposition with deposition of carbon occurs below the jet. Since the combustion takes place at constant pressure close to 1 bar, the enthalpy of combustion is measured under conditions near to those of the standard states. Flame calorimetry has the advantage that enthalpies of combustion are obtained for the gaseous state without the necessity of measuring enthalpies of vaporization in separate experiments and, moreover, completeness of combustion can be established by determination of both the water and the carbon dioxide produced. 

In spite of these advantages, flame-calorimetry has largely been used only for Cg to Cs hydrocarbons and before 1963 the only organic oxygen 

The application of flame calorimetry is limited to moderately volatile substances. Pell and Pilcher have used it for a substance boiling at 81 (tetrahydropyran) and by raising both the temperature at which the carrier gas (argon) is saturated with organic vapour and the operating temperature of the calorimeter above 25 C it should be possible to increase the scope of the method. 

---

Fluorine flame calorimetry is a logical extension of oxygen flame calorimetry in which a gas is burned in excess of gaseous oxidant (214). The decision does not reach that of the oxygen flame calorimeter in which, for example, Affj(H20) was determined with a standard deviation of 0.01%. Combustions of H2, NH3 (8), and fluorinated hydrocarbons are typical applications, but the uncertain nonideality corrections of HF(g) prevent full realization of the inherent accuracy. 

As referred to in the previous chapter, in bomb combustion calorimetry the reaction proceeds inside a pressure vessel—the bomb—at constant volume, and in this case the derived quantity is Ac U°. In flame calorimetry the reaction occurs in a combustion chamber, which is in communication with the atmosphere, and the measurements lead to ACH°. The methods of combustion calorimetry will be described in the following paragraphs. 

In flame calorimetry, it is not easy to measure directly with good accuracy the mass of reactants consumed in the combustion. Therefore, the results are always based on the quantitative analysis of the products and the stoichiometry of the combustion process. In the case of reaction 7.73, the H20 produced was determined from the increase in mass of absorption tubes such as M, containing anhydrous magnesium perchlorate and phosphorus pentoxide [54,99], When organic compounds are studied by flame combustion calorimetry, the mass of C02 formed is also determined. As in bomb calorimetry, this is done by using absorption tubes containing Ascarite [54,90]. 

The first reported attempt to use fluorine in calorimetric measurements is probably Berthelot and Moissan s study of the reaction between K.2SC>3(aq) and F2(g), in 1891 [ 19,120]. Modem fluorine bomb calorimetry, however, was started in the 1960s by Hubbard and co-workers [110,111,121], while in the same period Jessup and Armstrong and their colleagues [ 109,115-117] developed the method of fluorine flame calorimetry to a high degree of accuracy and precision. 

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... 

As illustrated in this section, the problems associated with using fluorine in combustion calorimetry seem to have been largely overcome. The fluorine bomb and flame calorimetry methods have been perfected to such an extent that, provided the chemistry of the process under study is well characterized, results of very good accuracy and precision can be obtained routinely. 

G. Pilcher. Oxygen Flame Calorimetry. In Experimental Chemical Thermodynamics, vol. 1 Combustion Calorimetry, S. Sunner, M. Mansson, Eds. IUPAC-Pergamon Press Oxford, 1979 chapter 14. 

D. A. Pittam, G. Pilcher. Measurements of Heats of Combustion by Flame Calorimetry. Part 8. Methane, Ethane, Propane, n-Butane and 2-Methylpropane. J. Chem. Soc. Faraday Trans. 11972, 68, 2224-2229. 

G. Pilcher, H. A. Skinner, A. S. Pell, A. E. Pope. Measurements of Heats of Combustion by Flame Calorimetry. Part 1. Diethyl Ether, Ethyl Vinyl Ether and Divinyl Ether. Trans. Faraday Soc. 1963, 59, 316-330. 

R. C. King, G. T. Armstrong. Constant Pressure Flame Calorimetry with Fluorine II. The Heat of Formation of Oxygen Difluoride. J. Res. Nat. Bur. Stand. 1968, 72A, 113-131. 

Fletcher and Pilcher ( ) derived the enthalpy of formation by measuring the enthalpy of combustion of methyl chloride by flame calorimetry. They reported a value A H (CHgCl, g, 298.15 K) -19.59 0.16 kcal mol" where the error, according to the authors, is twice the standard deviation of the mean including the uncertainty in calibration, measurement, determination of ignition energy, and values of auxiliary data used. 

In support of their results, Fletcher and Pilcher ( ) have compared the enthalpies of formation of C2HgCl(g) and i-C H Cl(g) as determined by equilibrium studies (2nd and 3rd law) with their work via flame calorimetry. The agreement is... 

The value chosen as the enthalpy of formation is AjH (CHgCl, g, 298.15 K) - -20.0 0.5 kcal mol". This value is close to the median value and is arbitrarily chosen as it is uncertain whether the flame calorimetry or hydrogenation studies are the more accurate. Slightly more weight was given to the flame calorimetry work due to favorable comparisons with equilibrium studies in related compounds. An error of t 0.5 kcal mol" is chosen so as to encompass the current A H (298.15 K) experimental values even though the actual experimental error is considerably less. 

Gas phase flame calorimetry has been used for alkanes up through C5 liquid combustion has been used for determining enthalpies of formation of the alkanes from C4 through Cl6, and solid combustion, for Cjg and higher. 

Pittam, D.A. and Pilcher, G. (1972) Measurements of heats of combustion by flame calorimetry. Part 8. Methane, ethane, propane, n-butane and 2-methylpropane./. Chem. Soc., Faraday Trans. 1,68,2224-2229. 

Although there have been various early attempts to use fluorine as the oxidant in flame calorimetry, the problem of corrosion restricted these efforts to the use of fluorine as a minor component of the reaction mixture. More recent work at the National Bureau of Standards has overcome many of the difficulties and Armstrong and Jessup have described a calorimeter in which volatile substances can be burnt in excess fluorine. Because the... 

R. A. Fletcher and G. Pilcher, Measurements of heats of combustion by flame calorimetry part 6. Formaldehyde and glyoxal, Trans. Faraday Soc. 66 794 (1970). 

---