Combustion Calorimetry with Fluorine: Constant Pressure Flame Calorimetry

The Heat of Combustion of Beryllium in Fluorine

An experimental determination of the energies of combustion in fluorine of polytetrafluoroethylene film and powder and of mixtures of beryllium with polytetrafluoroethylene gives for reaction (1) Δ H 25 C ° = - 1022.22 kJ mol-1 (-244.32 kcal mol-1) with an overall precision of 0.96 kJ mol-1 (0.23 kcal mol-1) at the 95 percent confidence limits. The total uncertainty is estimated not to exceed ±3.2 kJ mol-1 (±0.8 kcal mol-1). The measurements on polytetrarluoroethylene give for reaction (2a) and reaction (2b) Δ H 25 ° C ° = - 10369.7 and -10392.4 Jg-1, respectively. Overall precisions expressed at the 95 percent confidence limits are 3.3 and 6.0 Jg-1, respectively. (1)Be ( c ) + F 2 ( g ) = BeF 2 (  amorphous  ) (2a)C 2 F 4 ( polymer powder ) + 2 F 2 ( g ) = 2 CF 4 ( g ) (2b)C 2 F 4 ( polymer film ) + 2 F 2 ( g ) = 2 CF 4 ( g ) Be2C and Be metal were observed in a small carbonaceous residue from the combustion of the beryllium-polytetrafluoroethylene mixtures. Methods of analysis for these substances were developed. Gases resulting from the solution of the solid residues in aqueous KOH were analyzed for H2 and CH4 by differential absorption in molecular sieves at low temperatures.

The Heat of Formation of Boron Carbide

The standard heat of combustion in fluorine of a boron carbide sample having the composition B4.222C was determined from the heats of combustion of polytetrarluoroethylene and of boron carbide-polytetrafluoroethylene mixtures. The energy of the combustion reaction was measured in an isothermal-jacket bomb calorimeter. From the experimental data, we calculate -17.1 kcal mol-1 for the heat of formation of boron carbide. By combining all probable errors, we estimate our overall experimental uncertainty to be 2.7 kcal mol-1. The value for the heat of formation of boron carbide is for the phase represented by the formula B4.222C.

Heats of Formation of Aluminum Diboride and α-Aluminum Dodecaboride

The energies of combustion of AlB2 and α-AlB12 were measured in a bomb calorimeter using fluorine as the oxidant. Major problems of this investigation were the assessment of the state and distribution of impurities in the samples and the establishment of the stoichiornetry of the aluminum boride phase. We obtain -16±3 kcal mol-1 and -48±10 kcal mol-1 for the heats of formation of AlB2 and α-AlB12, respectively. The uncertainties cited are the overall experimental errors. Their magnitudes are chiefly due to uncertainties in the impurity correction applied and the uncertainties in the heats of formation of the combustion products.

The Heat of Formation of Boron Trifluoride by Direct Combination of the Elements

The energy of combination of crystalline boron in gaseous fluorine was measured in a bomb calorimeter. The experimental data combined with reasonable estimates of all known errors may be expressed by the equation:B ( c ) + 3 / 2 F 2 ( g ) = BF 3 ( g ) , Δ H f 298 ° = - 271.03 ± 0.51 kcal mol - 1 This result is compared with other recent work on and related to the heat of formation of boron trifluoride.

The Heats of Combustion of Polytetrafluoroethylene (Teflon) and Graphite in Elemental Fluorine

Bomb calorimetric measurements are reported for the combustion in fluorine of polytetrafluoroethylene (Teflon) and graphite-polytetrafluoroethylene mixtures. Mass spectrometric examination of the product gases showed CF4 to be the only major product with C2F6(g) present in only very small amounts. The completeness of combustion of the graphite was determined by chemical analysis of combustion residues and found to range from 97 to better than 99 percent. From the combustion data, the heats of formation Δ H f 298 ° [ CF 4 ( g ) ] and Δ H f 298 ° [ C 2 F 4 ( solid polymer ) ] were determined to be -222.87 ±0.38 kcal mol-1 and -197.82 ±0.39 kcal (gfw C2F4)-1, respectively. The uncertainties are estimates of the overall experimental errors. A previously reported value for the heat of formation of AlF3(c) is adjusted to be consistent with the present work. An evaluation of other data on CF4 is presented. The heat of formation of CF4(g) is combined with other work to derive the heats of formation of HF solutions at three specific concentrations.

Heat of Formation of Aluminum Fluoride by Direct Combination of the Elements

The standard heat of formation of aluminum fluoride was calculated from the heats of combustion of Teflon and of aluminum-Teflon mixtures in fluorine. The heat measurements were made in a bomb calorimeter. (1)C 2 F 4 (  solid polymer  ) + 2 F 2 ( g ) = 2 CF 4 ( g ) (2)Al ( c ) + 3 / 2 F 2 ( g ) = AlF 3 ( c ) For reaction (1), Δ H 298 ° was found equal to -10,350.7 ±0.4 Jg-1 (- 247.43 ±0.01 kcal monomole-1), and for reaction (2) Δ H 298 ° = Δ H f 298 ° was calculated to be -1507.8 ±1.2 kJ mole-1 (- 360.37 ±0.29 kcal mole-1). The latter uncertainties are the computed standard deviations of the means. The calculated standard heat of formation of aluminum fluoride is estimated to be accurate to within ±6.6 kJ mole-1 (1.6 kcal mole-1). The measurements on Teflon were combined with existing data and the heat of formation of carbon tetra-fluoride was calculated to be -221.8 kcal mole-1.