|Fenning 1933||R. W. Fenning and F. T. Cotton, Proc. Roy. Soc. (London) A 141, 17-28 (1933)|
A Bomb Calorimeter Determination of the Heats of Formation of Nitrous Oxide and Carbon Dioxide
|note COb||Fenning 1933 parallels Awbery 1933 by measuring the enthalpy of combustion of CO and N2O in a bomb calorimeter. The amount of gases was determined through pV measurements. They find the enthalpy of combustion of CO at 20° C and constant volume as 67364 +- 35 cal15/mol, combustion of N2O in CO 87401 +- 58 cal15/mol and combustion of hydrogen in N2O 87520 +- 26 cal15/mol. The combustion of hydrogen in oxygen is used for calibration, and Fenning 1933 takes it as 68320 cal15/mol at 25° C and constant pressure or 67486 cal15/mol at 20° C and constant volume. They also quote Rossini 1931a as giving 68313 +- 10 cal15 at 25° C and 1 atm. Rossini 1931 (Rather than Rossini 1931a) indeed gives 68313 cal15/mol, 285890 abs. J/mol and 285775 int. J/mol. Rossini uses a conversion of 1.0004 = 0.9996 from int. J to abs. J (current conversion is 1/0.99984) and 4.185 cal15 to 1 abs.J. (current is 4.18580). Fenning 1933 give at the end of the paper conversions of cal15 = 4.186 abs. J and 1 int. J = 1.0039 abs. J. All the results of Fenning 1933 are given in cal15. Hence, the most practical approach is to devise an overall conversion factor that takes into account also the slight change in the enthalpy of formation of water, as well as the difference in R that was used for pV metering. The best current available enthalpy of formation of water is -285825 +- 40 J/mol for 18.01528 g H2O. Using 159 J/mol to convert to 20° C produces 285984 J/mol at constant pressure, or with RT = 2437.4 J/mol, for a total of 3656 J/mol for 1.5 RT, produces 282328 J/mol at 20° C and constant volume. Fenning 1933 used 67486 cal15/mol; hence the overallconversion factor is 4.183505. For the combustion of CO in O2, they obtain 67364 +- 35 cal15/mol at 20° C and constant volume. With the above conversion factor this becomes 281817.6 +- 146 J/mol at constant volume or -283036.3 +- 146 J/mol at constant pressure and 20° C. For comparison, this would correspond to 283070 J/mol at 25° C. Alternatively, one could take the enthalpy of combustion of water at 20° C and constant volume and convert to the 1932 atomic scale (H2O is 18.0156 g) and obtain 282333 J/mol, or a factor of 4.18358. With this factor, their combustion of CO becomes 283041.4 J/mol at 20° C and constant pressure, which can be converted by 44.0095/44.00 to the current molecular weight of CO2 to 283103.5 J/mol at 20° C or 283134 J/mol at 25° C. Using instead the conversion 28.01010/28.00 produces 283143.4 J/mol at 20° C or 283177 J/mol at 25° C. Note that CODATA Key Vals quote Fenning 1933 enthalpy of CO as -110.36 +- 0.25 kJ/mol, implying that the enthalpy of combustion used was -283.15 +- 0.21 kJ/mol, which seems to correspond to the average of conversions using either CO or CO2. However, our interpretation giving -283.036 +- 0.146 J/mol at 20° C or -283.070 +- 0.146 J/mol is slightly closer to the values of Awbery 1933 and Rossini|
(see note CO and note COa), avoids the confusion of which molec. weight to use for correction, and is probably more correct in view of the fact that the amount were determined by pV metering.