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The Heats of Combustion Methane and Carbon Monoxide

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The Heats of Combustion Methane and Carbon Monoxide RP260 THE HEATS OF COMBUSTION OF METHANE AND CARBON MONOXIDE By Frederick D. Rossini ABSTRACT With the use of the calorimetric apparatus and procedure employed for deter- mining the heat of formation of water (v. supra p. 1), the heats of combustion of methane and carbon monoxide have been measured. The heat of combustion of CH 4 to form liquid H 2 and gaseous CO2 is found to be 889,700 ±300 international joules per mole at 30° C. and a constant pressure of 1 atmosphere, which is equivalent to 890, 160 ±300 international joules per mole at 25° C. and 1 atm. The heat of combustion of CO to form C02 is found to be 282,925 ± 120 inter- national joules per mole at 30° C. and a constant pressure of 1 atm., which is equivalent to 282, 890 ±120 international joules per mole at 25° C. and 1 atm. For comparison with the older work, see Figures 1 and 2. CONTENTS Page I. Introduction 37 II. Apparatus and calorimetric procedure 38 III. Molecular weights, constants, units, and factors 38 IV. Methane 39 1. Chemical procedure 39 2. Correction experiments 41 3. Combustion experiments 42 4. Data of previous investigators 42 V. Carbon monoxide 44 1. Chemical procedure 44 2. Correction experiments. 45 3. Combustion experiments 46 4. Data of previous investigators 47 VI. Conclusion 48 VII. Acknowledgment 49 I. INTRODUCTION The heats of formation of methane and carbon monoxide are com- puted from their heats of combustion because of the difficulties inherent in the problem of measuring the heat effect of their syn- thesis from or decomposition into the elements. Accurate values of the heats of formation of methaDe and carbon monoxide are particu- larly desirable at this time because industrial syntheses of many useful compounds involve one or both of these gases, and a knowledge of the heat effect of a given reaction permits calculation of the change of the equilibrium with temperature. The usually accepted values for the heats of combustion of methane and carbon monoxide are based primarily upon the measurements made over 50 years ago by Thomsen. Because of the comparatively large uncertainty (one-fourth to one-third per cent) attached to Thorn- sen's average values, and because calorimetric measuring apparatus, methods, and devices have improved greatly in accuracy and precision since his time, it was desirable that new determinations be made. 37 38 Bureau of Standards Journal of Research vol. ej II. APPARATUS AND CALORIMETRIC PROCEDURE It was found that the caiorimetric apparatus which was used in this laboratory to determine the heat of formation of water * could be used, without alteration, for burning methane and carbon monoxide in oxygen. The reaction vessel and other apparatus have been described in detail. 1 The caiorimetric procedure in the experiments involving combus- tion of methane and carbon monoxide was exactly similar to that employed in the combustion of hydrogen and the experiments here reported were carried out immediately after the completion of the latter combustions. The data from the calibration experiments at 30° C, listed as Set II in the experiments on hydrogen, were used in conjunction with the combustion experiments on methane and carbon monoxide. While the curves showing the variation of the calorimeter tempera- ture with time in the combustion experiments on hydrogen were practically identical with those in the calibration experiments, those for the combustion experiments involving methane and carbon mon- oxide were somewhat different. In the calibration experiments, the temperature rise of about 2.8° C. occurred in 12 minutes, whereas for the combustion experiments on methane and carbon monoxide it was 2.8° C. in 10 minutes and 1.3° C. in 12 minutes, respectively. Since, however, detailed observations in the "fore" and "after" periods were made in each experiment, and since the resistance coils in the thermometer bridge were accurately calibrated, the determina- tion of the true temperature rise in each experiment can be in error by only a negligible amount. III. MOLECULAR WEIGHTS, CONSTANTS, UNITS, AND FACTORS The atomic weights of O, H, and C are taken as 16.000, 1.0078, and 12.000, respectively. 2 For the reaction between CH 4 and 2 to form liquid H2 and 3 is, gaseous C02 , A(7P is taken as 93 joules per mole per degree, that the heat evolved in the reaction at constant pressure decreases 93 joules per mole per degree. For the reaction between and to form , A(7 is taken CO 2 C02 p as — 7 joules per mole per degree. 3 The heat evolved in this reaction at constant pressure increases 7 joules per mole per degree. The correction to 1 atmosphere pressure is made by the use of the thermodynamic equation. p w = nRTlog (1) 6 j^ where n is the number of moles decrease in gaseous volume, R is the gas constant in joules per degree, T is the absolute temperature, P is the pressure in mm of Hg. For the combustion of methane, w is taken as —6.7 joules per mole per mm increase in pressure, while for the conbustion of carbon monoxide w is —1.7 joules per mole i Rossini, B. S. Jour. Research, 6, p. 1; 1931. 2 Baxter, 1930 Report of the Committee on Atomic Weights, J. Am. Soc, 52, p. 861; 1930. s International Critical Tables, 5, p. 80. Rossini] Heats of Combustion of Methane and Carbon Monoxide 39 per mm increase in pressure. These corrections are strictly valid only for small changes in pressure. The pressures in the reaction chamber for the combustion experiments are given in Tables 4 and 8. The unit of energy and all other constants, units, and factors are the same as those given in the previous paper. 4 IV. METHANE 1. CHEMICAL PROCEDURE The methane used in this investigation, obtained through the cour- tesy of the Fixed Nitrogen Research Laboratory, had been prepared synthetically from carbon monoxide and hydrogen and was specially purified by the Du Pont Ammonia Co. The purifying procedure included a fractional distillation to remove other hydrocarbons, hydrogen, and carbon monoxide. Before entering the reaction vessel, the methane passed succes- sively through tubes containing (1) "ascarite" (sodium hydroxide- asbestos mixture) for removing C0 2 and other acidic oxides, (2) "dehydrite" (Mg(C10 4 ) 2 .3H 2 0) for removing water vapor, and (3) P 2 5 for removing the last traces of H 2 0. The ratio of carbon to hydrogen in the methane was determined by burning the gas in the reaction vessel in an excess of oxygen and absorbing the H 2 in " dehydrite" and the C0 2 in "ascarite." For this absorption, two glass-stoppered U-tubes, similar to that used in the hydrogen combustions, were used in series. The first tube contained " dehydrite" and P 2 5 , and the second " ascarite" and P 2 5 . The first tube absorbed only the H 2 while the second ab- sorbed all the C0 2 . Because of the large amount of heat evolved when " ascarite" absorbs C0 2 , the second tube was placed in a beaker of water. The procedure employed in determining the mass of H 2 formed has been described in detail. 6 The mass of C0 2 absorbed was determined in the same manner. When "ascarite" absorbs C0 2 , two reactions occur: 2NaOH (hydrated) + C0 2 = Na 2 C0 3 (hydrated) + H 2 (water of hydration) (2) NaOH (hydrated) + C0 2 = NaHC0 3 + H 2 (water of hydration) (3) 6 The molecular volumes were taken as follows: NaHC0 3 , 38.2 3 3 3 cm ; NaOH (hydrated), 18.8 cm +15.6 cm for each mole of water 3 3 of hydration; Na 2 C0 3 , 41.8 cm +15.6 cm for each mole of water of hydration. The increase in volume of the "ascarite" is 0.45 ±0.02 3 cm per gram of C0 2 absorbed. Following a calculation similar to 7 one already described, the mass of C0 2 absorbed becomes, when the absorber is filled with oxygen at 25° C. and 1 atmosphere. m (C0 2) = (1 .00000 + 0.00059 - 0.00014) X Am = 1 .00045Am (4) where m is the~mass of the brass weights. The uncertainty in this factor is not more than 2 in 100,000. When the absorber is filled 4 See footnote 1, p. 38. • See footnote 1, p. 38. » Calculated from densities and molecular weights' given in International Critical Tables, 1, pp. 150, 151. 7 See footnote 1, p. 38. — — : 40 Bureau of Standards Journal of Research [Vol. with hydrogen, as it was in some experiments, the mass of C0 2 is obtained from the following equation m (Co2) = (1.00000 + 0.00004 - 0.00014) X Am = 0.99990Am (5) The results of four determinations of the ratio of H 2 to C0 2 are given in Table 1. When the methane was burned in oxygen accord- ing to the equation CH4 + 20 2 = 2H 2 + C0 2 (6) it was found that the ratio of half the moles of H 2 to the number of moles of C0 2 formed was 0.99885 ± 0.00019. Table 1. Ratio of H2 to CO2 in the combustion products of methane Ratio: Deviation Experiment Mole of 2 Mole of CO2 1/2 moles H2 H from mean moles COa 1 0. 125614 0. 062886 0. 99875 -0. 00010 ' 2 . 111594 . 055855 . 99896 .00011 3 . 081449 . 040761 .99911 .00026 4 . 115400 . 057783 . 99857 -. 00028 . 99885 ±. 00019 The excess of C02 could be explained by the presence in the methane of a small amount of CO.
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