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The Equilibrium CO + H2O = CO2 + H2
When hydrocarbon gases are fired with oxygen in certain proportions the cooled products consist of hydrogen, water, and the oxides of carbon in various proportions. The relation between the amounts of the different constituents is given by the equation
where Kt is the equilibrium constant at temperature t° C. In 1884 Dixon obtained the value Kt = 4 in his experiments on the inflammation of mixtures containing carbon monoxide, hydrogen, and oxygen. This was supported by the results of Smithells and Ingle on the composition of the intereonal gases of hydrocarbon flames, and has recently been confirmed by Andrew, who studied the combustion of hydrocarbon gases in oxygen and obtained values for Kt ranging from 3.47 to 4.12, the mean value for several series of experiments being approximately 4.
Owing to experimental difficulties a high degree of concordance is not to be expected, and the above results may be regarded as showing reasonable agreement. From Andrew's results it would appear that the value of Kt is largely independent of the initial nature of the hydrocarbon-oxygen mixture and of the pressure of the gas before ignition. It is apparently, therefore, also largely independent of the maximum flame temperature, since this would vary with the pressure and composition of the mixture to be ignited. Theoretically, however, some change with temperature is to be expected, and Hahn calculates, from thermodynamical principles, that this change is given by the equation
log Kt = -2232/T – 0.08463 log T – 0.0002203T + 2.5084
where T is the absolute temperature. The following values for Kt have been derived from this equation:
and these agree with reasonable approximation with the results of Haber and others. Possibly in Andrew's experiments the differences in temperature were not sufficient to produce variations in Kt large enough to exceed the experimental error. Andrew did not attempt to calculate his flame temperature, but believes that in all cases it would be higher than 1600° C. On the other hand, a more likely explanation and one favoured by Andrew is that Kt, as determined experimentally, " does not correspond with the maximum flame temperature, but is characteristic of some hypothetical temperature, the equilibrium condition at which corresponds with the integration of the chemical changes which occur in a rapidly cooling mixture from higher to atmospheric temperatures. This purely hypothetical temperature, which may be referred to as the temperature of final reaction (since it may be supposed that the gases are in equilibrium, and cease to further react at this temperature), is identified both on thermodynamic and experimental grounds between the limits 1500° and 1600° C."
If this is accepted the results indicate that the equilibrium
CO + H2O ⇔ CO2 + H2
sets in with very great rapidity.
where Kt is the equilibrium constant at temperature t° C. In 1884 Dixon obtained the value Kt = 4 in his experiments on the inflammation of mixtures containing carbon monoxide, hydrogen, and oxygen. This was supported by the results of Smithells and Ingle on the composition of the intereonal gases of hydrocarbon flames, and has recently been confirmed by Andrew, who studied the combustion of hydrocarbon gases in oxygen and obtained values for Kt ranging from 3.47 to 4.12, the mean value for several series of experiments being approximately 4.
Owing to experimental difficulties a high degree of concordance is not to be expected, and the above results may be regarded as showing reasonable agreement. From Andrew's results it would appear that the value of Kt is largely independent of the initial nature of the hydrocarbon-oxygen mixture and of the pressure of the gas before ignition. It is apparently, therefore, also largely independent of the maximum flame temperature, since this would vary with the pressure and composition of the mixture to be ignited. Theoretically, however, some change with temperature is to be expected, and Hahn calculates, from thermodynamical principles, that this change is given by the equation
log Kt = -2232/T – 0.08463 log T – 0.0002203T + 2.5084
where T is the absolute temperature. The following values for Kt have been derived from this equation:
| Temp. ° C | 1005 | 1205 | 1405 | 1600 |
| Kt | 1.65 | 2.54 | 3.43 | 4.24 |
and these agree with reasonable approximation with the results of Haber and others. Possibly in Andrew's experiments the differences in temperature were not sufficient to produce variations in Kt large enough to exceed the experimental error. Andrew did not attempt to calculate his flame temperature, but believes that in all cases it would be higher than 1600° C. On the other hand, a more likely explanation and one favoured by Andrew is that Kt, as determined experimentally, " does not correspond with the maximum flame temperature, but is characteristic of some hypothetical temperature, the equilibrium condition at which corresponds with the integration of the chemical changes which occur in a rapidly cooling mixture from higher to atmospheric temperatures. This purely hypothetical temperature, which may be referred to as the temperature of final reaction (since it may be supposed that the gases are in equilibrium, and cease to further react at this temperature), is identified both on thermodynamic and experimental grounds between the limits 1500° and 1600° C."
If this is accepted the results indicate that the equilibrium
CO + H2O ⇔ CO2 + H2
sets in with very great rapidity.
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