Anomalous Pressures and Vibrations in Gas Explosions. Determination of the Dissociation Energy 2H2O⇌2OH+H2

Abstract

Using thermodynamic functions of gases derived from band spectra, calculations have been made of theoretical explosion pressures in mixtures of hydrogen and oxygen containing various inert gases which are compared with explosion pressures observed experimentally by different investigators. Details of methods of calculation are given. In carefully dried mixtures of hydrogen and oxygen with excess hydrogen, the observed pressures are too low. This effect is eliminated by the addition of small amounts of water vapor to the original mixture which, according to an hypothesis advanced by Wohl and von Elbe, quenches the luminescence from OH radicals formed in the flame front. In mixtures containing excess oxygen or nitrogen the observed pressures are too high. The existence of a lag in the excitation of the vibrational energy levels of oxygen and nitrogen during the explosion period, as advanced by Wohl and Magat, is offered as explanation. The excitation lag is greater in oxygen than in nitrogen. Explosions of mixtures of gaseous ozone diluted with oxygen do not show this effect, even though the explosion time for approximately the same explosion temperature hardly differs from that of a hydrogen‐oxygen explosion which shows the excitation lag. This is explained on the assumption that there is no appreciable lag in the transfer of vibrational energy between like molecules. A similar excitation lag apparently does not exist in H₂O and H₂. The phenomenon of time‐variable heat capacities within the duration of an explosion may be used to explain the intense audible vibrations observed by the authors and others on exploding slow burning lean mixtures of hydrogen with air or oxygen within a certain concentration range. The possible bearing of this effect on the phenomenon of engine knock is mentioned. Explosions were made of mixtures of hydrogen and oxygen with helium as inert gas, which are free from both anomalous effects mentioned above and the energy of dissociation of the reaction 2H₂O⇌2OH+H₂ was calculated from the data obtained with the aid of accurate thermodynamic functions of the gases involved. The average of the results of six explosions was found to be 126,000±2000 calories per 2 moles of H₂O. From this value one obtains $$\begin{array}{c}{\mathrm{H}}_2\mathrm{O}\rightleftharpoons \mathrm{H}+\mathrm{OH}\text{−(114,400±1000)\hspace{0.17em}}\mathrm{calories}\\ \mathrm{OH}\rightleftharpoons \mathrm{H}+\mathrm{O}\text{\hspace{0.17em} −(104,160±1000)\hspace{0.17em}}\mathrm{calories}.\end{array}$$