The thermal decomposition of nitrous oxide at pressures up to forty atmospheres

Abstract

Nitrous oxide decomposes to nitrogen and oxygen at velocities which can be conveniently measured at temperatures between 600° and 850° C. M. A. Hunter investigated the reaction by streaming the gas through a porcelain bulb in a furnace and measuring the decomposition for different times of passage. No attempt was made to determine whether the reaction is homogeneous or heterogeneous. The effect of wide variation of pressure was not used to determine its order, since the reaction was followed only over small ranges of decomposition at atmospheric pressure. From the velocity of decomposition, however, bimolecular constants were obtained which could be represented by the equation: ln k = 24·12 - 31800/T, where k is the bimolecular velocity constant and T the absolute temperature. If this equation holds, the activation energy of the bimolecular reaction is 62,040 cal./gm. mol. A much more thorough examination of the reaction was made by Hinshelwood and Burk, who measured the rate of reaction by following the pressure increase at constant volume in a silica bulb. The reaction was proved to be homogeneous. The initial pressure was varied between 50 and 500 mm. Hg, and it was found that the reciprocal of the half-lives when plotted against the initial pressures gave a straight line. true bimolecular reaction requires the straight line 1/ t ½ = ka , where t ½ is the half-line, and k the velocity constant, and a the initial concentration. The line through the experimental points showed a small intercept on the 1/ t ½ axis for which no explanation was offered at the time. From the variation of the bimolecular constants between 565° and 852° C. the activation energy of the reaction was calculated to be 58,450 cal./gm. mol. If the reaction were a bimolecular one dependent on immediate decomposition at each activating collision of the molecules the number of molecules reacting per second should be equal to Z x e -E/RT , where Z is the number of molecules colliding per second and E is the activation energy. From the observed rate of reaction at 1000° K. a value of 55,000 cal./gm. mol. was found for the activation energy. The fairly close agreement between the two values of the activation energy, 58,450 and 55,000 cal./gm. mol. and the manner in which the half-life varied with pressure provided good grounds for believing the reaction to be a simple bimolecular one, dependent only on collisions between the molecules.