SECOND-ORDER UNIMOLECULAR KINETICS IN THE THERMAL DECOMPOSITION OF HYDROGEN PEROXIDE VAPOR

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Abstract
The thermal decomposition of hydrogen peroxide vapor has been reinvestigated by the static method as a function of initial pressure at pressures up to 22 mm Hg, and in the presence of inert gas (helium, oxygen, and water) up to 100 mm Hg. In each case the apparent first-order rate constant increased linearly with pressure. It is demonstrated that under the present experimental conditions the pyrolysis of hydrogen peroxide shows behavior typical of an elementary unimolecular reaction in its low-pressure, second-order region. The reaction was accompanied by a heterogeneous decomposition which in the presence of foreign gas became inhibited. Helium was used as inhibitor over the temperature range 430–470 °C, which permitted calculating the activation energy for activation with peroxide and with helium. The results can be satisfactorily accounted for by assuming a critical energy of 47–50 kcal and five effective classical oscillators for activation with peroxide and three with helium, provided deactivation occurs on every collision. Kinetic evidence against this assumption is briefly discussed.