Shock Waves in Chemical Kinetics: Further Studies on the Rate of Dissociation of Molecular Iodine

Abstract

The rate of the dissociation reaction, $$\mathrm{M}+{\mathrm{I}}_2\mathrm{[open\; phi]}{\displaystyle {lim}_{k_R}^{k_D}}\mathrm{M}+\mathrm{I}+\mathrm{I},$$ has been measured by the shock tube method for argon, helium, nitrogen, oxygen, and carbon dioxide as inert gases, M, in the temperature range 1000°—1600°K. The shock wave results by themselves and the comparison of the shock wave measurements with the room temperature measurements of k_R by flash photolysis both show that k_R has a negative temperature coefficient. The absolute value of this negative temperature coefficient derived from the shock wave measurements is greater than the value derived from comparison of the average high temperature result with the room temperature result for any particular gas. This may be due to experimental error in the determination of dk_R/dT at the high temperatures, but it is believed that the values of k_R determined in the middle of the temperature range studied are reliable. The experimental evidence indicates that, for the measurements with CO₂, the rate of vibrational equilibration is so fast that the observations made here pertain entirely to vibrationally equilibrated CO₂. Evidence from other experiments indicates that the rate of vibrational relaxation in oxygen is such that most of the dissociation reaction occurs in relaxed O₂, but that nitrogen remains vibrationally unexcited under the conditions of the dissociation reactions studied here. The ratio, k_R, 12/k_R,A, of the efficiencies of iodine and argon as third bodies is not greater than 30 at 1300°K whereas it is 250–600 at room temperature. The hypothesis is proposed that in general the ratio k_R, gas/k_R,A for complex gases will decrease with increasing temperature.