Density Gradient Measurements of HCl Dissociation in Shock Waves

The dissociation of HCl in mixtures of 5%, 10%, and 20% HCl in Ar in the temperature range 3500–7000°K and in pure HCl in the range 3000–4000°K has been studied behind incident shock waves. Dissociation rates were determined by measurement of postshock density gradients using a laser-beam deflection technique. The postshock density gradient profiles exhibit the same highly characteristic ``dip'' observed previously with O2-rare gas mixtures [J. Chem. Phys. 55, 4017 (1971)], and the magnitude of the gradient at this point is again interpreted as a measure of the initial dissociation rate. The rate coefficients thus obtained for the mixtures of 5%, 10%, and 20% HCl exhibit a high degree of precision and yield the result kHCl–Ar = 4.78 × 1013 exp(-82 700 / RT) cc/mole· sec, with R in calories/mole·degree, for the rate coefficient due to HCl–Ar collisions, in good agreement with other reported values. These same data yield an apparent rate coefficient due to HCl–HCl collisions, kHCl–HCl, which satisfactorily describes the data obtained for pure HCl, with kHCl–HCl ≅ 10 kHCl–Ar. The temperature dependence of kHCl–HCl departs from Arrhenius form, with the apparent activation energy increasing at lower temperatures.