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 $k_{\mathrm{HCl}–\mathrm{Ar}}{\text{= 4.78 × 10}}^{13}\hspace{0.17em}\exp \text{(‐82\hspace{0.17em}700 / RT)\hspace{0.17em}}\mathrm{cc}/\mathrm{mole}·\sec$ , with R in calories/mole·degree, for the rate coefficient due to $\mathrm{HCl}–\mathrm{Ar}$ collisions, in good agreement with other reported values. These same data yield an apparent rate coefficient due to $\mathrm{HCl}–\mathrm{HCl}$ collisions, $k_{\mathrm{HCl}–\mathrm{HCl}}$ , which satisfactorily describes the data obtained for pure HCl, with $k_{\mathrm{HCl}–\mathrm{HCl}}{\text{≅ 10 k}}_{\mathrm{HCl}–\mathrm{Ar}}$ . The temperature dependence of $k_{\mathrm{HCl}–\mathrm{HCl}}$ departs from Arrhenius form, with the apparent activation energy increasing at lower temperatures.