Kinetics of Thermal Decomposition of Aqueous Perchloric Acid

Thermal decpmposition of 1.1-6.6 m aqueous perchloric acid at 295-322" yields oxygen, chlorine, and a small quantity of hydrochloric acid. The initial rates of decomposition have been measured, using a perchlorate ion activity electrode, and show 3.5 order dependence on the stoichiometric perchloric acid concentration; the corresponding rate coefficient and activation energy at 300.0" are 3.1 x lo-' tn-2.5 s-' and 84 kcal mole-', respectively. The order of reaction with respect to time is less than 3.5 and depends upon the initial perchloric acid concentration; this effect is due to acceleration of the decomposition by the reaction products, notably chloride ion. It is suggested that the ratedetermining step in the decomposition of aqueous perchloric acid is the reaction of two undissociated HC10, molecules to give H 2 0 , CIO,, and C10,.