The heats of formation of fluoro- and chloro-sulphuric acids and their thermodynamic stabilities

The heat of formation of fluorosulphuric acid determined by mixing sulphur trioxide with excess of hydrogen fluoride in fluorosulphuric acid was 189·4 ± 0·6 kcal./mole at 25°. The heat of formation of chlorosulphuric acid determined by hydrolysing the acid was 142·9 ± 0·4 kcal./mole. The reaction of hydrogen chloride with excess of sulphur trioxide in fluorosulphuric acid yielded a confirmatory value of 143·5 ± 0·6 kcal./mole. The heats are combined with free energy functions to calculate the possible dissociation of the acids and these are compared with experiment.