Enthalpy of dissociation of nitric acid in aqueous sulphuric acid and its relation to the activation energies of encounter-controlled nitrations

The enthalpy of dissociation of nitric acid to nitronium ion and hydroxide ion is estimated to be +(43 ± 4) kJ mol–1 in 68·3 w/w% sulphuric acid and +(19 ± 5) kJ mol–1 in 79·2 w/w% sulphuric acid. These values are used to show that the high energies of activation observed in the limiting nitrations of reactive compounds in sulphuric acid are consistent with the interpretation that these reactions are controlled by the rate of encounter of the nitronium ion with the aromatic molecule.