KINETICS OF REVERSIBLE CHLORINE HYDROLYSIS - TEMPERATURE-DEPENDENCE AND GENERAL ACID BASE-ASSISTED MECHANISMS

Rate constants of the forward reaction for the reversible hydrolysis of Cl2(aq) at mu = 0.50 M increase from 1.9 s-1 at 0.0-degrees-C to 30.5 s-1 at 30.0-degrees-C. The activation parameters at 15.0-degrees-C are DELTAH(double dagger) = 63 +/- 3 kJ mol-1 and DELTAS(double dagger) = -8 +/- 4 J mol-1 K-1 for the forward reaction and DELTAH(double dagger) = 27 +/- 1 kJ mol-1 and DELTAS(double dagger) = -71 +/- 9 J mol-1 K-1 for the reverse reaction. The DELTAH- values at mu = 0.50 M decrease markedly with increase in temperature (DELTAC(p) = -537 +/- 7 J mol-1 deg-1), and therefore the DELTAH(double dagger) and DELTAS(double dagger) values are not constant over a range of temperatures. The equilibrium constant, K = ([HOCl][H+][Cl-])/[Cl2(aq)], equals 1.04 X 10(-3) M2 at 25.0-degrees-C, mu = 0.50 M. The K value depends on the ionic strength as well as the temperature. In the reversible hydrolysis reaction, general bases (A-) assist the hydrolysis rate (the Bronsted beta value is 0.58 +/- 0.06) and general acids (HA) assist the reverse reaction (the Bronsted a value is 0.40 +/- 0.05): Cl2 + H2O + A- reversible HOCl + Cl- + HA (k2/k-2).