Kinetic Solvent Isotope Effect: A Simple, Multipurpose Physical Chemistry Experiment

The spontaneous hydrolysis of acetic anhydride in H 2 O and D 2 O is a convenient undergraduate physical chemistry experiment that is used to teach practical kinetics, and to demonstrate the kinetic solvent isotope effect. The reaction is monitored by UV-Vis spectroscopy (disappearance of acetic anhydride) by employing a new approach, namely, by measuring the absorbance difference between cuvettes containing the reaction in H 2 O (reference beam) and D 2 O (sample beam) in a double-beam spectrophotometer. This technique allows determination of rate constants in both solvents, k H 2O and k D 2O, respectively, from a single kinetic run by using commercial curve fitting programs. Advantages of the differential absorbance method are instrument time saving, and introduction of the students to the application of curve fitting. To test the quality of the calculated rate constants, the students determine k H 2O by conductance measurement, and compare results of both methods. The use of two experiments to study the reaction illustrates the variety of experimental techniques that can be employed for rate measurements.