Abstract
The spontaneous hydrolysis of acetic anhydride in H2O and D2O 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 H2O (reference beam) and D2O (sample beam) in a double-beam spectrophotometer. This technique allows determination of rate constants in both solvents, kH2O and kD2O, 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 kH2O 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.
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