醋酸甲酯物性介绍.doc

KINETICS: THE HYDROLYSIS OF METHYL ACETATE INTRODUCTION Methyl acetate hydrolyzes to form acetic acid and methanol, according to the following reaction: (1) The reaction is extremely slow in pure water, but is catalyzed by both hydronium and hydroxide ions. In this experiment the kinetics of the reaction catalyzed by HCl will be studied. Hydrolysis reactions of this type are reversible; the back reaction is called esterification. For any equilibrium the overall rate is the difference in rates of the forward and reverse reactions: (2) If, however, the reaction is not allowed to proceed very far to the right, the concentrations of products will be very small and the reverse rate term can be neglected. Thus, during the early stages of the reaction the rate law is written: (3) The reaction is first-order with respect to both methyl acetate and water and of order q (to be determined in the experiment) with respect to H3O+. For dilute solutions the [H2O] remains essentially constant during the reaction and equation (3) simplifies to (4) The [H3O+] also remains constant during the reaction, because the HCl is not consumed and its presence suppresses the ionization of acetic acid. Thus, the reaction is pseudo-first-order and equation (4) can be further simplified to give (5) where kH, the acid-dependent rate constant, is equal to kF[H3O+]q. The value of q can be determined if kH is evaluated for more than one concentration of HCl. The integrated form of the first-order rate law is (6) where C0 and Ct are the molar concentrations of methyl acetate initially present and at time t. The initial concentration (C0) is readily calculated from the known solution volumes and the density of methyl acetate. The value of Ct can be obtained by taking the difference between the initial concentration and the number of moles per liter of methyl acetate consumed at time t. From the stoichiometry of the reaction the number of moles per liter of methyl acetate consumed is equal