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Unformatted text preview: Experiment 4: Polarimetry-Kinetics of Mutarotation of Glucose Author: Ben Rainey Group 2 Section 3, T 2:00 PM 5:00 PM Partner: Ben Huffer October 19, 2010 Abstract : In this experiment, the kinetics of mutarotation of glucose was studied using polarimetry. The goals of the experiment were to determine the rate constants for the forward and reverse reactions, the equilibrium constant for the reaction, and the activation energies for the forward and reverse reaction. A digital polarimeter (ADP 220) was used to measure the amount of rotation of polarized light that two forms of D- glucose provided. The data collected was analyzed using a linear least squares fitting to determine the sum of the rate constants, k 1 and k 2 , for the forward and reverse reaction respectively. Ratios relating the equilibrium constants to rate constants were used to determine the equilibrium constants, K(T 1 ) and K(T 2 ), with T 1 being 32C and T 2 being 40C, as 1.704 and 1.602, respectively. These ratios were also used to determine the individual rate constants k 1 (T 1 ), k 2 (T 1 ), k 1 (T 2 ), k 2 (T 2 ) as 0.0592 min-1 , 0.0347 min-1 , 0.0740 min-1 , and 0.0462 min-1 respectively. Activation energies E 1 and E 2 for the forward and reverse reactions, respectively, were determined to be 22.16 kJ/mol and 28.43 kJ/mol. Polarimetry is a relatively simply yet highly effective way to study a rate process. I. Introduction This experiment had three main objectives. Objective one was to determine the rate constants for both the forward and reverse reactions. Objective two was to determine the equilibrium constant for the mutarotation of glucose. Objective three was to determine the activation energies for the forward and reverse reaction. With the mutarotation of Experiment 4: Polarimetry glucose, rotation of polarized light can be used to determine the concentration of glucose at different times. The amount of rotation observed is proportional to the concentration of the molecule in the medium and the path length of the polarized light in the medium. Glucose has two common isomers, (A) and (B). The isomers can interconvert in aqueous solutions at room temperature. The equilibrium constant K for the reaction can be related to the equilibrium concentrations of A and B, Aeq and Beq respectively, as well as the rate constants for the forward and reverse reactions, k1 and k2 , respectively: = = K AeqBeq k1k2 (4-1) Laboratory work has determined that both -glucose and -glucose follow first order kinetics. Therefore, the interconversion of the isomers is both a reversible and unimolecular reaction. From first order kinetics of this reversible process and from recognition that t is linearly related to At , it can be derived (see Experiment 4, pages 2-3 in lab notebook for detailed derivation): lnt- 0- = - + k1 k2t (4-2) where 0 , t , and are the rotations at t=0, t=time t, and t=, and t is the time. Time has been reached once equilibrium is established. has been reached once equilibrium is established....
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