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CH10_Page_40 - hfl M3i_i K R a c K l —l In 2 =...

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Unformatted text preview: hfl M3i_i K. R a c K l- —l In 2 = 221.4)(“3 _J-mol_ l _ 1 J'l'lfllD—IS'IE‘; 33141-14 ' -mol‘ 293 K ass K = 43.32? Ksm = 3359x1045 cream.” =14.4v4 Simfi PKH‘MK =14.t}0 for regular water <2 FEM“ 214474, substitution of D for H causes reactants, i.e., the associated molecules, to be favored more in heavy Twater than in regular water even though we expect I) and H to be chemically equivalent. We can suggest that this observation is another example of the kinetic isotope effect where the zero-point energy of the CHI) bond is lower than that of the 0-H bond (see the solutions to Exercises 1155 and 1?.35 for further discussion). If bond dissociation is the rate-limiting step in the mechanism, and if only the vibrational energy of the ground state of the water molecules is changed appreciably, then the activation barrier for dissociation is bigger in heavy water since the zero-point energy is lower due to the heavier mass of D. This increase in the activation energy for bond dissociation causes the rate of dissociation to decrease white the rate of association is not changed appreciably. The position of equilibrium then favors the associated D20 molecules more than it does in the H20 molecules. 10.13:] {a} LEI “in IA . 334 ...
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