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14 Chemical Kinetics Solutions to Exercises 14.90 Just as the π electrons in are attracted to the surface of a hydrogenation catalyst, the nonbonding electron density on S causes compounds of S to be attracted to these same surfaces. Strong interactions could cause the sulfur compounds to be permanently attached to the surface, blocking active sites and reducing adsorption of alkenes for hydrogenation. 14.91 (a) Living organisms operate efficiently in a very narrow temperature range; heating to increase reaction rate is not an option. Therefore, the role of enzymes as homo- geneous catalysts that speed up desirable reactions without heating and undesir- able side-effects is crucial for biological systems. (b) catalase: + nitrogenase: N₂ 2NH₃ (nitrogen fixation) (c) This model for enzyme kinetics is similar to the mechanism detailed in Sample Exercise 14.15, where step one is a fast equilibrium and step two is slow and rate- determining. This model assumes that the rate of the bound substrate, ES, being chemically transformed into bound product is slow and rate-determining. 14.92 The individual structure of each enzyme molecule leads to a unique coiling and folding pattern. The resulting shape and electronic properties of the active site in each enzyme leads to its substrate specificity. 14.93 Let k and Ea equal the rate constant and activation energy without the enzyme (uncata- lyzed). Let kc and Eac equal the rate constant and activation energy with the enzyme (catalyzed). A is the same for the uncatalyzed and catalyzed reactions. The difference in activation energies is kc = 1.0 10⁶ s⁻¹, k = 0.039 s⁻¹, T = 25°C = 298 K. According to Equation [14.20], In k = + In A. Subtracting In k from In kc In Carbonic anyhdrase lowers the activation energy of the reaction by 42 kJ. 14.94 (a) (NH₂)₂C=O(aq)+ (b) According to Equation [14.20], In k = + In A. Subtracting In k from In kc = 294 Ea 373 Dividing through by 294: 20.5239(8.314)(373) = Ea (1.27)Eac = = Ea (1.27) Dividing though by 373: 50,167 = = (Ea - is somewhere between 50 kJ and This is a substantial reduction in activation energy for the enzyme-catalyzed reaction. 421