1 (292)

Prévia do material em texto

292 Chapter 14 Chemical Kinetics 14.111 Using the energy diagram shown and using Hess's law, we can see that the activation energy for the decomposition is equal to the acti- vation energy for the formation reaction plus the heat of formation of 2 moles of HI, or formation = decomposition + formation decomposition So formation = 185 kJ + 2 = 174kJ. activation activation E energy energy Check: Because the reaction is endothermic, we expect the activation energy in the reverse direction to be less than that in the forward direction. I₂ (2 mol)(-5.65 kJ/Mol) 2 HI Reaction progress Note: Energy axis is not to scale. Challenge Problems 14.113 (a) Because the rate-determining step involves the collision of two molecules, the expected reaction order would be second-order. (b) The proposed mechanism is + CH3NC* + CH3NC (fast) CH₃CN (slow) CH₃NC CH₃CN So the sum matches the overall reaction. is the activated molecule. Because the second step is the rate-determining step, Rate = k₃ [CH₃NC*] Because is an intermediate, its concentration cannot appear in the rate law. Using the fast equilibrium in the first step, we see that = or [CH₃NC*] = Substitut- ing this into the first rate expression, we get Rate = which simplifies to Rate = k[CH₃NC]. This matches the experimental observation of first order, and the mechanism is valid. 14.115 Rate = and Rate = d[A] , so d[A] = Moving the A terms to the left and the t and constants to the right, we have d[A] = kdt. Integrating, we get [A] [A]₀ d[A] [A] = 0 kdt. When we evaluate this integral, = kt = kt + A or [A], 1 = kt + 1 which is the desired integrated rate law. 14.117 For this mechanism, the overall reaction is the sum of the steps in the mechanism: CICO(g) + + CO(g) + + No overall reaction is given. Because the third step is the rate-determining step, Rate = k₅[ Because CICO is an intermediate, its concentration cannot appear in the rate law. Using the fast equilibrium in the second step, we see that = or [CICO] = k₃ k₄ Substituting this expression into the first rate Copyright © 2017 Pearson Education, Inc.