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14 Chemical Kinetics Solutions to Exercises = 77°F = = 298 K; = = 5°C = 278 K, k₁/k₂ = 40 The values are amazingly consistent, considering the precision of the data. (b) For a first order reaction, t1/2 = 0.693/k, k = k₁ at 298 K = 0.693/2.7 yr = 0.257 = yr⁻¹ = = 2.835 10³; k₂ = 10³ = 9.066 = 9.1 yr⁻¹ Reaction Mechanisms (section 14.6) 14.69 (a) An elementary reaction is a process that occurs in a single event; the order is given by the coefficients in the balanced equation for the reaction. (b) A unimolecular elementary reaction involves only one reactant molecule; the acti- vated complex is derived from a single molecule. A bimolecular elementary reac- tion involves two reactant molecules in the activated complex and the overall process. (c) A reaction mechanism is a series of elementary reactions that describe how an overall reaction occurs and explain the experimentally determined rate law. 14.70 (a) The molecularity of a process indicates the number of molecules that participate as reactants in the process. A unimolecular process has one reactant molecule, a bi- molecular process has two reactant molecules and a termolecular process has three reactant molecules. (b) Termolecular processes are rare because it is highly unlikely that three molecules will simultaneously collide with the correct energy and orientation to form an ac- tivated complex. (c) An intermediate is a substance that is produced and then consumed during a chemical reaction. It does not appear in the balanced equation for the overall reaction. 14.71 A transition state is a high energy complex formed when one or more reactants collide and distort in a way that can lead to formation of product(s). An intermediate is the product of an early elementary reaction in a multistep reaction mechanism. A transi- tion state occurs at an an energy maximum or peak of a reaction profile such as Figure 14.20. An intermediate exists at an energy minimum or trough of a reaction profile. Every reaction, single- or multi-step, has a transition state. Only multistep reactions have intermediates. 415