Thermo_ISM_ch19 - Thermodynamics by Engel

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19-1 Chapter 19: Complex Reaction Mechanisms Questions on Concepts Q19.1) How is a simple reaction different from a complex reaction? A simple reaction is any reaction that consists of the single reaction step. A complex reaction is any reaction that occurs in two or more elementary steps. Q19.2) For a reaction mechanism to be considered correct, what property must it demonstrate? In order to be valid, a proposed reaction mechanism must agree with the experimentally determined rate law. Q19.3) What is a reaction intermediate? Can an intermediate be present in the rate law expression for the overall reaction? A reaction intermediate is a chemical species that is formed during the course of a reaction, but decays as the reaction proceeds resulting in product formation. The reaction intermediates must be consumed prior to the completion of the reaction, therefore reaction intermediates will not appear in the overall reaction expression. Q19.4) What is the preequilibrium approximation, and under what conditions is it considered valid? The pre-equilibrium approximation is somewhat related to the steady state approximation. The pre-equilibrium approximation assumes a complex reaction of the form f p r k k k AB I P → + ← The principle assumption of the pre-equilibrium approximation is that equlibrium is established between reactants A, B and the intermediate species I. I then undergoes reaction to form the product P. This approximation is valid when the reaction rate for the decomposition of I into A and B is much faster than the decomposition of I into P (in other words, k f >> k p ). Q19.5) What is the one main assumption in the Lindemann mechanism for unimolecular reactions? The principle assumption of the Lindemann reaction is that an activated complex is formed during the course of the reaction. This activated complex can then
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Chapter 19/Complex Reaction Mechanisms 19-2 deactivate resulting in reformation of the reactant, or it can decompose resulting in product formation. This activated complex assumption allows for the decomposition to be a linear function of the concentration of the activated reactant; therefore, the Lindemann mechanism is consistent with the experimentally determined unimolecularity of the reaction. Q19.6) How is a catalyst defined, and how does such a species increase the reaction rate? A catalyst is a chemical species that increases the rate of a chemical reaction without being consumed during the reaction. The catalyst serves to set up an additional reaction coordinate for which the activation energy is reduced relative to the uncatalyzed reaction. Q19.7) What is an enzyme? What is the general mechanism describing enzyme catalysis? An enzyme is essentially a biological catalyst. The "lock and key" model of enzyme activity is discussed in the text where the catalyst binds a substrate in a very specific location and orientation. Once the enzyme-substrate complex is formed, the product is produced and free enzyme is regenerated.
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Thermo_ISM_ch19 - Thermodynamics by Engel

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