C401Ch12LN3 - Chapter 12: Chemical Kinetics, Part 3 The...

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Chapter 12: Chemical Kinetics, Part 3 The Arrhenius Equation Arrhenius discovered that most reaction-rate data obeyed an equation based on three factors: The number of collisions per unit time. The fraction of collisions that occur with the correct orientation. The fraction of the colliding molecules that have energy equal to or greater than E a . From these observations Arrhenius developed the Arrhenius equation : Where k is the rate constant, E a is the activation energy, R is the ideal-gas constant (8.314 J/K•mol), and T is the temperature in K. A is called the frequency factor . It is related to the frequency of collisions and the probability that a collision will have a favorable orientation. It is also related to molecular size and shape. Both A and E a are specific to a given reaction . E a and A may be determined experimentally. If we have data from 2 different temperatures we can find E a mathematically. We use the Arrhenius equation: If we have data from 3 or more different temperatures, we can find Ea and A graphically. We can do this because of the second way of representing the Arrhenius Equation: If we graph ln k vs. 1/T, we obtain a straight line, with a slope of –E a /R and a y-intercept of ln(A).
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Reaction Mechanisms The balanced chemical equation provides information about substances present at the beginning and end of the reaction. But it doesn’t tell you the path or the steps of the rxn. The reaction mechanism is the process by which the reaction occurs. Mechanisms provide a picture of which bonds are broken and formed during the course of a reaction. They also tell you the individual steps in a chemical rxns. Elementary Steps • Elementary steps are any processes that occur in a single step. The number of molecules present in an elementary step gives the molecularity of that elementary step. Unimolecular : one molecule in the elementary step. Bimolecular : two molecules in the elementary step. Termolecular : three molecules in the elementary step. It is not common to see termolecular processes (statistically improbable). A multistep mechanism consists of a sequence of elementary steps. The elementary steps must add to give the balanced chemical equation. Some multistep mechanisms will include intermediates . These are species that appear in an elementary step but are neither a reactant nor product.
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This note was uploaded on 06/25/2008 for the course CHEM 400-401 taught by Professor Dr.samples during the Fall '06 term at American River.

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C401Ch12LN3 - Chapter 12: Chemical Kinetics, Part 3 The...

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