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The Collision Model

Temperature and Rate. The Collision Model The higher the temperature, the more energy available to the molecules and the faster the rate. Complication: not all collisions lead to products. In fact, only a small fraction of collisions lead to product. The Orientation Factor

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The Collision Model

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  1. Temperature and Rate • The Collision Model • The higher the temperature, the more energy available to the molecules and the faster the rate. • Complication: not all collisions lead to products. In fact, only a small fraction of collisions lead to product. • The Orientation Factor • In order for reaction to occur the reactant molecules must collide in the correct orientation and with enough energy to form products.

  2. Temperature and Rate • The Orientation Factor • Consider: • Cl + NOCl  NO + Cl2 • There are two possible ways that Cl atoms and NOCl molecules can collide; one is effective and one is not.

  3. Temperature and Rate The Orientation Factor

  4. Temperature and Rate • Activation Energy • Arrhenius: molecules must posses a minimum amount of energy to react. Why? • In order to form products, bonds must be broken in the reactants. • Bond breakage requires energy. • Activation energy, Ea, is the minimum energy required to initiate a chemical reaction.

  5. Temperature and Rate • The Arrhenius Equation • Arrhenius discovered most reaction-rate data obeyed the Arrhenius equation: • k is the rate constant, Eais the activation energy, R is the gas constant (8.314 J/K-mol) and T is the temperature in K. • A is called the frequency factor. • A is a measure of the probability of a favorable collision. • Both A and Ea are specific to a given reaction.

  6. Temperature and Rate • Determining the Activation Energy • If we have a lot of data, we can determine Ea and A graphically by rearranging the Arrhenius equation: • From the above equation, a plot of ln k versus 1/T will have slope of –Ea/R and intercept of ln A.

  7. Reaction Mechanisms • The balanced chemical equation provides information about the beginning and end of reaction. • The reaction mechanism gives the path of the reaction. • Mechanisms provide a very detailed picture of which bonds are broken and formed during the course of a reaction. • Elementary Steps • Elementary step: any process that occurs in a single step.

  8. Reaction Mechanisms • Elementary Steps • Molecularity: the number of molecules present in an elementary step. • Unimolecular: one molecule in the elementary step, • Bimolecular: two molecules in the elementary step, and • Termolecular: three molecules in the elementary step. • It is not common to see termolecular processes (statistically improbable).

  9. Reaction Mechanisms • Rate Laws for Elementary Steps • The rate law of an elementary step is determined by its molecularity: • Unimolecular processes are first order, • Bimolecular processes are second order, and • Termolecular processes are third order. • Rate Laws for Multistep Mechanisms • Rate-determining step: is the slowest of the elementary steps.

  10. Reaction Mechanisms Rate Laws for Elementary Steps

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