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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 - PowerPoint PPT Presentation
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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.
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.
Temperature and Rate
The Orientation Factor
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.
Temperature and Rate
The Arrhenius Equation• Arrhenius discovered most reaction-rate data obeyed the
Arrhenius equation:
– k is the rate constant, Ea is 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.
Temperature and Rate
RTEa
Aek
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.
Temperature and Rate
ARTE
k a lnln
• 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.
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).
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.
Reaction Mechanisms
Rate Laws for Elementary Steps
Reaction Mechanisms