CH 302 1st Edition Lecture 21Outline of Last Lecture I. Reaction MechanismsII. Elementary StepsIII. Unimolecular ReactionsIV. Bimolecular ReactionsV. Rate Limiting StepsVI. Mechanisms and Rate LawsVII. IntermediatesOutline of Current Lecture I. Arrhenius TheoryII. Reaction CoordinationIII. Activation EnergyIV. Arrhenius LawV. Reaction CoordinationVI. CatalystsCurrent LectureArrhenius Theory- The Arrhenius theory and reaction coordinates relate to a particular perspective on chemical kinetics that imagines the path taken by the reactants as they are converted to the products. - This change is not instantaneous and is typically slowed by the fact that in order to covert from reactants to products, the system must first pass through a higher energy state known as the transition state. - The size of the energy barrier controls the temperature dependence of the rate constant.Reaction Coordination- A reaction coordinate is a path that links the reactant molecules and the products molecules. - In many reactions, we can directly envision this coordinate as the length of a particular bond or bonds. - In other cases, the reaction coordinate is used merely to represent some unknown coordinate. - The key is that there are many many potential paths between reactants and products.- The reaction coordinate represents the lowest energy path.- For example, in the reaction of CH3Cl + OH- to form CH3OH and Cl-, the mechanism of this reaction is a single step in which the CH3Cl collides with the OH- and forms the products. - We can envision a reaction coordinate for this reaction which is the lengthening of the C-Cl bond (the one that is breaking) and the shortening of the C-O bond (the one that is forming).Activation Energy- The height of a barrier along the reaction pathway is the activation energy. - The size of the activation energy depends on which way you approach the barrier as the energy on either side of the barrier could be higher or lower. - Typically, we envision reactions proceeding left to right along the reaction coordinate, so often, the activation energy is only noted for the forward reaction. - The activation energy on the diagram below shows the barrier to be 102.6 kJ mol-1. Barriers are measured in energy per mole (typically kJ mol-1)Arrhenius Law- The rate of a reaction depends on the height of the barrier (the activation energy) because only a small fraction of the number of molecules at a given temperature have sufficient energy to get over the barrier. - At a given temperature, there is a distribution of kinetic energies for all the molecules ina sample (this is the Boltzmann distribution). - This molecular kinetic energy can describe things like vibrations of molecules even for samples like solids that might not be "moving".- This distribution has a long tail to high energy. - So there are always a few molecules with sufficient energy to get over the barrier (energies greater than Ea). - As the temperature is raised, the distribution shifts to higher energy and so more molecules have sufficient energy to get over the barrierReaction Coordination- We can relate our reaction coordinate picture to the chemical mechanism.For example, if we have a mechanism that has three steps, we would expect this reaction to have three barriers. We would also expect the reaction to have two intermediates.- For example, the following picture illustrates a mechanism that occurs in three steps andforms two intermediates at points B and C (A is reactants and D is products). - Note: Each letter does not literally have to be a single chemical species, instead point B isall the intermediate chemical species that exist after the first step.Catalysts- Catalysts can either be in the same phase as the chemical reactants or in a distinct phase.- Catalysts in the same phase are called homogeneous catalysts, while those in different phases are called heterogeneous catalysts.- For example, if we have Pt metal as a catalyst for the reaction of hydrogen gas and ethene gas, then the Pt is a heterogeneous catalyst. However, an enzyme in solution catalyzing a solution phase biochemical reaction is a homogeneous
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