CH 302 1st Edition Lecture 20Outline of Last Lecture I. Integrated Rate LawsII. First OrderIII. Zero OrderIV. Second Order V. Pseudo First OrderOutline of Current Lecture I. Reaction MechanismsII. Elementary StepsIII. Unimolecular ReactionsIV. Bimolecular ReactionsV. Rate Limiting StepsVI. Mechanisms and Rate LawsVII. IntermediatesCurrent LectureReaction Mechanisms- One of the reasons for studying chemical kinetics is not simply to learn how to make reactions go faster, but the fact that by studying reaction rates macroscopically in the lab, we can gain insight into the molecular details of how the chemistry is actually happening.- The steps involved in a chemical reaction make up what we call a mechanism. They are a breakdown of what actually happens during the course of the reaction (what bonds break, what intermediate species are formed, what is the role of a catalyst....).Elementary Steps- The mechanism of a chemical reaction is typically written down as a series of elementarysteps. - The steps themselves are characterized by their "molecularity". - The molecularity is a way of stating exactly how many molecules are involved in the elementary step. - One molecule reacting by itself is a unimolecular reaction, or you say that the molecularity is one. - Two molecules reacting is called a bimolecular step (the molecularity is two). - Generally speaking only uni- and bimolecular steps are proposed in reaction mechanisms- The sum of these steps is the overall reaction.Unimolecular Reactions- Unimolecular steps in a reaction mechanism are elementary steps in which a single molecule does something (typically breaks a bond). - The key is that these steps involve only a single reactant species. - Thus the name unimolecular.- Radioactive decay is an example of a unimolecular process.- Chemical reactions can also be unimoleuclarBimolecular Reactions- Bimolecular reactions are elementary reaction steps that involve two reactant species. - The two molecules collide and then react. For this elementary reaction step the two species (CH3Br and OH-) collide and react. - The rate for bimolecular steps is 2nd order overall and first order with respect to each of the reactants since the number of collisions depends on the concentrations of both of the molecules. Rate Limiting Steps- Understanding how the different steps in a mechanism contribute to the overall rate of areaction can be complicated if we demanded to know the rate with exquisite precision. - However, if we simply want to get an understanding of the rate that is nearly (but not perfectly) correct, we can make an important simplification: the rate of the overall reaction is determined by the rate of the slowest step in the mechanism. - The slowest step in the mechanism is called the rate determining step or rate limiting step.Mechanisms and Rate Laws- Putting this all together, we can now compare mechanisms and empirical rate laws. - Any given mechanism will predict the overall rate law for the reaction. - We can compare this predicted rate law with the empirical law observed in the lab and see if the "mechanism is consistent with the rate law". - This lets us know if the mechanism is plausible. If the proposed mechanism predicts the observed rate law, then this is a possible mechanism for what is actually happening in the reaction. Verifying that the proposed mechanism is exact would require further measurement to validate it. - However, if the predicted rate law is not consistent with the experiments (they are different), then we know this mechanism is not correct. - In this way, the macroscopic measurements of rate in the lab give us insight into specifically how the chemistry is actually taking place on a molecular level.Intermediates- An intermediate is a chemical species involved in a mechanism that does not appear inthe overall reaction. - An intermdiate is typically both a product and reactant in different elementary steps of the overall chemical
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