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CHAPTER 13: CHEMICAL KINECTICSIntroductiono Chemical kinetics examines the factors which can influence the rate of a reactiono The rate at which a reaction progresses can be influencedby several factors:TemperaturePressureReactant concentrationThe addition of a catalystMechanical forceCollision Theoryo Molecules A and BC must physically collide with each other o The collision between A and BC must occur with sufficient energysuch that the bond between B and C breakso Reactant molecule A must collide with the C side of the compound BC. In other words, collisions must occur with the correct spatial orientationRates of Chemical Reactionso For any balanced chemical reaction: o aA + bB à cC + dD o The average reaction rate is given by the equation:¿−1 a∆[A]∆ t=−1b∆[B]∆ t=1c∆[C]∆ t=1 d∆[D]∆ to o The reaction rate of a chemical species is proportional to its stoichiometric coefficient.Rate Lawso A rate law is a mathematical equation which relates the rate of a reaction to the concentration of the reactant(s)Integrated Rate Lawso Integrated rate laws quantify the relationship between reactant concentration and time.o If several concentrations are determined at different times, the rate constant k can then be graphically determinedo It is of interest to know the half-life (t1/2) of a reactiono The time required for a reaction to go to half-completion o In terms of radioactive isotopes, this is the time required for a sample to decay to half its initial amount
Reaction Mechanismso A reaction mechanism represents the step-by-step reactions which, when combined, add up to an overall net reactiono The overall reaction is only as fast as its slowest elementary step.o The slowest step is called the rate-limiting step. o Recall that equilibrium is reached when the rate of the forward equals the rate of the reverse reaction. We called this a dynamic process.Reaction Rates and Temperatureo In general, as temperature increases, so does the reaction rate o In order for a reaction to occur, reactant molecules must collide with sufficient energy AND in the proper orientationCatalysiso A catalyst lowers the activation energy of a reaction by assisting reactant molecules to be in the proper orientationo Homogenous catalysts are in the same chemical state as the compounds they will react with (the substrates)o Heterogeneous catalysts are in different chemical states than their substratesdo not appear in the overall balanced chemical equationcan be reused many times over before losing efficacyare used very early on in a mechanism, and regenerated close to or during the last stepdo not change ∆Hrxn (i.e., percent conversion)increase the rate of both the forward and reverse reactions equallyoften provide a different mechanism than the uncatalyzed reactionSummaryo Kinetics studies the factors that can influence the rate of a chemical reactiono For a reaction to occur, reactant molecules must physically collide with sufficient energy and in the proper orientationo The method of initial rates and the use of integrated rate laws can help solve for the rate orders and for the rate constanto The slowest step of a reaction mechanism dictates the overall reaction rate
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