Chapter 13 Chemical Kinetics This chapter introduces the concept of rates of chemical reactions For the exam you should be able to Differentiate between average and instantaneous rates of chemical reactions Average reaction rate is the change in concentration of reactant or product as a function of time m s concentration increases with increasing time Instantaneous reaction rate is the average rate over an infinitesimally small time variable Rate decreases as a function of decreasing concentration Define the rate constant k is used for the rate constant the rate constant is known as the proportionality between the reaction rate and the concentration of the reactant k is affected only by a change in temperature unit for k is 1 s increase in temperature increase in rate increase in rate constant Use the concepts of stoichiometry to write reaction rate expression in terms of the disappearance of reactants and the appearance of products 2A B two moles of A disappear for each mole of B that forms the rate at which B forms is one half the rate at which A disappears Rate 1 2 A T Sketch the rate of reaction versus concentration of reactant for zero and first order reactions Reaction order is the sum of the powers to which all reactant concentrations appearing in the rate law are raised 1st order A product rate A t rate law says rate k A 1 s Use rate data to determine rate laws and rate constants A rate law is a the measure of the rate of the reaction to get information regarding the rate constant and reaction order raised to some powers aA bB cC dD Rate k A x B y where x and y are experimentally determined Deduce overall rates of reactions based on the units of a given rate constant 1st order units are inverse seconds 1 s or s 1 2nd order units are M 1 s 1 3rd order units are m 2 s 1 Use the integrated forms of the differential rate laws to determine the time required for the concentration of a reactant to change a desired amount given the initial concentration and the rate constant for zero first and second order reactions See recitation worksheets and practice tests for examples and explanations Use the integrated forms of the differential rate laws to derive mathematical expressions for the half life of zero first and second order reactions See recitation worksheets and practice tests for examples and explanations Use collisional theory to explain the dependence of reaction rates on temperature and activation energy Chemical reactions occur as a result of collisions between reacting molecules therefore it is the collision theory of chemical kinetics The rate of the reaction is directly proportional to the number of molecular collisions per second or to the frequency of molecular collisions If the concentration were doubled then the number of collisions would double Also if the initial kinetic energy is large the colliding molecules will vibrate so strongly that they break some of the chemical bonds Use potential energy profiles to determine whether or not a reaction is endothermic or exothermic A potential energy profile is a diagram used to describe the mechanism of a reaction This diagram is used to show the concepts of activation energy and the arrhenius equation and to show the changing potential energy between the reactant and product that occur during a chemical reaction The activated complex is highly unstable when there is a high potential energy PAGE 591 Explain why an activated complex transition state is unstable and short lived When molecules collide they form an activated complex AKA transition state it is a temporary species formed by the reactant molecules as a result of the collision before they form the product They are short lived because or unstable when the products are less stable than the reactants because the reacting mixture from the surroundings will absorb the heat and thus have an endothermic reaction also it is an unstable arrangement of atoms that exists momentarily at the peak of the activation energy barrier Because of its high energy the activated complex exists for an extremely short period of time Perform calculations using the Arrhenius equation i e relative rates calculating activation energies etc See recitation worksheets and practice tests for examples and explanations By rearranging this equation any variable can be updated it can be rearranged to Activation energy is the minimum amount of energy required to initiate a chemical reaction Demonstrate that the sum of elementary steps is the overall reaction for a reaction mechanism and that intermediates appear in the reaction mechanism but not in the overall reaction molecular level Elementary steps is a series of simple reactions that represent the progress of the overall reaction at the Intermediates appear in the mechanism of the reaction but not the overall balanced equation therefore if itself between step one and step two the net reaction does not contain it coefficients Step 1 NO NO N2O2 Step 2 N2O2 O2 2NO2 Overall 2NO O2 2NO2 so the N2O2 was the intermediate because it cancels with The rate law rate k NO 2 O and the order is determined by the stoichiometric Define molecularity of unimolecular bimolecular and termolecular reactions Molecularity is the number of molecules reacting in an elementary step Unimolecular 1 molecule participates in reaction Bimolecular 2 molecules participating in reaction Termolecular 3 molecules reacting participating in reaction the simultaneous encounter of all 3 molecules is far less likely very rare reaction Evaluate the likelihood of a proposed mechanism based on the molecularity of the elementary steps and the theoretical rate law from the rate determining step Based on proposed mechanism the 3rd termolecular is the most unlikely if the rate is not consistent with the observed rate law it is not a reasonable mechanism The slow step is also always the rate determining step Relate the importance of the rate determining step in determination of reaction mechanisms Reaction mechanisms are based off of elementary steps satisfying two requirements 1 The sum of the elementary steps must give the overall balanced equation for the reaction 2 The rate determining step should predict the same rate law as it determined expirimentaly Describe what catalysts do to a reaction rate and how they do it A catalyst is a substance enzyme biological catalyst that increases the rate of the reaction by lowering the activation energy and not being consumed in the reaction also an
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