GENERAL CHEMISTRY CHEM 162
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This 13 page Class Notes was uploaded by Carmela Kilback on Wednesday September 9, 2015. The Class Notes belongs to CHEM 162 at University of Washington taught by Staff in Fall. Since its upload, it has received 22 views. For similar materials see /class/192605/chem-162-university-of-washington in Chemistry at University of Washington.
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Date Created: 09/09/15
Chapter 15 Chemical Kinetics ALL reactions bi ochemical chemical inorganic organic gas phase solution solid etc cxc ept nuclear reactions aAbB products 151 Reaction Rates Laws An Introduction 156 Reaction Mechanisms 15 7 The SteadyState Approximation 158 A Model for Chemical Kinetics 159 Catalysis 1 Reac on Mechanisms reactants to products aA bB 45A gt products eg NO2 CO NO C02 3 Rate law from experiment Rate N02 m n uuuer Proposed rxn mechanism N02g t N02g k gtN03g N0 5 N03 5 C0 5 k 2gt N02 3 C02 5 Overall Reaction N02 5 co 5 NOg co2 g N03 5 an 39ntermediate neither a reactant nor a product a ElementarV step i a reaction whose rate law can be written from its molecularig a Molecularig i the number of species ie reactants that must collide to produce the reaction indicated by that step a Examples of Elementary Steps TABLE 157 Examples of Elementary Steps and Corresponding Rate Laws Elementary Step Molecularity Rate Law A gt products Um39molecular Rate kA A A 4 products Bimolccular Rate HA 2A gt products A B A products Bimolccular Rate IzIAB A A B gt products Tcrmolecular Rate kA2Bl 2A B 4 products A B C gt products Termolccular Rate leIABC A The rate law for an elementary step follows directly from the molecularity of that step ie unimolecular step 5 Rate k A Reaction Mechanism a A series of elementary steps must satisfy two criteria 1 Sum of elementary steps must equal the overall balanced equation for the an 2 The mechanism must agree with the experimentally determined rate law N02 g t N02 g gt N03 g N0 g N03g C0 g gt N02 g t C02 g N02 g C0 g gt N0 g t C02 g a Ratedetermining step A wonderful concept k1 k2 k3 A A gt B C gtD W E fast slow In a series of multistep reactions the slowest step is the ratedetermining step rate of overall reaction is equal to the ratedetermining step Noam N0g 1 N02fg N02fg k1 N020 C020 Noz g COM m o C n a c n e m A m m 0 S k Noug Nogg WNoxg Noltg k N03 g CO g T N02 g CoZ g Ratedetermining step is a bimolecular rxn Rate k1 NOZ2 Same as the experimentally observed rate 2 Proposed mechanism may be correct since it meets the 2 criteria of a reaction mechanism gt However it is not always very easy to identify the slowest step and hence the ratedetermining step 157 The SteadyState Approximation a A systematic way to determine the ratedetermining step in a reaction mechanism 2N0 g Hzg gtNZO g HZOg Proposed mechanism k 2N0 gtk1 N20Z 71 k N20Z H2 Zgt NZO HZO Assume the concentration of the N202 intermediate is constant ie d N202 0 dt Rate of production of NZOZ rate of consumption of NZO2 le Oz 1lt1NOlZ dlglzozl krlNZOZ klezOzl Hz t t k1 NOZ k4 N202 k2 N202 Hz SteadyState Condition 1 A Model for Chemlcal Kmehcs aAbB gt products Rate Alanna Fig1510 A plotshowing the exponential dependence constant on the absolute tempe i Concentration of reactants affects reaction rates i Other factors that affect reaction rates temp erature ra e riture presence ofa catalyst gt Chemical reactions speed up when the temperature is increased evidence comes from measuring reaction rates at different temperatures 7 rate constant k Vs T shows a strong temperature dependence 139th a The exact temperature dependence ofkT is k A exp emRT e Arrhenius egiation k rate constant R gas constant eg 831 JmolK T absolute temp in Kelvin K Ea activation energy Jmol ie the minimum or threshold energy necessary for a reaction to occur also called energy barrier to reaction A preexponential factor or frequency factor for the rxn anhvsir all A H quot39 39 39 39 in k rEaR 1m 1n A 1quot k If plot is linear Arrhenius equation is obeyed by the rxn 7 Many reactions in science chemistry physics biology etc at least over some temp range a Arrhenius equation k A exp iEaRT is obtained from the collision model of chemical reactions Molecules must collide to react Not all intermolecular collisions result in reaction 7 collision With proper orientation may result in rxn i a threshold minimum energy required eg 2 BrNO g gt 2 NO g Br2 g ONWB T Ir 1 139 x I A Iunsu anr L 2 5 o g l S ZErNO I J 39 Exothermlc rxn rcuclunl 2N0 Br IAl tin man amnion prugrc Orientation of reactants in collision 8 8 a m 9 h Nn reaction Example 158 Find the Ea ofthe Ixn emu K 2 N205 E 4 N02 5 02 s 539 L qu P mm T 1 C k 5quot T K lT Iltquot lnk 4K 4 x mm 30 7300 40 50 7900 60 7 4mm Slope rEaR 7 712 x 104K R 83145 JK mol 2 Ea 10 x 105 Jmol 100 kJmol r I 1 m 12 Finding the Activation Energy from Two Data Points Instead of a plot of lnk vs UT and getting Ea from the slope you can calculate it from In k rEaRT In A 1 Ink rEaRTZJrlnA 2 21 1n km EaR1T 7 1T2 Example Notintextbook ZHIQE HAS 125 Rate k HI2 951 X 10 9 Lmols 110 X 10 5 Lmols 1n 110X10 5 Ea 1 r 1 51 X 10 9 83145 JmolK 500 K 600 K 176 kJmol What is Ea 15 9 Catalysis a Recall that we can speed up chemical reactions by increasing temperature as described by the Arrhenius laW k A exp iEaRT T1 a T2 T2 gt T1 Ratez kz expE 1 7 Rate1 k1 R T1 T2 a However it is not always practical or convenient to increase reaction rates by increasing temperature a Catalysis i the use of catalysts to speed up reactions Without changing temperature Catalyst a substance that speeds up a reaction Without being consumed itself 39 Ratecalzly5l up to 106to 1045 Rate wo catalyst Enzymes i biological catalysts 7 Nearly all biological rxns are catalyzed by enzymes eg Carbonic anhydrase catalyses the removal of the metabolic product carbon from cells enzyme COZ H20 2 HCO H r Industrial Reactions 7 Most industrial chemical processes are based on catalyzed rxns 7 catalyst is in the same phase as reacting molecules Eg in solution liquid or gas Heterogeneous 7 catalyst is in a phase different from the a Catalysts reactants e solid catal st in reacti uid mixture Examples of Homogeneous Catalysis g y Hg 7 liquid or gas phase systems i typically acids or bases metal salts enzymes Nitric Oxide Catalyzed Production Destruction of Ozone Lower Atmosphere N0 g 2 02 g gt N02 g hv light NOZg gt NOg 0g 02g 0g 03 g 32 02 g 03 g A NO is a catalyst in the production of ozone Upper Atmosphere N0 g 03 g gt N02 g 02 g 0 g N02 g gt NO g 02 g 0g 03g gt 202g gt NO catalyzes the depletion of ozone layer NO in the atmosphere comes from emissions from car engines Figure 1516 Exhaust gases from an automobile are passed through a catalytic converter to minimize air pollu on Exhaust manifold Exhaust gases Exhaust gases C0r gt NO a Cululylic cnnvcl lcr N3 Nox SOX The big air pollutants How Catalysts Work A The dramatic effect of a catalyst on the the activation energy Figure 1515 Energy plots for catalyzed and f given reaction uncatalyzeol pathways or a Unenlalyzed palhway Catalyzed i pll lway Enevgy Producls JAE l Reaction pmgn 39ns a given energy Number M e W n glv Figure 1516 Effect ofa catalyst on the number of reacti onproducing collisions err e cnlllslons unci alyzcd En luncnlnlyzodl Energy an 1culdyzcltl Energy How Catalysts W0 k Tame 210 The change in rate of some typical reactians seen when catalyst ls added to the reaction mixture Rate 0 E El Enhancement Uncalalyzed Catalyzed Cnlculaled m Catalyst kcaImot kenml 500 K P 44 4 JD Au 5s 29 lo o I 53 34 In SHIRT Tub adapted I39Ivrn Mala HI Band H937 Heterogeneous Catalysis o o mm m 1 Adsorption and activation ofreactants 2 Migration on the surface gt 3 Reaction of adsorbed species on the surface 4 Desorption ofpmducts Fig 1517 Heterogeneous catalysis h ofthe hydrogenation of ethylene Example Activation Energy bB gt Products The rate constant for this reaction increases from 100 S391 to 1000 s1 when the temperature is increased from 3000 K to 4000 K What is the activation energy for the reaction in kJmol R 8314 JmolK Since lnk Ea RT In A InkERT lnA k E l l 2 a 2 1n2a Ink1 Ea RTl In A k1 R T1 T2 k Rlnk 8314 JmoloK1n l l l 1 7 777 T1 T2 300K 400K 2297 kJmol or 230 kJmol Example Reaction Mechanism Assume the following proposed reaction mechanism is correct Cl2 a 2 Cl slow 1 H2 Cl 6 HCl H fast 2 H Cl 6 HCl fast 3 a Write the overall balanced equation for the reaction described by this mechanism b What is the rate law of the overall reaction that is consistent with this mechanism c How would the rate of the reaction be affected if the initial concentration of Cl2 is doubled d How would the rate of the reaction be affected if the initial concentration of H2 is doubled nz What is the molecularity of step 2 Example Reaction Mechanism Assume the following proposed reaction mechanism is correct 37 7 0 P F Cl2 a 2 Cl slow 1 H2 Cl 6 HCl H fast 2 H Cl 6 HCl fast 3 Write the overall balanced equation for the reaction described by this mechanism H2 C12 6 2HCl What is the rate law of the overall reaction that is consistent with this In echanism 7 Rate kClZ How would the rate of the reaction be affected if the initial concentration of Cl2 is doubled Rate will double How would the rate of the reaction be affected if the initial concentration of H2 is doubled Rate will be unaffected What is the molecularity of step 2 B im ole cular
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