Chemistry 1B, Fall 016 Topic 3 Chemistry 1B Fall 016 Topic 3 [more] Chemical Kinetics goals for topic 3 inetics and mechanism of chemical reaction energy profile and reaction coordinate activation energy and temperature dependence of rate constant catalysis 1 have already covered worsheet 10 sections I-IV chemical inetics elementary reactions reaction mechanisms reaction energy profile and the reaction coordinate diagram 3 4 this video and : worsheet 10 sections I-VI and VII (6-10 goals for video 13 elementary reactions inetics and the mechanism of a chemical reaction energy profile and reaction coordinate 5 6 1
Chemistry 1B, Fall 016 Topic 3 inetics and mechanism of reaction elementary reactions NO (g + CO(g NO(g + CO (g at T< 500K if the reaction was a collision between a NO molecule and a CO molecule one might expect for the differential rate law: dno [ ] NO [ ][ CO] but the observed rate of reaction is: dno [ ] NO [ ] You might as? dno [ the observed inetics ] NO [ ] NO (g + CO(g NO(g + CO (g at T< 500K is not consistent with this stoichiometric equation in representing the actual molecular collisions involved in the reaction o?? but first let s loo at reactions where the stoichiometric equation DOES represent the actual collision process. these are called ELEMENTARY REACTIONS 7 8 inetics and mechanism of reaction mechanism of the reaction NO (g + CO(g ô NO(g + CO (g overall reaction (T<500K bac to the question: so what s up DOC?? NO (g + CO(g NO(g + CO (g at T< 500K answer: the above (stoichiometric equation NO colliding with CO does NOT represent the molecular mechanism (molecular steps of the reaction in actuality: the reaction taes place in two steps NO (g + NO (g NO(g + NO 3 (g NO 3 (g + CO(g CO (g + NO (g NO (g + CO(g NO(g + CO (g net reaction overall reaction 9 NO (g + NO (g NO(g + NO 3 (g NO 3 (g + CO(g CO (g + NO (g The two steps are the actual molecular processes by which the reaction occurs and is the MECHANISM the reaction They represent ELEMENTARY reactions The combination of elementary reactions must sum to the overall stoichiometry Species [e.g. NO 3 (g] appearing in the steps of the mechanism, but not the overall reaction are REACTION INTERMEDIATES 10 how to get rate law from mechanism of the reaction NO (g + NO (g NO(g + NO 3 (g T<500K NO 3 (g + CO(g CO (g + NO (g rates for individual steps come directly from the stoichiometry of the ELEMENTARY reactions how to combine rates of elementary reactions to get overall rate equation LOTS OF TRICKS (but we don t have time for many our ONE example will be reaction followed by rapid reaction (lie above reaction 11 how to get rate law from mechanism of the reaction NO (g + CO(g NO(g + CO (g T<500K NO (g NO(g + NO 3 (g NO 3 (g + CO(g CO (g + NO (g since these are ELEMENTARY reactions: dno [ ] dno [ 3] [ NO ] dco [ ] [ NO3] [ CO] since >> as soon as reaction produces NO 3 the reaction occurs the total rate is the just the rate of the step total rate= [NO ] ( nd order in NO, 0 th order in CO from slide #5: the observed rate of reaction is: d [ N O ] [ N O ] 1
Chemistry 1B, Fall 016 Topic 3 factoids about rate laws and mechanisms the actual process of determining mechanism of a reaction in general the rate law cannot be written in terms of the stoichiometric coefficients of the overall reaction the rate law for an elementary reaction IS determined by the molecularity (stoichiometry of the individual step the overall rate law is obtained from combining rate laws of elementary reactions an overall reaction where the measured rate law and the stoichiometric coefficients match MAY or MAY NOT be an elementary reaction 13 does mechanism fit measured rate law? yes no may be the correct mechanism other mechanisms may give same overall rate; use additional chemical experiments to verify try again Ascertain stoichiometry and measure rate of reaction by initial rates or integrated rate expression Use chemical smarts to propose ( invent steps of mechanism Write rate laws for elementary reactions and combine rate laws to get differential rate expressions for overall reaction 14 DNA synthesizer inetics the reaction coordinate and course of a chemical reaction and your thought Chem 1B inetics was complicated!! elementary reaction BrNO NO +Br molecularity? bimolecular E Æ E arev E reac 006 by National Academy of Sciences on = 1. 10 7 M 1 s 1, off = 0.06 s 1, 1 = 1.5 10 7 M 1 s 1, 1 = 50 s 1, = 50 s 1, = 3 s 1, 3 = 9.5 10 5 M 1 s 1, 3 = 100 s 1, 4 = 150 s 1, 4 = 40 s 1, 5 = 100 s 1, 5 = 9.5 10 5 M 1 s 1, 6 = 4 s 1, 6 = 4 s 1, 7 = 60 s 1, 7 = 1.45 10 4 M 1 s 1. Pourmand N et al. PNAS 006;103:6466-6470 15 energy vs reaction coordinate (progress of reaction reactants products transition state (maxima of E vs reaction coordinate [N---Br bonds breaing, Br---Br bond forming] activation energy (E a =E transition state E reactants reverse activation energy (E a rev = E transition state E products E reaction ( H= (E products E reactants 16 multi-step reaction NO (g + CO(g ô NO(g + CO (g at T< 500K NO (g + NO (g NO(g + NO 3 (g NO 3 (g + CO(g CO (g + NO (g ( stable Reaction Intermediate is at a relative minimum along curve http://chemwii.ucdavis.edu/@api/dei/files/916/=intermediate.bmp 17 18 3
Chemistry 1B, Fall 016 Topic 3 our last video!! onto more inetics!!! 19 0 Zumdahl figure 15.8 NO (g + CO(g ô NO(g + CO (g elementary reactions NO (g + NO (g NO 3 (g + NO (g an elementary reaction represents the molecules colliding in an actual step of the overall reaction step1 step the rate law for an elementary reaction is determined by the stoichiometry of the elementary reaction (the order of each reactant is its stoichiometric coefficient NO 3 (g + CO(g CO (g + NO (g the overall order of an elementary reaction is its molecularity 1 the sum of the individual elementary reactions must yield the equation for the overall reaction elementary reactions and molecularity elementary or not??? C N ( particle nuclear decay 14 1 14 6 7 elementary reaction 14 d 6C 14 1 6C first-order molecularity unimolecular if A + B C d[ A] A B IS an elementary reaction then [ ][ ] termolecular TRUE O ( g NO( g O ( g NO ( g 3 d O3 ( g O3NO second-order Cl( g Cl ( g Cl ( g 3 d Cl( g Cl Cl 3 third-order bimolecular termolecular improbable 3 if the measured rate law for A + B C da [ ] A [ ][ B] third order then A + B ö C MAY IS or MAY NOT be an elementary reaction a multi-step mechanism for reaction may have a set of elementary reactions that coincidentally leads to the third-order rate law is FALSE TRUE 4 4
Chemistry 1B, Fall 016 Topic 3 collision theory and effect of temperature on rate constant Temperature dependence of rate constant Catalysis zpe E RT a collision frequency (collisions per sec probability that a collision will have energy > E a (will get over activation barrier at transition state and become products orientation factor will colliding molecules be in the right orientation to react? 5 6 Arrhenius equation temperature dependence of rate constant (HW 10 #69 Z15.73 zpe RT T T1 Ae Ae RT RT1 Ae RT pre exponential factor (slight dependence on T will be ignored Svante August Arrhenius (19 February 1859 October 197 was a Swedish scientist, originally a physicist, but often referred to as a chemist, and one of the founders of the science of physical chemistry. E a 1 1 a RT E T e R T T1 e RT1 T e 1 e T T1 1 1 R T T1 E 1 1 a ln T ln T 1 R T1 T eqn 15.11 7 now T1 and E a get T or measure T1 and T get E a 8 Example (by request A first-order reaction with activation energy E a =50 J mol -1 has a rate constant of 300 =3.0 10-1 sec -1 at 300K. What is the rate constant at 310K? What is the ratio of 310 / 300? 9 30 5
Chemistry 1B, Fall 016 Topic 3 adage raise 10K ô twice as catalysis catalyst? substance that increases the rate of a chemical reaction without being consumed itself E a º 50 J mol -1 at T=98 K catalysis changes the reaction pathway catalyzed pathway has lower E a but same E=E products -E reactants 31 3 Catalysis and activation energy general types of catalysis enzymatic catalysis e.g. carbonic anhydrase catalyzes caranh CO + H O HCO 3 + H + removing CO formed in cells during metabolism surface (heterogeneous catalysis e.g. platinum and rhodium metallic particles in catalytic converter converts CO, NO, N O and NO to CO and N Pt ( metal NO (g + CO (g Rh ( metal CO (g + N (g homogeneous catalysis (ozone depletion e.g. O(g + O 3 (g O (g E a =17.1 J mol -1 uncatalyzed http://www.columbia.edu/cu/biology/courses/c005/purves6/figure06-14.jpg catalyzed by Cl from CCl F +h CClF + Cl Cl(g + O 3 (g ClO(g + O (g =.1 J mol -1 catalyzed O(g + ClO (g Cl(g + O (g =0.4 J mol -1 catalyzed O(g + O 3 (g O (g 33 34 O(g + O 3 (g O (g OH one last example of mechanism: + H 3 CI OH + I vs OH +( CI ( COH + I H H O(g + O 3(g O (g E a=17.1 J mol -1 uncatalyzed OH + H C I H C OH + I ClO is reaction intermediate H reaction rate= SN [OH - ] [ CI] second-order H bimolecular SN ClO + O +O Cl(g + O 3 (g ClO(g + O (g =.1 J mol-1 catalyzed O(g + ClO (g Cl(g + O (g =0.4 J mol-1 catalyzed O(g + O 3(g O (g C I I + C + + OH C OH SN 1 http://www.entchemistry.com/lins/kinetics/pediagrams.htm 35 reaction rate= SN1 [C 4 H 9 I] no [OH ] [OH ] 0 followed by first-order 36 6
Chemistry 1B, Fall 016 Topic 3 chemical inetics (chapter 15 inetic measurements: how will a reaction proceed NC - H 3 C-I Catalysis NC- + I - Kinetics: How a reaction proceeds, and the molecular steps involved in a reaction [the mechanism of a reaction]. http://www.bluffton.edu/~bergerd/classes/cem1/sn-e/sn_alternate.html 37 38 goals for lecture 3 inetics and mechanism of chemical reaction energy profile and reaction coordinate activation energy and temperature dependence of rate constant catalysis Finis!!! BUT one MORE THING 39 40 p the orientation factor ( ONBr ô NO +Br two-step, then, first-order with reaction intermediate HW#10 probs 67 and 68 what relative orientation of two ONBr molecules will lead to a reaction? (Br------Br bond formation yes yes no http://guweb.gonzaga.edu/faculty/cron/chem40/images/reaction_coordinate_sn1.gif the more particular the reaction is about alignment of colliding molecule the smaller the p factor 41 4 7
Chemistry 1B, Fall 016 Topic 3 Example (by request example continued A first-order reaction with activation energy E a =50 J mol -1 has a rate constant of 300 =3.0 ä 10-1 sec -1 at 300K. What is the rate constant at 310K? What is the ratio of 310 / 300? ln T ln A RT 1. ln ln A R (310 K. 310 ln 300 ln A R (300 K (same A and E a 310 1 1 ln ( E a ( 300 R 310K 300K 3 1 5010 Jmol 1 1 1 1 8.3145 ( 310 300 Jmol K K K ln ( 0.6466 310 300 310 300 e (3.0 10 sec 1.91 0.574sec 0.6466 1 1 1 ln ( 0.6466 1.91 310 310 300 300 note unitless subtract: 1-310 1 1 ln ( E a ( 300 R 310K 300K 3 1 5010 Jmol 1 1 1 1 8.3145 Jmol K ( 310K 300K note unitless 43 Old adage: raise T by 10º (300 310 double reaction rate (E a 50 J/mol 44 8