Chemistry 1B, Fall 2012 Lecture 23

Transcription

1 Chemistry 1B Fall 01 [more] Chemical Kinetics 1 kinetics and mechanism of reaction NO (g) + CO(g) ô 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 [ ] kno [ ][ CO] but the observed rate of reaction is: dno [ ] kno [ ] You might ask? 1

2 goals for lecture 3 kinetics and mechanism of chemical reaction energy profile and reaction coordinate activation energy and temperature dependence of rate constant catalysis 3 kinetics and mechanism of reaction question: what s up DOC?? NO (g) + CO(g) ô NO(g) + CO (g) at T< 500K answer: the above (stoichiometric) equation does NOT represent the molecular mechanism (molecular steps of the reaction) actually: the reaction takes place in two steps NO (g) + NO (g) ô NO(g) + NO 3 (g) NO 3 (g) + CO(g) ô CO (g) + NO (g) NO (g) + CO(g) ô NO(g) + CO (g) slow fast overall reaction 4

3 mechanism of the reaction NO (g) + CO(g) ô NO(g) + CO (g) overall reaction (T<500K) k slow NO (g) + NO (g) ô NO(g) + NO 3 (g) k fast NO 3 (g) + CO(g) ô CO (g) + NO (g) slow fast 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 5 how to get rate law from mechanism of the reaction T<500K k slow NO (g) + NO (g) ô NO(g) + NO 3 (g) slow k fast NO 3 (g) + CO(g) ô CO (g) + NO (g) fast rates for individual steps come directly from the stoichiometry of the ELEMENTARY reactions (HW#7 66) how to combine rates of elementary reactions to ge overall rate equation LOTS OF TRICKS (but we don t have time for many) our ONE example will be slow reaction followed by rapid reaction (like above reaction) 6 3

4 how to get rate law from mechanism of the reaction T<500K k slow NO (g) ô NO(g) + NO 3 (g) slow NO (g) + CO(g) ô NO(g) + CO (g) k fast NO 3 (g) + CO(g) ô CO (g) + NO (g) fast since these are ELEMENTARY reactions: dno [ ] dno [ 3] kslow[ NO ] dco [ ] k fast[ NO 3][ CO ] since k fast >> k slow as soon as slow reaction produces NO 3 the fast reaction occurs the total rate is the just the rate of the slow step total rate= k slow [NO ] ( nd order in NO, 0 th order in CO) from slide #: the observed rate of reaction is: d [ N O ] k [ N O ] 7 factoids about rate laws and mechanisms 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 8 4

5 the actual process of determining mechanism of a reaction does mechanism fit measured rate law? Ascertain stoichiometry and measure rate of reaction by initial rates or integrated rate expression yes no Use chemical smarts to propose ( invent ) steps of mechanism may be the correct mechanism other mechanisms may give same overall rate; use additional chemical experiments to verify try again Write rate laws for elementary reactions and combine rate laws to get differential rate expressions for overall reaction 9 DNA synthesizer kinetics and your thought Chem 1B kinetics was complicated!! 006 by National Academy of Sciences k on = M 1 s 1, k off = 0.06 s 1, k 1 = M 1 s 1, k 1 = 50 s 1, k = 50 s 1, k = 3 s 1, k 3 = M 1 s 1, k 3 = 100 s 1, k 4 = 150 s 1, k 4 = 40 s 1, k 5 = 100 s 1, k 5 = M 1 s 1, k 6 = 4 s 1, k 6 = 4 s 1, k 7 = 60 s 1, k 7 = M 1 s 1.) Pourmand N et al. PNAS 006;103:

6 the reaction coordinate and course of a chemical reaction elementary reaction BrNO ö NO +Br molecularity? bimolecular E Æ E arev ΔE reac energy vs reaction coordinate (progress of reaction reactants öproducts) transition state (maxima of E vs reaction coordinate) activation energy (E a =E transition state - E reactants ) reverse activation energy (E arev = E transition state - E products ) ΔE reaction (ΔH)= (E products - E reactants ) 11 multi-step reaction ( stable ) Reaction Intermediate is at a relative minimum along curve 1 6

7 NO (g) + CO(g) ô NO(g) + CO (g) at T< 500K NO (g) + NO (g) ô NO(g) + NO 3 (g) NO 3 (g) + CO(g) ô CO (g) + NO (g) slow fast 13 collision theory and effect of temperature on rate constant k zpe E RT a collision frequency 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? 14 7

8 Arrhenius equation k zpe k Ae E RT a a E RT pre exponential factor 15 temperature dependence of rate constant (HW 7-68 Z#15.67) k k k k k T T 1 1 Ae Ae 1 E E a a RT RT E a 1 1 a RT E T e R T T1 e Ea RT1 T e T k e T 1 Ea 1 1 R T T1 E 1 1 a ln kt ln k T 1 R T1 T eqn know k T1 and E a get k T or measure k T1 and k T get E a 16 8

9 17 Example (by request) A first-order reaction with activation energy E a =50 kj mol -1 has a rate constant of k 300 =3.0 ä 10-1 sec -1 at 300K. What is the rate constant at 310K? What is the ratio of k 310 /k 300? 18 9

10 adage raise 10K ô twice as fast E a º 50 kj mol -1 at T=98 K 19 catalysis catalyst? substance that increases the rate of a chemical reaction without being consumed itself catalysis changes the reaction pathway catalyzed pathway has lower E a but same E=E products -E reactants 0 10

11 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) ö O (g) E a =17.1 kj mol -1 uncatalyzed catalyzed by Clÿ from CCl F +hn öcclf + Clÿ Cl(g) + O 3 (g) ö ClO(g) + O (g) Ea =.1 kj mol-1 catalyzed O(g) + ClO (g) ö Cl(g) + O (g) Ea=0.4 kj mol-1 catalyzed O(g) + O 3 (g) ö O (g) 1 O(g) + O 3 (g) ö O (g) ClO is reaction intermediate ClO + O +O 11

12 one last example of mechanism H H OH - + H C I ô H C OH + I - H H reaction rate= [OH - ] [ CI] second-order possibly bimolecular OH - + C I ô C OH + I - reaction rate= [C 4 H 9 I] first-order not bimolecular no [OH - ], [OH - ] 0 3 one last example of mechanism H H OH - + H C I ô H C OH + I - H H SN reaction rate= [OH - ] [ CI] second-order possibly bimolecular C slow I ô I - + C + + OH - fast ô C OH SN 1 reaction rate= [C 4 H 9 I] no [OH - ] first-order not bimolecular 4 1

13 chemical kinetics (chapter 15) N C - H 3 C-I N C- + I - Kinetics: How fast a reaction proceeds, and the molecular steps involved in a reaction [the mechanism of a reaction]. 5 kinetic measurements: how fast will a reaction proceed Catalysis 6 13

14 goals for lecture 3 kinetics and mechanism of chemical reaction energy profile and reaction coordinate activation energy and temperature dependence of rate constant catalysis 7 on to intermolecular forces!!! last topic 8 14

15 Zumdahl figure 15.8 NO (g) + CO(g) ô NO(g) + CO (g) NO (g) + NO (g) ô NO 3 (g) + NO (g) slow step1 step NO 3 (g) + CO(g) ô CO (g) + NO (g) fast 9 elementary reactions an elementary reaction represents the molecules colliding in an actual step of the overall reaction 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) the overall order of an elementary reaction is its molecularity the sum of the individual elementary reactions must given the equation for the overall reaction 30 15

16 elementary reactions and molecularity (HW7 prob 66; Z#15.51) C N ( particle) nuclear decay 14 k elementary reaction 14 d 6C 14 k 1 6C first-order molecularity unimolecular O ( g) NO( g) O ( g) NO ( g) k 3 d O3 ( g) k O NO 3 second-order bimolecular Cl( g) Cl ( g) Cl ( g) d Cl ( g) k3 k Cl Cl 3 third-order termolecular improbable 31 elementary or not??? if A + B ö C da [ ] IS an elementary reaction then ka [ ][ B] trimolecular TRUE if the measured rate law for A + B ö C da [ ] ka [ ][ B] third order then is A + B ö C IS MAY or MAY NOT be an elementary reaction FALSE TRUE a multi-step mechanism for reaction may have a set of elementary reactions that coincidentally leads to the third-order rate law 3 16

17 p the orientation factor ( ONBr ô NO +Br ) will this relative orientation of two ONBr molecules lead to a reaction? yes yes no the more particular the reaction is about alignment of colliding molecule the smaller the p factor 33 two-step, slow then fast, first-order with reaction intermidiate HW#7 probs 67 and

18 effect of catalyst on E a