on-line kinetics 3!!! Chemistry 1B Fall 2013 1
on-line kinetics 3!!! Chemistry 1B Fall 2013 Mechanism of a chemical reaction Elementary reactions Activation energy and reaction coordinate diagram 2
Chemistry 1B Fall 2013 Lecture 23 [more] Chemical Kinetics 3
kinetics and mechanism of reaction NO 2 (g) + CO(g) NO(g) + CO 2 (g) at T< 500K if the reaction was a collision between a NO 2 molecule and a CO molecule one might expect for the differential rate law: d[ NO ] dt 2 k[ NO ][ CO] but the observed rate of reaction is: d[ NO ] dt 2 2 k[ NO ] 2 2 You might ask? 4
goals for lecture 23 kinetics and mechanism of chemical reaction energy profile and reaction coordinate activation energy and temperature dependence of rate constant catalysis 5
kinetics and mechanism of reaction question: what s up DOC?? NO 2 (g) + CO(g) NO(g) + CO 2 (g) at T< 500K answer: the above (stoichiometric) equation NO 2 colliding with CO does NOT represent the molecular mechanism (molecular steps of the reaction) actually: the reaction takes place in two steps NO 2 (g) + NO 2 (g) NO(g) + NO 3 (g) NO 3 (g) + CO(g) CO 2 (g) + NO 2 (g) slow fast net reaction NO 2 (g) + CO(g) NO(g) + CO 2 (g) overall reaction 6
mechanism of the reaction NO 2 (g) + CO(g) NO(g) + CO 2 (g) overall reaction (T<500K) k slow NO 2 (g) + NO 2 (g) NO(g) + NO 3 (g) k fast NO 3 (g) + CO(g) CO 2 (g) + NO 2 (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 7
how to get rate law from mechanism of the reaction T<500K k slow NO 2 (g) + NO 2 (g) NO(g) + NO 3 (g) slow k fast NO 3 (g) + CO(g) CO 2 (g) + NO 2 (g) fast rates for individual steps come directly from the stoichiometry of the ELEMENTARY reactions (HW9 #71) 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 slow reaction followed by rapid reaction (like above reaction) 8
how to get rate law from mechanism of the reaction NO 2 (g) + CO(g) NO(g) + CO 2 (g) T<500K k slow 2 NO 2 (g) NO(g) + NO 3 (g) slow k fast NO 3 (g) + CO(g) CO 2 (g) + NO 2 (g) fast since these are ELEMENTARY reactions: d[ NO2 ] 2 kslow[ NO2 ] dt d[ CO] k fast[ NO 3][ CO ] dt d[ NO 2 dt 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 2 ] 2 (2 nd order in NO 2, 0 th order in CO) 3 ] d[ NO ] dt 2 from slide #4: the observed rate of reaction is: 2 k[ NO ] 2 9
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 10
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 11
DNA synthesizer kinetics and your thought Chem 1B kinetics was complicated!! 2006 by National Academy of Sciences k on = 1.2 10 7 M 1 s 1, k off = 0.06 s 1, k 1 = 1.25 10 7 M 1 s 1, k 1 = 250 s 1, k 2 = 50 s 1, k 2 = 3 s 1, k 3 = 9.5 10 5 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 = 9.5 10 5 M 1 s 1, k 6 = 4 s 1, k 6 = 4 s 1, k 7 = 60 s 1, k 7 = 1.45 10 4 M 1 s 1.) Pourmand N et al. PNAS 2006;103:6466-6470 12
the reaction coordinate and course of a chemical reaction elementary reaction 2BrNO 2NO +Br 2 molecularity? bimolecular E E a rev ΔE reac energy vs reaction coordinate (progress of reaction reactants products) transition state (maxima of E vs reaction coordinate) [N---Br bonds breaking, Br---Br bond forming] activation energy (E a =E transition state E reactants ) reverse activation energy (E a rev = E transition state E products ) ΔE reaction (ΔH)= (E products E reactants ) 13
multi-step reaction ( stable ) Reaction Intermediate is at a relative minimum along curve http://chemwiki.ucdavis.edu/@api/deki/files/916/=intermediate.bmp 14
NO 2 (g) + CO(g) NO(g) + CO 2 (g) at T< 500K NO 2 (g) + NO 2 (g) NO(g) + NO 3 (g) NO 3 (g) + CO(g) CO 2 (g) + NO 2 (g) slow fast 15
END OF KINETICS 4 AND onto more kinetics!!! 16
on-line kinetics 4!!! Chemistry 1B Fall 2013 17
collision theory and effect of temperature on rate constant k zp e Ea RT 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? 18
Arrhenius equation k zp e Ea RT k A e Ea RT pre exponential factor 19
temperature dependence of rate constant (HW 9 #73 Z15.73) k k k k T 2 T 1 A e A e Ea Ea RT RT E RT E 1 1 T e R T T T a a 2 2 2 1 1 e E a RT 1 2 1 e k T k e T 2 1 Ea 1 1 R T T 2 1 Ea 1 1 ln kt ln k 2 T1 R T T 1 2 eqn 15.11 know k T1 and E a get k T2 or measure k T1 and k T2 get E a 20
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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 10-1 sec -1 at 300K. What is the rate constant at 310K? What is the ratio of k 310 /k 300? 22
adage raise 10K twice as fast E a 50 kj mol -1 at T=298 K 23
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 24
general types of catalysis enzymatic catalysis e.g. carbonic anhydrase catalyzes CO 2 + H 2 O caranh HCO 3 + H + removing CO 2 formed in cells during metabolism surface (heterogeneous) catalysis e.g. platinum and rhodium metallic particles in catalytic converter converts CO, NO, N 2 O and NO 2 to CO 2 and N 2 Pt ( metal) 2NO (g) + 2CO (g) Rh ( metal) 2CO 2 (g) + N 2 (g) homogeneous catalysis (ozone depletion) e.g. O(g) + O 3 (g) 2O 2 (g) E a =17.1 kj mol -1 uncatalyzed catalyzed by Cl from CCl 2 F 2 +h CClF 2 + Cl Cl(g) + O 3 (g) ClO(g) + O 2 (g) Ea =2.1 kj mol-1 catalyzed O(g) + ClO (g) Cl(g) + O 2 (g) Ea =0.4 kj mol-1 catalyzed O(g) + O 3 (g) 2O 2 (g) 25
O(g) + O 3 (g) 2O 2 (g) O(g) + O 3 (g) 2O 2 (g) E a =17.1 kj mol -1 uncatalyzed ClO is reaction intermediate ClO + O +O 2 Cl(g) + O 3 (g) ClO(g) + O 2 (g) Ea =2.1 kj mol-1 catalyzed O(g) + ClO (g) Cl(g) + O 2 (g) Ea =0.4 kj mol-1 catalyzed O(g) + O 3 (g) 2O 2 (g) http://www.kentchemistry.com/links/kinetics/pediagrams.htm 26
one last example of mechanism H H I OH + H C H C OH + I H H reaction rate= [OH ] [CH 3 CI] second-order possibly bimolecular CH 3 CH 3 OH + CH 3 C I CH 3 C OH + I CH 3 CH 3 reaction rate= [C 4 H 9 I] first-order not bimolecular no [OH ], [OH ] 0 27
one last example of mechanism H H OH + H C I H C OH + I H H SN 2 reaction rate= [OH ] [CH 3 CI] second-order bimolecular CH 3 CH 3 CH 3 CH 3 C I slow I + CH 3 C + + OH fast CH 3 C OH CH 3 CH 3 CH 3 SN 1 reaction rate= [C 4 H 9 I] no [OH ] first-order 28
chemical kinetics (chapter 15) N C - H 3 C-I N C-CH 3 + I - Kinetics: How fast a reaction proceeds, and the molecular steps involved in a reaction [the mechanism of a reaction]. http://www.bluffton.edu/~bergerd/classes/cem221/sn-e/sn2_alternate.html 29
kinetic measurements: how fast will a reaction proceed Catalysis 30
goals for lecture 23 kinetics and mechanism of chemical reaction energy profile and reaction coordinate activation energy and temperature dependence of rate constant catalysis 31
(last) intense week!! make mine a triple!!! 32
END OF KINETICS 4 AND Have a nice Thanksgiving Holiday!! after break onto intermolecular forces!!! 33
on to intermolecular forces!!! last topic 34
Zumdahl figure 15.8 NO 2 (g) + CO(g) NO(g) + CO 2 (g) NO 2 (g) + NO 2 (g) NO 3 (g) + NO (g) slow step1 step2 NO 3 (g) + CO(g) CO 2 (g) + NO 2 (g) fast 35
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 36
elementary reactions and molecularity (HW9 prob 71; Z#15.55) elementary reaction molecularity k1 C N particle 14 14 6 7 d C 14 6 14 k1 6 dt C ( ) nuclear decay first-order unimolecular O g NO g O g NO g k2 ( ) ( ) ( ) ( ) 3 2 2 d O ( g) 3 dt k O NO 2 3 second-order bimolecular k3 2 Cl ( g) Cl ( g) 2 Cl ( g) d Cl 2 dt ( g) 2 2 2 k Cl Cl 3 2 third-order termolecular improbable 37
elementary or not??? if A + 2B C da [ ] dt k A B IS an elementary reaction then 2 [ ][ ] trimolecular TRUE if the measured rate law for A + 2B C da [ ] 2 k[ A][ B] third order dt then is A + 2B C IS an elementary reaction A + 2B C MAY or MAY NOT be FALSE TRUE a multi-step mechanism for reaction may have a set of elementary reactions that coincidentally leads to the third-order rate law 38
p the orientation factor (2 ONBr 2NO +Br 2 ) 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 39
two-step, slow then fast, first-order with reaction intermidiate HW#7 probs 67 and 68 http://guweb2.gonzaga.edu/faculty/cronk/chem240/images/reaction_coordinate_sn1.gif 40
effect of catalyst on E a http://guweb2.gonzaga.edu/faculty/cronk/chem240/images/reaction_coordinate_catalyst_lg.gif 41
CO + NO 2 CO 2 + NO (T > 700 K) CO + NO 2 E CO 2 + NO http://www.kentchemistry.com/links/kinetics/pediagrams.htm 42
Catalysis and activation energy http://www.columbia.edu/cu/biology/courses/c2005/purves6/figure06-14.jpg 43
Fall 2013: Chemistry 1M Lab #12 S O 2-2 I - I 2 SO 2-2 8 2 4 44