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U-M CHEM 451 - Kinetics
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CHEM 451 1st Edition Lecture 5Outline of Last Lecture I. Conformational Degrees of FreedomII. Ligand-induced modulation; folding free energy landscapea. Macroscopic, Mesoscopic, and Microscopic statesIII. ATCase as an example of dynamic allosteric controlIV. Dynamic long-range interactionsV. Introduction to Chemical KineticsOutline of Current Lecture VI. Rate constanta. StoichiometryVII. Rate Lawsa. Half LifeVIII. Elementary Reaction StepsCurrent LectureRate constant: describes the reaction rate regardless of the time (universally applicable)- Measure using flow/stop flow method - We measure instantaneous rate – slope of tangential of concentration vs. time.o How much reactant has disappeared over a certain interval of time? o Instantaneous rate changes over time- Describes the time evolution independent of the time or path of reactionStoichiometry: 2A C-− 12d[A]dt=d[C]dt- Rate of disappearance of reagent is proportional to the rate of formation.- We can always measure proportions that relate to the compound of interest.These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.Rate Laws: Second order rate law approaches time axis more slowly than first order; characteristically different than first order.Half-Life:- For a zero order reaction A products, rate = k: t½ = [Ao] / 2k- For a first order reaction A products, rate = k[A]: t½ = 0.693 / k- For a second order reaction 2A products or A + B products (when [A] = [B]), rate = k[A]2: t½ = 1 / k [Ao]Consider A + B  C- 2nd order: −d[A]dt=k [ A][B]- Choose [A]0 = [B]0−d[A]dt=k [ A]2- OR Choose [A]0<<< [B]0o [B] ≈ CONSTANT!o−d[A]dt=k'[A]; k'=k [B ]0o This is known as pseudo-first orderFor elementary reaction steps the rate law can be directly inferred.- Unimolecular elementary step: A  producto “A” has enough vibrational energy to fall aparto Rate = −d[A]dt=k [ A]o- Bimolecular elemtary step: A + B  productso 2 molecules bump together. A higher concentration of BOTH will increase the likelihood of “bumping” togethero Rate = −d[A]dt=k [ A][B]- Elementary steps are PART of an overall reaction.- Two related but distinct concepts:o Order of rate law: sum of exponentials in terms of reactant concentrationso Molecularity: number of particles coming together to react in an elementary step- In biochemistry, we can choose whether an elementary step is unimolecular or bimolecularo Example: 3 reactant moleculeso It is very unlikely to have one step only.o The probability that 3 molecules collide at the same time is low.- Reverse steps: BOTH are independent elementary steps!Example: 2NO(g) + O2(g)  2NO2(g)1. 2NO(g)  N2O2(g)d[N2O2]dt=k1[NO]22. N2O2(g)  2NO(g)−d[N2O2]dt=k−1[NO]23. N2O2(g) + O2 (g)  2NO(g)−d[N2O2]dt=k2[N2O2] [O2]−d[N2O2]dt=12d[NO2]dtd[NO2]dt=2 k2[N2O2] [O2]Problem: We cannot control the concentration of the


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U-M CHEM 451 - Kinetics

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