ex2s10 - NAME: MONKEY CHEMISTRY 419, SPRING, 2010 (2103)...

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NAME: MON“KEY” CHEMISTRY 419, SPRING, 2010 (2103) Midterm Examination 2, April 22, 2010 Answer each question in the space provided; use back of page if extra space is needed. Answer questions so the grader can READILY understand your work; only work on the exam sheet will be considered. Write answers, where appropriate, with reasonable numbers of significant figures. You may use only the "Student Handbook," a calculator, and a straight edge . 1. (10 points) In the figure is shown the potential-energy surface for the reaction A BC C AB . Indicate the following things by putting points or lines with labels: (a) the transition state (or saddle point) (b) the entrance to the reactant channel for the reaction as written (c) the exit from the product channel for the reaction as written (d) a reactive event (e) an unreactive event. (Be sure to label any points or lines you may draw on the graph carefully.) The entrance and exit channels are determined by which diatomic molecule starts (and it will have the smallest separation); in this case R AB must be small. The transition state is the saddle point in the surface. A reactive event starts in the reactant channel and ends in the product channel. An unreactive event starts in the in reactant channel and ultimately moves back out through the reactant channel. DO NOT WRITE IN THIS SPACE # 1________/10 # 2________/15 # 3________/20 # 4________/15 # 5________/15 # 6________/10 # 7________/15 ============= TOTAL PTS /100
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Score for Page NAME: CHEM 419, Midterm Exam 2, Spring, 2010, page 2 2. (15 points) Lindemann developed a simple mechanism to explain certain unimolecular reactions. The mechanism involves the interaction of the reactant molecule (A) with a “mediator” (M) to produce an excited-state molecule (A * ), which ends up reacting to give product (P). The overall reaction is simple: P A Here is the mechanism: ] [ ] ][ [ ] ][ [ * 3 3 * * 2 2 * 1 1 * 2 1 A k v P A M A k v M A M A M A k v M A M A k k k    (a) To the right of each step (as indicated), write an equation for the velocity of the step in terms of the concentrations and rate constant. (b) Assume that the excited-state molecule is a reactive intermediate. Give an expression for the steady-state concentration of the reactive intermediate in terms of other concentrations and rate constants. 0 ] [ ] ][ [ ] ][ [ ] [ 0 ] [ * 3 * 2 1 3 2 1 * * A k M A k M A k v v v dt A d dt A d Solving this equation gives the steady-state concentration: ] [ ] ][ [ ] [ 2 3 1 * M k k M A k A (c) Give the rate law for the Lindemann mechanism as a function of concentrations and rate constants. The rate of reaction is given by the rate of creation of product, which happens only in step 3.
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ex2s10 - NAME: MONKEY CHEMISTRY 419, SPRING, 2010 (2103)...

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