Assignment5Solutions - Chemistry 350 Assignment 5:...

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Chemistry 350 Assignment 5: Solutions December 5 th 2005 Question 1 Q19.2 For a reaction mechanism to be considered correct, what property must it demonstrate? In order to be valid, a proposed reaction mechanism must agree with the experimentally determined rate law. Q19.10 What is the difference between a homogeneous and a heterogeneous catalyst? For a homogeneous catalysis both the catalyst and the substrate exist in the same phase. Heterogeneous catalysis occurs when the catalyst and the substrate molecules exist in different phases. Q19.15 What depopulation pathways occur from the first excited singlet state? For the first excited triplet state? There are three main pathways for decay of the first excited singlet state: emitting a photon or fluorescence, non-radiative decay or internal conversion, and intersystem crossing to the triplet state. The first excited triplet state decays by emitting a photon or phosphorescence, and non-radiative decay or internal conversion. Question 2 P19.1 A proposed mechanism for the formation of N 2 O 5 from NO 2 and O 3 is 1 2 2 3 3 2 32 25 NO O NO O NO NO M N O M k k ! "" #! ! ! ""
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Determine the rate law expression for the production of N 2 O 5 given this mechanism. $ % $ %$ % $% $ %$ % $ %$ 25 2 2 3 3 2 2 3 2 2 NO NO NO NO NO O NO NO d k dt d kk dt & &’ % 3 Applying the steady state approximation to the intermediate NO 3 and substituting back into the differential rate expression for N 2 O 5 yields: $ % $ %$ % $ %$ $ %$ % $ %$ % % $ %$ % $ %$ % 3 1 2 3 2 2 3 2 2 3 1 2 3 1 33 2 2 2 3 1 2 2 3 2 1 2 3 NO 0 NO O NO NO NO NO NO O NO O NO NO NO O NO O d dt k k d k dt k k k k & & & & & () & *+ ,- & The mechanism predicts that the reaction is first order in NO 2 and O 3 , second order overall. P19.4 The hydrogen-bromine reaction corresponds to the production of HBr from H 2 and Br 2 as follows: . This reaction is famous for its complex rate law, determined by Bodenstein and Lind in 1906: 22 H Br 2HB ! ! r $ % $ %$ % 12 2 HBr H Br HBr 1 Br dk m dt & ! where k and m are constants. It took 13 years for the correct mechanism of this reaction to be proposed, and this feat was accomplished simultaneously by Christiansen, Herzfeld, and Polyani. The mechanism is as follows:
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1 1 2 3 4 2 2 2 2 Br 2Br Br H HBr H H Br HBr Br HBr H H Br k k k k k ""# .""" / /! "" #! / "" / ! / "" / Construct the rate law expression for the hydrogen-bromine reaction by performing the following steps: a) Write down the differential rate expression for [HBr]. b) Write down the differential rate expressions for [Br] and [H]. c) Because Br and H are reaction intermediates, apply the steady-state approximation to the result of part (b). d) Add the two equations from part (c) to determine [Br] in terms of [Br 2 ]. e) Substitute the expression for [Br] back into the equation for [H] derived in part (c) and solve for [H]. f) Substitute the expressions for [Br] and [H] determined in part (e) into the differential rate expression for [HBr] to derive the rate law expression for the reaction.
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This note was uploaded on 04/11/2010 for the course CHEM 1101 taught by Professor Leroy during the Spring '10 term at University of Toronto- Toronto.

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Assignment5Solutions - Chemistry 350 Assignment 5:...

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