Lecture9_S08

Lecture9_S08 - Lecture 9 How do reactions occur? Directly?...

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How do reactions occur? Directly? A + B C One bimolecular elementary step Molecularity = # of reactants in the reaction (uni-, bi-, ter-) Single molecular event leading to reaction Or does the reaction happen indirectly? A I I + B C Step 1: intermediate is formed Step 2: intermediate is consumed Sum: A + I + B I + C Net Chemical equation A + B C So, Intermediate does not appear in net chemical equation Lecture 9 equation: Rule #1: the rate law usually describes the slowest elementary step in a multistep reaction mechanism For the proposed mechanism: Step 1: NO 2 + NO 2 k 1 Subscript Label meaning for step 1 NO 3 + NO slow Step 1: NO 3 + CO k 2 NO 2 + CO 2 fast NO 2 + NO 2 + NO 3 + CO NO 3 + NO 2 + NO + CO 2 Intermediate (formed and consumed) Not an intermediate it is a reactant Net Chemical equation: NO 2 + CO NO + CO 2 Since elementary step 1 is slow, it is rate-determining: Rate = k 1 [NO 2 ][NO 2 ] = k 1 [NO 2 ] 2 (Second order) Review: Half life = 1/(k 1 [NO 2 ] o )
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Rule #2: rate law is written in terms of reactants if an intermediate appears in the rate law expression
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Lecture9_S08 - Lecture 9 How do reactions occur? Directly?...

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