Reaction Mechanisms and Rate Slides and Notes - Reaction Mechanisms and Rate In many mechanisms one step(called the rate-determining step is

Reaction Mechanisms and Rate Slides and Notes - Reaction...

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Reaction Mechanisms and Rate In many mechanisms, one step (called the rate-determining step ) is significantly slower than all others. Overall reaction can only proceed as fast as its slowest step (analogy to automobile traffic flow). Consider a mechanism with first step as rate-determining step. Example: Rate law found experimentally to be rate = k obs [NO 2 ][F 2 ] Possible mechanism: NO 2 + F 2 k 1 NO 2 F + F (slow) NO 2 + F k 2 NO 2 F (fast) Overall rate determined by first step 2 NO 2 + F 2 2 NO 2 F
Reaction Mechanisms and Rate Mechanisms in which rate-determining step is after one or more fast steps often revealed by reaction order more than 2, non-integral order, or inverse concentration dependence of a species. Example: Rate law found experimentally to be rate = k obs [NO] 2 [O 2 ] Alternative mechanism: NO + NO k –1 k 1 N 2 O 2 (fast equilibrium) N 2 O 2 + O 2 k 2 2 NO 2 (slow) Rate law consistent with single termolecular step, but these are rare. 2 NO + O 2 2 NO 2
Reaction Mechanisms and Rate ] 2 2 ] .
Problem Consider the following reaction: The following three-step mechanism is proposed: What rate law is predicted by this mechanism? NH3+OClk–1k1NH2Cl +OH(fast equilibrium)NH2Cl +NHk2N2H5++Cl(slow)N2H5++OHk3N2H4+H2O (fast)2 NH3(aq)+OCl(aq)N2H4(aq)+H2O(l)+Cl(aq) 3
Reaction Mechanisms and Rate An inverse dependence of overall rate on a concentration is often a clue of a rapid equilibrium before rate-determining step. Similarly for fractional orders of reaction. Example: Initial rate of this reaction (before HBr buildup) is rate = k obs [H 2 ][Br 2 ] 1/2

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