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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 =kobs[NO2][F2]Possible mechanism: NO2+F2k1→NO2F +F (slow)NO2+F k2→NO2F (fast)Overall rate determined by first step 2 NO2+F2→2 NO2F
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 =kobs[NO]2[O2]Alternative mechanism: NO +NO k–1k1N2O2(fast equilibrium)N2O2+O2k2→2 NO2(slow)Rate law consistent with single termolecular step, but these are rare. 2 NO+O2→2 NO2
Reaction Mechanisms and Rate ]22].
Problem Consider the following reaction: The following three-step mechanism is proposed: What rate law is predicted by this mechanism? NH3+OCl–k–1k1NH2Cl +OH–(fast equilibrium)NH2Cl +NHk2→N2H5++Cl–(slow)N2H5++OH–k3→N2H4+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 =kobs[H2][Br2]1/2