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Reaction Mechanisms and RateIn many mechanisms, one step (called therate-determining step) issignificantly slower than all others.Overall reaction can only proceedas 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 berate=kobs[NO2][F2]Possible mechanism:NO2+F2k1→NO2F+F (slow)NO2+Fk2→NO2F (fast)Overall ratedetermined byfirst step2 NO2+F2→2 NO2F
Reaction Mechanisms and RateMechanisms in which rate-determining step is after one or more faststeps often revealed by reaction order more than 2, non-integral order,or inverse concentration dependence of a species.Example:Rate law found experimentally to berate=kobs[NO]2[O2]Alternative mechanism:NO+NOk–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 RateSince second step determines overall rate, we can writeHowever, concentration of N2O2intermediate cannot be controlled.since second step is slow, we can assume elementary reactionsbeforerate-determining step are in equilibrium.Hence, for 1st step,rate=k2[N2O2][O2][N2O2][NO]2=k1k–1=K1OR[N2O2]=K1[NO]2Substiuting, we have rate=k2K1[NO]2[O2]This is consistent with observed reaction order, withkobs=k2K1.
ProblemConsider 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+NH3k2→N2H5++Cl–(slow)N2H5++OH–k3→N2H4+H2O (fast)2 NH3(aq)+OCl–(aq)→N2H4(aq)+H2O(l)+Cl–(aq)
Reaction Mechanisms and RateAn inverse dependence of overall rate on a concentration is often aclue of a rapid equilibrium before rate-determining step.Similarlyfor fractional orders of reaction.Example:Initial rate of this reaction (before HBr buildup) israte=kobs[H2][Br2]1/2