CHEM 104 1st Edition Lecture 17 When a mechanism s first step is the slow rate determining step this is automatically the rate when this is not the case solving for the rate is more complicated we ll only ever solve for rates when the mechanism involves a fast equilibrium step followed by the slow step Example of mechanism with fast equilibrium step followed by slow step 2NO Br2 2NOBr Experimental rate law rate k NO 2 Br2 Proposed mechanism Step 1 NO Br2 k1 k 1 NOBr2 Step 2 NOBr2 NO k2 2NOBr Rate k2 NOBr2 NO Can t use this rate because there s an intermediate involved in it Rate of NOBr2 formation k1 NO Br2 Rate of NOBr2 consumption k2 NOBr2 NO k 1 NOBr2 First portion of consumption rate is small compared to the second so it can be ignored Rate of NOBr2 formation rate of NOBr2 consumption K1 NO Br2 k 1 NOBr2 NOBr2 k1 k 1 NO Br2 Rate k2 NOBr2 NO k2 k1 k 1 NO Br2 NO k NO 2 Br2 Catalysis Catalyst participates in reaction and is regenerated at the end changes mechanism lowers activation energy causing reaction to occur faster Example rate of cis butane to trans butane without a catalyst k cis rate with I2 catalyst k cis I2 1 2 Catalysis can be homogenous or heterogeneous i e reaction between gases can occur on a solid Enzymes highly efficient catalysts speed up reactions 10 7 1014 times very selective for one reaction or one type of reaction o Example aldehyde dehydrogenase catalyzes oxidation of aldehydes o Substrate reactant molecule whose reaction is catalyzed by the enzyme o Active site part of the enzyme that interacts with the substrate
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