Preequilibrium Approximation (26.2) Some reaction mechanisms involve intermediate reactions that are reversible – Inverse temperature dependence of the reaction rate can indicate the presence of a reversible process (i.e., as temp. goes up, the reaction slows down) If the reversible step obtains equilibrium faster than the rate of decay of the resulting intermediate, then a simple approximation can be used – The intermediate concentration is approximated using the equilibrium constant The rate of formation of nitrogen dioxide from nitric oxide and oxygen shows inverse temp. dependence due to a reversible step – Initial reversible step (formation of nitric oxide dimer) is endothermic

Kinetics of Catalysis (26.3) Catalysts lower the activation energy of a reaction, thus increasing the rate – Catalysts typically form a substrate-catalyst complex (SC) which facilitates the reaction – Catalysts are regenerated during the course of the reaction Rate of formation of product depends on the rate of decay of the substrate-catalysts complex – The SSA and preequilibrium approximation can be used to come up with a rate law – K m is the composite constant (value depends on rate constants of given reactions and on how you solve for the rate law) We can use mass balance to eliminate the dependence on catalyst concentration

Enzyme Catalysis (26.4) Enzymes are catalysts, so their kinetics can be explained in the same fashion Enzymes – Rate law for enzyme catalysis is referred to as the Michaelis-Menten rate law – K m is called the Michaelis constant When the substrate concentration is very large, the rate depends only on the concentration of enzyme and the turnover rate (k 2 ) – Equilibrium between reactants and enzyme-substrate complex (ES) is pushed to the right (i.e., only ES exists, no free enzyme) – When [S] >> K m, the maximum rate is achieved (R max ) One can get K m by realizing that it is equal to [S] when the rate is half of the maximum rate – If K m is small, then the enzyme tightly binds the substrate (equilibrium is shifted to ES)

Enzymes as Catalysts