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Unformatted text preview: chem exam 2.oo3 Topic EXAM 2 START 16 16.7 Reaction Mechanisms: Steps in the overall reaction reaction mechanism reaction intermediate elementary reactions and molecularity elementary reactions elementary steps unimolecular reaction bimolecular reaction the rate determining step of a reaction mechanism rate limiting step rate determining step correlating the mechanism with the rate law mechanisms with a slow initial step mechanisms with a fast initial step summary The mechanisms of most common reactions consist of two or more elementary steps, reactions that occur in one step and depict a single chemical change. The molecularity of an elementary step equals the number of reactant particles and is the same as the reaction order of its rate law. Unimolecular and bimolecular steps are common. The rate- determining or rate-limiting (slowest) step determines how fast the overall reaction occurs, and its rate law represents the overall rate law. Reaction intermediates are species that form in one step and react in a later one. The steps in a proposed mechanism must add up to the overall reaction, be physically reasonable, and conform to the overall rate law. If a fast step precedes a slow step, the fast step reaches equilibrium, and the concentrations of the intermediates in the rate law of the slow step must be expressed in terms of reactants. 16.8 catalysis: speeding up a chemical reaction catalyst homogenous catalysis homogenous catalyst heterogenous catalysis heterogenous catalyst Topic hydrogenation catalysis in nature cellular catalysis: the function of enzymes enzyme active site atmospheric catalysis: depletion of the ozone layer summary a catalyst is a substance that increases the rate of a reaction without being consumed. It accomplishes this by providing an alternative mechanism with a lower activation energy. Homogenous catalysts function in the same phase as the reactants. Heterogenous catalysts act in a different phase from the reactants. The hydrogenation of carbon- carbon double bonds takes place on a solid catalyst, which speeds the breakage of the H-H bond in H 2 . Enzymes are biological catalysts with spectacular efFciency and speciFcity. Chlorine atoms derived from CC molecules catalyze the breakdown of stratospheric ozone. problems 56, f8, 64, 67, 79, 66 17 17.1 the equilibrium state and the equilibrium constant equilibrium constant (K) summary The rate and extent of a reaction are not necessarily related. When the forward and reverse reactions occur at the same rate, the system has reached dynamic equilibrium and concentrations no longer change. The equilibrium constant (K) is a number based on a particular ratio of product and reactant concentrations. K is small for reactions that reach equilibrium with a high concentration of reactant(s) and large for reactions that reach equilibrium with a low concentration of reactant(s)....
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