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Unformatted text preview: Reaction Kinetics: Elementary Ideas 25th February 2010 1 Rate Laws Returning to our hypothetical model reaction: i i + j j k k + l l In practice, the rate of reaction is treated as follows, thus introducing the concept of the rate constant, k : R = k N reactants Y i =1 [ i ] i i definition of variables: k = rate constant (sometimes called rate coefficient; can be determined experimentally; can be estimated theoreticallytransition state theory) [ i ] = concentration of REACTANT species i i = Reaction order of REACTANT species i At this time, we are NOT associating the species reactant order i to stoichiometric coefficient! p = i i = overall reaction order NOTE: at this point, the experimental rate equation is written with species reaction orders that have NO inherent relation to the stoichiometric coefficients ( i , j , k , etc ) of the chemical reaction of interest! 1 2 Elementary Reactions Consider the following overall reaction: 2 N 2 O 5 4 NO 2 + O 2 The predicted rate law for this reaction (which happens to be in good agree- ment with experiment) is: rate = k 1 k 2 k- 1 + 2 k 2 [ N 2 O 5 ] = k effective [ N 2 O 5 ] (1) We observe that the overall rate is NOT second order in [ N 2 O 5 ] as one might quickly jump to conclude. This is because the mechanism under- lying this overall chemical transformation is complex ; that is, there are several mechanistic steps involved, with certain intermediates involved as well. We will say more about reaction mechanisms later. For the present,...
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