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kinet_02172011_part2

# kinet_02172011_part2 - Reaction Kinetics Elementary Ideas...

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Reaction Kinetics: Elementary Ideas 25th February 2010 1 Rate Laws Returning to our hypothetical model reaction: ν i i + ν j j ν k k + ν 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 ] γ i i definition of variables: k = rate constant (sometimes called rate coefficient; can be determined experimentally; can be estimated theoretically–transition state theory) [ i ] = concentration of REACTANT species ”i” γ i = Reaction order of REACTANT species ”i” At this time, we are NOT associating the species reactant order γ i to stoichiometric coefficient! p = Σ i γ 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 ( ν i , ν j , ν k , etc ) of the chemical reaction of interest! 1

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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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kinet_02172011_part2 - Reaction Kinetics Elementary Ideas...

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