This preview shows pages 1–3. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.View Full Document
Unformatted text preview: Reaction Kinetics: Elementary Ideas 18th 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 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 , ) 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, we mention them in order to emphasize that the stoichiometric coefficients...
View Full Document