1Physical ChemistryLecture 6Mechanisms of chemical reactionsChemical kineticsUnderstand the nature of reactionsMeasure concentration changes with timeDetermine effects of various parameters (e.g. pressure, temperature, relative concentration, presence of other chemical substances)Explain observed changes in chemical termsRequires integration of theory of chemical action with experimental resultsDevelopment of a mechanismof reactionDistinguish between possible mechanisms by comparison of prediction to observed resultsArrhenius theoryEmpirical equation of Arrhenius (van’t Hoff)Useful parameterization of temperature-dependent rate constantsA pre-exponential factorEaactivation energyOften seen in empirical analysis of kinetic dataActivation energy related to the likelihood of a reactive event)/exp()(RTEATkaExample Arrhenius plotTREATka1ln)(lnUseful linear formSlope of plot gives the activation energy directlyMany rate constants seem to obey this relationshipNon-Arrhenius behaviorArrhenius behavior is empiricalModern chemical-reaction theory does not readily predict the Arrhenius equationSurprising how often Arrhenius behavior is seenSeen in analysis because it is simpleRate constants of some reactions do not obey Arrhenius’s simple equationExample: decomposition of diacetyleneIndicates complexity of the chemical reactionElementary reactionsChemical reactions are often more complex than presented in the balanced equationDo not always occur as a single step from initial state to finalstateOrder is not necessarily the stoichiometric coefficientSome reactions do occur in a single step --elementary reactionsGenerally involve simple mono- or bimolecular interactionsMay involve “unusual” species, i.e. species that are not thought of because they are not very stableOrder in elementary reactions is the stoichiometry number, which is called the molecularity11212222][2BrHkvHBrBrHOHkvOHOH
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