CH353 - Physical Chemistry I
Spring 2015, Unique 51170

Lecture Summary, 5 May 2015

Unimolecular Reactions: Unimolecular reactions proceed through the Lindemann mechanism:

  1) R + R --> R* + R
  2) R* --> P

where the first elementary step is rate determining and the steady-state approximation is valid.  Depending on the concentration of the reactant R, this reaction will either be first order in [R] or second order in [R].   

Radical Chain Reactions: Radical chain reactions are initiated when an unstable radical is generated, perhaps through absorption of UV radiation.  This radical is then propagated through a series of elementary steps in which new radicals are generated.  By setting up a series of elementary reversible reactions and applying the steady-state approximating to our radial intermediates, we were able to derive the experimentally observed rate law for the radical chain reaction H2 + Br2 --> 2 HBr, which at first looked hopelessly complex.   

Homogeneously Catalyzed Reactions:  Catalyzed reactions proceed at a faster rate than an uncatalyzed reaction because of the addition of a molecule which is neither made nor consumed in the reaction and does not appear in the reaction stoichiometry.  The catalyst changes the mechanism of the reaction versus the uncatalyzed version (i.e. results in a different series of elementary steps) and does appear in the expression for the rate law of the reaction.  The process of solving an analytical rate law using elementary reactions, the SSA, or FE is the same for catalyzed reactions as we have already seen for uncatalyzed reactions.