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Activation Energy, Catalysts

activation energy (Ea) - minimum amount of energy needed for reaction to occur

  • kinetic energy of colliding molecules used to break bonds
  • activated complex (transition state) - atomic arrangement at the point of highest energy
  • lower activation energy >> faster reaction rate
  • f = e-Ea/RT
    • f = fraction of molecules w/ energy equal to or greater than Ea
    • R = gas constant
    • T = absolute temperature
  • Arrhenius equation - takes into account 3 factors (activation energy, # of collisions, fraction of collisions w/ correct orientation)
    • k = Ae-Ea/RT
    • A = frequency factor constant
  • ln (k1/k2) = Ea/R (1/T2 - 1/T1)
    • must knowk2 from T2 (another temperature) to calculate k1 and T1)

reaction mechanisms - process by which reaction occurs

  • can describe the order in which bonds are broken/formed
  • elementary steps (elementary processes) - single event/step
    • rate law based on molecularity for each elementary step
  • molecularity - describes # of reactant molecules in an elementary step
    • unimolecular - single molecule involved (rearrangement)
    • bimolecular - collision of 2 molecules
    • termolecular - simultaneous collision of 3 molecules (very rare)
  • multi-step mechanism - sequence of elementary steps
    • elementary steps add up to give overall chemical process
    • intermediate - substance that’s not reactant/product in overall reaction
    • rate-determining (limiting) step - slowest step, limits overall reaction rate
    • rate law of overall reaction = rate law of slowest step

catalyst - substance that changes reaction rate

  • doesn’t change during reaction
  • lowers overall activation energy
  • homogeneous catalyst - in same phase as reactants
  • heterogeneous catalyst - in different phase than reactants
    • often consists of metals or metal oxides
    • adsorption - binding of molecules to surface (1st step)
    • active sites - where reactants adsorb
  • enzymes - biological catalysts
    • usually large protein molecules
    • lock-and-key model - substrates fit into enzymes at specific location w/ certain shape
    • turnover number - # of catalyzed reactions occurring at an active site
Subject: 
Chemistry
Subject X2: 
Chemistry

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