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