CHEM 132

• Thermodynamics


  If a reaction takes place
• Kinetics


  How fast a reaction proceeds
• Reaction rate


  Change in concentration of a reactant or product per unit time
• Average reaction rate is always positive
• Rate of reaction


  Different than rate of consumption
• Instantaneous rate of change


  The reaction rate at a specific moment, the tangent of the line at that moment
• Rate law


  The action rate depending only on reactants
• Products very rarely appear in the rate law
• Value of exponents must be determined by experiment, not from the balanced equation
• Rate constant, k


  Units depend on the order of reaction (sum of exponents in rate law)
• Reaction rate


  Always concentration per time (M/s)
• Higher k


  Faster reaction because rate constant is directly proportional to rate
• Rate constant


  Depends solely on reaction and temperature
• Differential rate law


  Rate depends on concentration
• Integrated rate law


  Concentration depends on time
• Once differential rate law is known, integrated rate law can be determined
• Determining rate law


  Experiments to calculate exponents for each reactant
• Initial rate


  The instantaneous rate determined at the beginning of the reaction (t=0)
• Reaction concentrations are easy to control before the reaction begins
• Exponents (order) in rate laws are not at all related to coefficients in equation
• First order reaction


  Plot of ln[A] vs t is linear
• Half life


  Time required for the reactant to reach half its original amount
• First order reactions have constant half lives
• Zero order reactions


  Rate law is not dependent on the concentration of reactant, graph of [A]0 vs t is linear
• Second order reactions


  Graph of 1/[A] vs t is linear
• Reaction mechanism


  Series of elementary steps by which a chemical reaction occurs
• Elementary step


  Reaction whose rate law can be written from its molecularity
• Molecularity


  Number of species that collide to produce reaction indicated by that step
• Intermediate


  Species formed in an early step and consumed in a later step
• Catalyst


  Species consumed in an early step and reformed in a later step
• Transition state or activated complex


  Like an intermediate but not part of a mechanism
• Elementary steps


  • Unimolecular
  • Bimolecular
  • Termolecular
• Unimolecular


  One molecule and first order
• Bimolecular


  Collision of two species and second order
• Termolecular


  Collision of three species and is very rare because all three must hit at the same time
• Reaction mechanism is the sum of elementary steps that must given an overall balanced equation for the reaction
• Mechanisms must agree with experimentally determined rate law
• Rate determining step


  When multistep reactions have one step much slower than all others, the overall reaction is only as fast as the slowest step
• Equilibrium


  When reaction rates are equal in both directions
• Collision theory


  1. Molecules must collide

  2. With enough energy

  3. In the proper orientation