Chem ch 13

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Chem ch 13
2010-05-04 15:12:46
Chemistry Chang

Chang 13
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  1. chemical kinetics
    • the area of chemistry concerned with the speeds, or rates, at which a chemical reaction occurs
    • the speed of a reaction is not neccessarily related to the energy change
  2. reaction rate
    • the change in the concentration of a reactant or a product with time
    • units of M/s
    • always highest at the beginning of a reaction
    • 2A --> B
    • rate = - 1/2 ^[A] / ^t = ^[B] / ^ t (rate expression)
  3. activation energy
    • Ea - the minimum amount of energy required to initiate a chemical reaction
    • -must have a collision in order to have a reaction
    • -collisions must occur with enough kinetic energy to break the existing bonds
    • if energy is too small the molecules will merely bounce off each other intact
  4. rate law
    • expresses the realtionship of the rate of a reaction to the rate constant and the concentrations of the reactants raised to some powers
    • for rxn aA +bB --> cC
    • the rate law is: rate = k[A]^x[B]^y
    • x and y must be experimentally determined
    • sum of the exponents gives "order"of reaction
  5. rate constant
    • k rate =k[A]
    • every time a reaction takes place, at a constant temperature, the same value for k is obtained
  6. first order reactions
    • rate = k[A] = - ^[A] / ^ t
    • In [A]0 / [A]t = kt
    • [A] at time = 0, [A] at time = t
    • for gas-phase reactions we can replace the concentration terms with the pressures of the gaseous reactant
  7. half-life
    • t 1/2 - the time required for the concentration of a reactant to decrease to half of its initial concentration
    • at t 1/2, [A] = 1/2[A]0
    • t 1/2 = 0.693/k
    • in 1st order reaction it is independant of the [A]
  8. second-order reactions
    • A --> products
    • rate = - ^[A] / ^ t = k[A]2 (squared)
    • 1/[A]t = kt + 1/[A]0
    • half life = 1 / k[A]0
    • initial concentration does matter in 2nd order rxns
  9. zero order reactions
    • rate = k[A]0
    • rate = k
    • half life = [A]0 / 2k
  10. collision theory of chemical kinetics
    • molecules must collide in order to react
    • rate increases with the # of collisions per second
  11. activated complex
    • aka - transition state
    • a temporary species formed by the reactant molecules as a result of the collision before they form the product
  12. effect temperature has on reaction rate
    • more molecules have the kinetic energy necessary to overcome the acivation energy
    • molecules move faster so they have more collisions
    • collisions are more energetic
  13. The Arrhenius Equation
    • k = Ae ^(-Ea / RT) OR In k = In A - Ea / RT
    • Ea = activation energy in kJ/mol
    • Ae = frequency factor (we usually don't know)
  14. Arrhenius Equation with two temperatures and respective constants
    • In k1/k2 = Ea / R ( 1/ T2 - 1/ T1)
    • for any order reaction
  15. elementary steps
    a series of simple reactions that represent the progress of the overall reaction on the molecular level
  16. reaction mechanism
    • the sequence of elementary steps that leads to product formation
    • details how the reaction is thought to take place
    • reactions are hypotheses (always subject to revision)
  17. intermediates
    • appear in the mechanism of the reaction (elementary steps) but not in the overall balanced equation
    • they are canceled as you add the elementary steps
  18. rate-determining step
    the slowest step in the sequence of steps leading to product formation
  19. unimolecular reaction
    an elementary step in which only one reacting molecule participates
  20. bimolecular reaction
    an elementary step involving two molecules
  21. termolecular reaction
    • an elementary step involving 3 molecules
    • they are very rare
    • require 3 molecules to "slam" into each other at once
  22. molecularity of a reaction
    the number of molecules reacting in an elementary step
  23. catalyst
    • a substance that increases the rate of a chemical reaction without itself being consumed
    • reacts within an elementary step of the mechanism but is regenerated in a subsequent step
    • it works by allowing an alternative route which lowers the activation energy
  24. heterogeneous catalyst
    catalytic converter - gas vs solid
  25. homogeneous catalyst
    same phase
  26. enzyme catalysis
    biological catalyst
  27. catalyst vs intermediate
    • catalyst appears as a reactant and later gets spit back out
    • intermediate appears as a product and gets sucked back in