Chemical Kinetics

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Chemical KineticsRates of Reactions
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Reaction Rates

• Average rate
• Change of reactant or product concentrations
• over a specific time interval
• Initial rate
• Rate at the beginning of a reaction.

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Question?

• For the reaction
• The dependence of the concentration of H2 on
  time is shown below. Is the reaction rate faster
  at point A or point B?
• A, B, both rates are the same

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Rate Determining Step

• slowest step in a multi-step mechanism
• the step which determines the overall rate of the
  reaction

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Rate Law

• an expression which relates the rate to the
  concentrations and a specific rate constant

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Rate Law

• Reaction rate k Am Bn
• where m gt order with respect to A
• n gt order with respect to B
• overall order m n

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Question?

• The empirical rate law for the reaction
• is Rate kNO2F2.
• Which of the following mechanisms is consistent
  with this rate law?
• A) NO2 (g) F2 (g) ltgt NO2F(g) F(g) fast
  NO2(g) F(g) ---gtNO2F(g) slow
• B) NO2 (g) F2(g) ltgt NO2F(g) F(g) slow
  NO2(g) F(g) ---gt NO2F(g) fast
• C) F2(g) ltgt F(g) F(g) slow 2NO2(g)
  2F(g) ---gt 2NO2F(g) fast
• A, B, or C

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Order of Reaction

• The exponent of the concentration for a reactant
  implies the number of molecules of that species
  involved in the rate determining step
• first order, exponent equals one
• second order, exponent equals two

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Integrated Rate Laws

• A ---gt products
• rate - (DA/Dt) kAm
• average rate
• rate - (dA/dt) kAm
• instantaneous rate

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This plot of lnH2O2 vs. time produces a
straight line, suggesting that the reaction is
first order.
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Elementary Reactions

• Molecularity is the number of particles coming
  together in a reaction step.
• unimolecular - rearrangement of a molecule
• bimolecular - reaction involving the collision of
  two particles
• termolecular - reaction involving the collision
  of three particles (statistically unlikely)

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Unimolecular Reaction
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Bimolecular Reaction
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Collision Rate Model

• Three conditions must be met at the molecular
  level if a reaction is to occur
• the molecules must collide
• they must be positioned so that the reacting
  groups are together in a transition state between
  reactants and products
• and the collision must have enough energy to
  break bonds, form the transition state and
  convert it into products.

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Transition State Activated Complex or Reaction
Intermediates

• an unstable arrangement of atoms that has the
  highest energy reached during the rearrangement
  of the reactant atoms to give products of a
  reaction

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Activation Energy

• the minimum energy required to start a reaction

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Temperature Reaction Rate
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Catalyst

• A substance which speeds up the rate of a
  reaction while not being consumed
• Homogeneous Catalysis - a catalyst which is in
  the same phase as the reactants
• Heterogeneous Catalysis - a catalyst which is in
  the different phase as the reactants
• catalytic converter
• solid catalyst working on gaseous materials

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Question?

• Which curve illustrates the effect of a catalyst
  on the reaction diagram, given that it speeds up
  the rate of a reaction?
• A, B, C, D

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Reaction Mechanism

• A set of elementary reactions which represent the
  overall reaction

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Car Catalytic Converter

• catalyst
• H2O(g) HCs -------gt CO(g) H2(g)
  (unbalanced)
• catalyst
• 2 H2(g) 2 NO(g) ------gt N2(g) 2 H2O(g)
• catalyst
• HCs O2(g) --------gt CO2(g) H2O(g)
  (unbalanced)
• catalyst
• CO(g) O2(g) --------gt CO2(g) (unbalanced)
• catalyst Pt-NiO
• HCs unburned hydrocarbons

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Chain Mechanisms

• chain initiating step - the step of a mechanism
  which starts the chain
• chain propagating step(s) - the step or steps
  which keeps the chain going
• chain terminating step(s) - the step or steps
  which break the chain

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Chain Mechanisms

• combustion of gasoline in an internal combustion
  engine
• chain initiating step - additives which generate
  free radicals, particles with unpaired electrons
• chain propagating step(s) - steps which generate
  new free radicals
• chain terminating step(s) - steps which do not
  generate new free radicals

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Enzymes

• Any one of many specialized organic substances,
  composed of polymers of amino acids, that act as
  catalysts to regulate the speed of the many
  chemical reactions involved in the metabolism of
  living organisms.

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