Continuous Flow Method

1
Continuous Flow Method
Syringes are filled with A and B solution and
compressed at constant rate Flow rate must
exceed turbulent Limit Age of solution is
proportional to flow tube distance and flow
rate Only one time point (age) per measurement
position Large volumes required
Position of detector can be moved
2
Stopped Flow Method

• Syringes are filled with A and B solution and
  compressed to small amounts (50-200µl) then
  stopped
• Once the flow is stopped, solution ages with
  time (can be controlled)
• Detector sees more than one age
• Low dead times (1 ms)
• Expensive detection system

3
Relaxation methods
4
Relaxation Methods
5
Relaxation Methods
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Determining Mechanism from Rate Law

• If the rate law is
  , the total composition of the reactants
  in the rate limiting step is aAbB..
• If the rate law is
  , the total composition
  of the reactants in the rate limiting step is
  aAbB..-?M-nN..
• A rate law, to be properly interpreted according
  to rule 1, must be written in terms of the
  predominate species in the reaction medium.

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Determining Mechanism from Rate Law

• The number of positive terms in the rate law is
  the number of independent, parallel pathways.
  Negative terms represent the reverse reaction.
• A summation of n terms in the denominator implies
  a succession of n steps, all but nth are
  reversible.
• Species whose concentrations appear in
  single-term denominators are produced in the step
  prior to rate controlling step.

8
Determining Mechanism from Rate Law

• Adding up the steps in a mechanism must yield the
  net chemical reaction rapid reaction may follow
  the rate-controlling step.
• Alternative mechanisms leading to same pattern of
  activated complexes are not kinetically
  distinguishable.
• The order increases with increasing concentration
  when the reaction proceeds by parallel pathways
  but decreases when a series of steps occur.