Determination of Rate Laws:

The general form of the rate law will be as follows:

Inorder to determine the rate law or expression, the exponents m, n, and p must be found.

The data required for this may be presented in one of two forms.  You should be able to differentiate between the forms and be able to determine the rate law from the data provided.

 

[A]o and  initial rate	Use method of initial rates	Choose two experiments in which the initial concentration of only one reactant is changed.  Divide the rate expressions for both and solve for the exponent. Repeat process for all  reactants.  Note: for simplicity, place the largest rate/concentration values in the numerator.  As a simple rule of thumb:  if [A]o is doubled and.....  the rate remains the same (x 1)............zero-order in A the rate doubles (x 2)............................first-order in A the rate quadruples (x 4).......................second-order in A the rate times 8 (x 8).......................thrid-order in A the rate halves (x 1/2)....................... inverse first-order in A
[A] and time	Use integrated rate law	Two questions: (see flow chart)                                 i. Is t1/2 constant?                                  ii. Is plot of [A] vs t linear? Once the order of the reaction with respect to A is determined, the rate expression may be written.  For more complex reactions, additional experiments run under pseudo first-order conditions will need to be performed.

 
 
 
 
 
 
 

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  Pseudo First-order Conditions
 

To run a reaction under pseudo first-order conditions,
the concentration of one reactant is initially significantly smaller than the other reagents

For example: if you had three reactants you could set up a reaction such that [A] << [B]~[C]

In this situation, species 'A' may be completely consumed and yet the concentrations of the other reactants remain essentially constant.
  (they will change slightly, but not significantly enough to affect the reaction rate).

The concentration of 'A' is then measured vs. time.

From this data the order of the reaction with respect to [A] may be determined.

The experiment is then repeated for each reactant (ie. make [B] small in one and [C] small in another) to determine the order with respect to the other reactants.

 

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Created 02/20/99
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