Page 138 - [B._MURPHY,_C._MURPHY,_B._HATHAWAY]_A_working_meth

122 Chapter 8

Initial Rates
This method is much more common. The rate at the beginning of a
reaction is measured and the reactant concentrations are known most
accurately at this time.

Advantages:
1. No assumption is made about the form of the rate equation.
2. Accurate known concentrations are used.
3. Minimal interference by the presence of impurities or products.

Disadvantages:
1. Needs a number of separate experiments.
2. An extrapolation method is required, i.e. rates are measured by
drawing tangential lines.

WORKING METHOD FOR THE DETERMINATION OF THE
ORDER OF A REACTION BY THE METHOD OF INITIAL
RATES
1. Write down the chemical equation.
2. Determine the appropriate rate equation, taking care to include
the stoichiometry factors, VA, vB, VC, VD, etc.
3. Using the given data, determine each of the following ratios:
Rate 1 Rate 2 Rate 3
---
etc.
Rate 2 ’ Rate 3 ’ Rate 4 ’
4. From each ratio obtained in step 3, evaluate the partial orders, x,
y, z etc. for the reaction, remembering xo = 1, from the rules of
indices and the rules of logs:
log(AB) = log A + log B; log(A/B) = log A-log B; log A’ =p log A
5. Evaluate the overall order of the reaction, from the sum of the
partial orders: x + y + z + . . . etc.
6. Determine k, the specific rate constant for the reaction for each
set of data, Find the sum of these values, and divide to get an
average value.
7. Determine the correct unit of the rate constant, k.
8. Answer any riders to the question.