Unformatted text preview: Experiment 7. As we saw in connection with that experiment, the absorbance at an appropriate
wavelength (592 nm in this case) of a solution of a colored species is directly proportional to the
concentration of the colored species. With that relation, the equation above can be expressed as a
corresponding equation showing how the absorbance changes with time during the reaction. A = A” e‘1'"t A0 in the equation is the absorbance at the reference time, and A is the absorbance at the later
time t. Ifthe last equation above is converted to a different form it provides the basis for the test you
will use to see if the order with respect to bromphenol blue in the true rate law indeed has the
value one, as deduced from the proposed mechanism. The new form is obtained by dividing both
sides of the equation by A0, then taking the natural logarithm of both sides, and ﬁnally
multiplying both sides by minus one. The following equation results. In (A/Ao) = k't If the order with respect to bromphenol blue has the value one, a plot of ln (AIAO) versus time
will give points along a straight line beginning at the origin. The computer will make Such a plot
and see if the points are acceptably close to a straight line. If they are, the value of one for the
order with respect to bromphenol blue is established. The plot used to establish the order with respect to bromphenol blue can be used to determine
the order with respect to hydroxide ion. According to the equation that is its basis, the slope of
the straight line between the points is k'. If the order with respect to OH" in the true rate law is
one, k' equals k [OH‘]. You will carry out the reaction in three solutions with different
concentrations of OH. The computer will calculate k‘ from the plot for each solution, and then
divide k' by the OH concentration in the solution to obtain a corresponding value of the rate
constant k. If the order with respect to hydroxide ion is indeed one, the three values of k will be
nearly equal, scattering no more than the amount determined by experimental uncertainty. The computer will judge the scatter of calculated rate constants and indicate whether it is within
acceptable limits. It was pointed out at the beginning of this section that reaction rates depend on temperature.
That dependence is usually. so large that great care must be taken to keep the temperature as
nearly constant as possible when determining reaction orders. Before you measure any
absorbances the cuvettes holding the solutions will be temperatureequilibrated in a carefully
controlled water bath at 27°C, and they will be kept in the bath throughout the reaction except for
brief periods when they are removed to measure absorbances. For reactions involving ions in solution there is another important factor that must be
controlled in addition to the temperature. That additional factor is termed the ionic strength of
the solution. The reactivity of an ion in solution is inﬂuenced significantly by the concentrations
of all ions in the solution, even those that do not participate in the reaction. That is because all 119 ...
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 Fall '08
 TEDESCO

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