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ChemLab113-0086

# ChemLab113-0086 - Experiment 7 As we saw in connection with...

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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 temperature-equilibrated 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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