Full Lab Manual
Introduction & Goals
Chemistry & Background
In Your Write-up
Chemistry & Background
Kinetic studies have established that the mechanism of the initial reaction is not a simple exchange of the H2O and NO2– ligands. The reactive species is not the aqua complex, but rather its conjugate base, Co(NH3)5(OH)2+. This reacts with N2O3 to form the nitrito isomer and nitrous acid, HNO2, as shown below. In this mechanism, the oxygen atom of the original water ligand is marked with a star to help you keep track of it. The structure with dotted lines is a transition state between reactants and products. In this structure, the dotted lines show the formation of new bonds,
In the second step of the reaction, the NO2– ligand isomerizes. The nitro and nitrito linkage isomers exist in equilibrium. The equilibrium is shown below, with the two resonance structures of the nitro isomer shown explicitly:
In weeks 1 and 2, you used absorbance measurements to monitor the progress of a reaction over time. We will use the same approach here, but with the added feature of requiring two wavelengths to follow everything that is happening in the overall reaction.
Your goals are thus to determine the rate constant for the initial ligand exchange reaction and for the subsequent ligand isomerization reaction. To do so, it is useful to observe the two reactions sequentially, at different wavelengths and different temperatures. To monitor the first reaction, you should monitor absorbance at a wavelength where the overall rate of disappearance of the starting reactant Co(NH3)5(H2O)3+ can be monitored. This can be done at a wavelength where the absorbance of the nitrito and nitro complex are equal, such as 415 nm. To monitor the second reaction, the ligand isomerization, it is important to wait until the initial reaction is complete. Then it is useful to monitor a wavelength that has a large difference between the nitro and nitrito isomer absorbance, such as 450 nm.
The rate law for the initial ligand substitution reaction shown in the above mechanism has been found by experiment to be
The reaction conditions can be manipulated to make it possible to determine reaction orders independently. For example, if the concentrations of NO2– and HNO2 are much greater that that of Co(NH3)5(H2O)3+, such that they remain nearly constant throughout the course of the reaction, the conditions will be pseudo first order, with a rate law
where k' = k [NO2–] [HNO2]
Another way to determine the order of a reaction is to monitor the concentration of a reactant as a function of time and compare to the predictions of an integrated rate law for various possible reaction orders. The concentration vs. time data can be plotted in the form of various integrated rate laws, and the correct reaction order can be confirmed by a linear plot for the correct integrated rate equation, as you saw in week 1. The integrated rate law for a zeroth order reaction predicts a linear change in concentration with time, as you saw in weeks 1 and 2. In this case, a linear plot of concentration vs. time confirmed that the reaction was zeroth order. For a first order reaction, the integrated rate law gives a dependence of reactant concentration, c, with time of
The second step of the reaction, the isomerization from nitrito to nitro forms is also first order with respect to Co(NH3)5(H2O)3+.
This experimental observation gave rise to the predicted mechanism given earlier:
In this week's experiments, you will determine the rate constants and confirm the reactions orders predicted by the proposed mechanism. To confirm the reaction order with respect to Co(NH3)5(H2O)3+ for both steps in the reaction, we will measure absorbance vs. time. We first need to relate the integrated rate law, in terms of concentrations, to the absorbance values measured. We will use the relationship between concentration and absorbance, Beer's Law:
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