Benzophenone Lab Report

The chemical kinetics of Benzophenone solution at different pH were studied. The dips in voltage due to flashing the samples were converted into absorbance versus time graphs, which appeared to be smooth exponential decay curves (Figure 3). The inverse of the absorbance, however, was linearly proportional to time (figure 4). The slope of these graphes allowed calculating the observed decay rate of the deprotonated ketyl radical. As a result, it can be concluded that changing the pH as little as one unit has significant effect on the rate of decay of the deprotonated ketyl radial (kobs). For instance, at pH 13.44, the average kobs from three trials equaled 293250±514M-1s-1. On the other hand, at pH 12.76, the average kobs 5230417±42063M-1s-1.

The overall deduced pattern is that the higher the pH, the lower the concentration of the hydrogen ion, and the lower the observed decay rate for the deprotonated ketyl intermediate is. As discussed before, the flash photolysis of Benzophenone relies on basic environment that abstracts a proton from the ketyl radical. Thus, the higher the pH, the stronger the basic solution, and the faster it will deprotonate the intermediate. However, the rate of decay of the deprotonated intermediate was observed here, and the relationship between pH and decay rate becomes reversed. This is supported by the findings in this experiment. As shown in figure 5, the concentration of the hydrogen ion is linearly proportional to average kobs, and since the hydrogen concentration is inversely proportional to pH, then the pH is inversely proportional to the decay rate as explained

If, for example, it was positioned at the wall furthest from the flash, the molecules far away might not receive enough photon energy to becomes electronically excited. This hinders the ability to obtain intensity dips due to flashing the sample.
2) Purging with nitrogen gas ensures the removal of dissolved oxygen in the benzophenone samples. Since oxygen’s ground state is a triplet spin state, it can quench the reaction by reaction with the excited triplet state and returning it to the ground state by emitting a photon (phosphorescence). This does not allow the protonated ketyl radicals to forms and halts the reaction.
3) The ground state electronic configuration of oxygen molecule (O2) is: (σ1s)2(σ*1s)2 (σ2s)2(σ*2s)2(σ2pz)2(π2px)2(π2py)2(π*2px)1(π*2py)1. The 12 electrons are arranged in molecular orbitals (MO) rather than atomic orbitals (AO). It also has two unpaired electrons. Thus, the spin multiplicity can be calculated by n+1, where n is the number of unpaired electrons. Hence, the multiplicity of O2 is 3