5.3 Day 4 and 5 Procedures

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Day 4 and 5 - Kinetic Measurements @ 60C

The conversion of \([Co(NH_3)_5Cl]^{2+}\) to \([Co(NH_3)_5(H_2O)]^{3+}\) is accompanied by a change in color from a purplish-pink to a lighter, more orange-pink and a shift of the \(\lambda_{max}\) to higher energies. You will follow the conversion of \([Co(NH_3)_5Cl]^{2+}\) into \([Co(NH_3)_5(H_2O)]\) at 550 nm where \(\epsilon\) = 21M cm for \([Co(NH_3)_5(H_2O)]\). If you know the amount of \([Co(NH_3)_5Cl]^{2+}\) in the sample initially, you can calculate the concentration of \([Co(NH_3)_5(H_2O)]^{3+}\) when the reaction is over (at "infinite" time). The absorption at 550 nm at infinite time, \(A_{\infty}\), is simply the absorption after 3+ complete conversion to \([Co(NH_3)_5(H_2O)]\) . For a first order conversion of \([Co(NH_3)_5Cl]^{2+}\) into \([Co(NH_3)_5(H_2O)]^{3+}\) \(ln(A-A_{\infty}) = - kt + C_1\) where t is the time, k is the rate constant for the reaction, and \(C_1\) is a constant.

Weigh out two samples of \([Co(NH_3)_5Cl]Cl^2\) accurately (to 0.1 mg) by taring a 25- mL volumetric flask and then reweighing it plus the sample of \([Co(NH_3)_5Cl]Cl^2\). The amounts should be different, for example, ~20 mg and ~40 mg. Label both flasks. Dilute to the 25 mL mark with an aqueous solution of 0.5 M \(HNO_3\).

Remove a sample from each flask with a clean pipette and transfer to a glass cuvette. Select the scanning kinetics icon on the Cary 100 Spectrophotometer and follow the instructions outlined in Appendix 1, part B. Obtain a baseline spectrum of the 0.5 M nitric acid solution. Place the cuvettes in the sample holder of the Cary 100 Spectrometer and equilibrate for 15 minutes at 60C. IMPORTANT NOTE: There will be several teams using each instrument, requiring coordination of the start time for the kinetics runs. The spectrometer will record spectra every 20 minutes. Manually record the time and absorbance at 550 nm for each spectrum and record the location of all isosbestic points or transfer your data files to a USB drive for analysis.

Calculate the expected value for \(A_{\infty}\) and construct a table of \(ln(A - A_{\infty})\) and time t for the two solutions. Continue to follow the reactions until the value of \(A_{\infty}\) is approximately half the value of \(A_o - A_{\infty}\), where \(A_o\) is the expected absorption of \([Co(NH_3)_5Cl]Cl^2\) alone at 550 nm, which you know from the quantity you weighed out and ε for \([Co(NH_3)_5Cl]Cl^2\) 550 nm. The time at which \(A_o - A_{\infty}\) is half the value of \(A_o - A_{\infty}\) is the half-life for this first-order reaction, i.e., the time required for it to proceed halfway. After five half-lives the reaction is essentially

finished, but you will have time to follow the reaction for approximately only one half-life.

Note carefully the presence of one or more isosbestic points. An isosbestic point is found at some value of \(\lambda\) where the absorbance of the mixture does not change during the reaction. A "clean" set of isosbestic points is characteristic of a simple conversion of one absorbing compound into another.