ChemLab - Chemistry 6 - Coordination Chemistry 2

Coordination Chemistry 2

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This week, you will work with a partner, using a single sample of cobalt complex from one person's supply.

Prepare a solution of Co(NH₃)₅(H₂O)(NO₃)₃ by weighing 0.11 to 0.12 g of solid on the analytical balance and diluting with water to 25 mL in a volumetric flask. Make sure that the solid is completely dissolved. Invert and shake the stoppered flask several times for quantitative mixing.

Your experiments with this solution consist of the following kinetics runs and measurements:

1. One reaction performed at 70 °C, to determine the absorbance values at t_∞, when reaction is complete. This reaction is started at the beginning of the lab period and absorbances at 415 nm and 450 nm are recorded at the end of the lab period.

2. Measurements to determine the absorbance values at t_o. This is done at room temperature, with solution of the cobalt complex alone, at the correct concentration.

3. A second reaction to observe the absorbance vs. time at 415 nm. This reaction is performed at room temperature, and the data will be analyzed to determine the rate constant for the initial ligand substitution reaction.

4. A third reaction to observe the absorbance vs. time at 450 nm. This reaction is performed at 40–50 °C, and the data will be analyzed to determine the rate constant for the second part of the mechanism, the slower ligand isomerization, from nitrito to nitro.

Reaction to determine A_∞
Begin by making a reaction mixture that can be used to determine A_∞. Pipet 5 mL of your Co(NH₃)₅(H₂O)(NO₃)₃ solution and 5 mL of the supplied HNO₂/NaNO₂ solution into a small Erlenmeyer flask, and mix well. Cork the flask, and suspend it with copper wire in the 70 °C temperature bath supplied by your TA. You will measure the absorbance of this solution at the end of the lab period, after the reaction is complete.

Next, set up a water bath in an insulated beaker with a temperature between 40 and 50 °C. Monitor its temperature periodically, so it will be at a stable temperature when you need it. You will also need to set up a temperature bath at room temperature.

Measurement of A_o
Now measure the initial absorbance values, A_o, used in your data analysis. Use a Mohr pipet to transfer 3 mL of your cobalt solution from the volumetric flask to a cuvet, add 3 mL of distilled water, and mix the solution well. Calibrate the spectrometer using a blank of distilled water at 415 nm, and measure the solution's absorbance. Change the wavelength to 450 nm, and calibrate with a distilled water blank. Measure the solution's absorbance at this wavelength.

Reaction to observe initial ligand substitution
To observe the initial ligand substitution reaction, set the wavelength of the spectrometer to 415 nm. Pipet 6 mL of your Co(NH₃)₅(H₂O)³⁺ solution and 6 mL of the supplied HNO₂/NaNO₂ solution into a small Erlenmeyer flask. Mix well, and note the time, since this is t = 0 for your first kinetics run. Keep this solution at room temperature by suspending it in a room temperature water bath. Transfer some of the solution to a cuvet, also in a room-temperature water bath, and monitor the reaction at 415 nm at room temperature, every 3 min until the initial substitution reaction is complete. Calibrate the spectrometer using a blank of distilled water between readings.

Reaction to observe ligand isomerization
When your water bath reaches a stable temperature between 40 and 50 °C, suspend samples of your cobalt solution and the HNO₂/NaNO₂ solution in it, so the reactants can reach this temperature before you combine them. The slower ligand isomerization from the nitrito to the nitro form will be monitored at this higher temperature. When the reactant solutions have reached the bath temperature, pipet 6 mL of your Co(NH₃)₅(H₂O)³⁺ solution and 6 mL of the HNO₂/NaNO₂ solution into a small Erlenmeyer flask. Mix well, and note the time. Keep this solution suspended in your warm water bath. Transfer some of the solution to a cuvet, also in the water bath, and monitor the reaction at 450 nm, every 3 minutes until the reaction reaches equilibrium. Calibrate the spectrometer using a blank of distilled water between readings. You will need to look carefully at the shape of your plotted data to decide when the initial substitution reaction is complete and when the ligand isomerization begins. This point will correspond to t_o and A_o for the isomerization reaction.

Completing the A_∞ measurements
Retrieve your A_∞ reaction mixture from the 70 °C water bath. Calibrate the spectrometer using a blank of distilled water at 415 nm, and measure the solution's absorbance. Change the wavelength to 450 nm. Calibrate again with a distilled water blank and measure the solution's absorbance at this wavelength.