ChemLab - Chemistry 6 - Coordination Chemistry 1

Coordination Chemistry 1

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Procedure

This week's procedure is done individually, without a partner. Please work independently.

General Instructions
Refer to the stoichiometric net reaction as you first read through this procedure. You will be asked to calculate a final percent of theoretical yield (percent yield, for short) for your product. Thus, you must know the initial numbers of moles of all the reactants involved, to at least two significant figure accuracy. Keep this in mind as you weigh solids or measure solutions.

Solutions of reagents will be available in the lab in repipets set to deliver the appropriate amount into separate, clean beakers. Put a watch glass over the top of the beakers and take them to your hood. At the appropriate point in the procedure, add each reagent to your synthesis beaker, in your hood, not at the repipet. Do not remeasure the volume of any solution delivered by repipet, since greater accuracy is not necessary. Rinse the ammonia beaker with water as soon as you add it to your reaction beaker. Keep your reaction beaker in the hood at all times, to minimize fumes.

Synthesis Procedure
Weigh 3.6 g (0.012 mol) Co(NO₃)₂⋅6H₂O and 2.5 g (0.031 mol) NH₄NO₃ on an electronic top-loading balance (not on an analytical balance). Record the weights and place these solids in a 400 mL beaker (or 500 mL Erlenmeyer). Add no more than 10 mL hot water from a hot water bath (dippers made from 10-mL beakers will be provided) and swirl until the solids are dissolved. Place the beaker on a stirring plate in the hood and add 40 mL 6 M ammonia (0.24 mol).

Over a period of about 30 min add with stirring 25 mL (0.022 mol) 3% H₂O₂ in small (0.8 mL/min.) amounts. Best yields will be achieved if the 3% H₂O₂ is added via an eyedropper while stirring the solution. Addition can be more rapid at first than towards the end. Add a total of 15 mL during the first 15 minute period and a total of 10 mL during the final 15 minutes After addition is complete, allow the solution to sit with occasional stirring for 10 minutes or until bubbling stops, whichever comes first. The slow addition of H₂O₂ minimizes the extent of a non-productive side reaction that hydrogen peroxide undergoes by not allowing large concentrations of peroxide to build up. This side reaction causes oxygen gas to bubble out of the solution especially when relatively little unreacted cobalt(II) remains. It is the common mode of peroxide self-decomposition:

2 H₂O₂

2 H₂O + O₂ (g)

Slowly and carefully, add 30 mL (0.48 mol) 16 M HNO₃. Let the solution cool for 10 minutes. Add a volume of 95% ethyl alcohol (also called ethanol and abbreviated EtOH) approximately equal to the volume of solution already in the large beaker. The precipitate should become visible at this stage.¹ If it does not, notify your TA and review your procedure to this point. Allow the mixture to sit for 10 minutes to allow time for precipitation to proceed and then collect the solid by vacuum filtration on a Büchner funnel, as described in the Techniques section.

Rinse out solid remaining in the beaker with ethanol and twice rinse the solid on the filter. Pass air through the solid by suction for a few minutes to remove most of the ethanol. After filtration, the solid should be first weighed and then recrystallized, following the procedure described below.

Procedure for the Recrystallization of Aquapentaammine Cobalt(III) Nitrate
The main impurity in the initial preparation is the nitrate, Co(NH₃)₅NO₃(NO₃)₂, rather than the desired aqua complex, Co(NH₃)₅H₂O(NO₃)₃. To remove this impurity, the sample is first dissolved in a warm basic solution. The nitrate complex is rapidly converted to the hydroxo complex:

Co(NH₃)₅NO₃²⁺ + OH^–

Co(NH₃)₅OH²⁺ + NO₃^–

Then the solution is made acidic. Co(NH₃)₅OH²⁺, which is the conjugate base of the weak acid Co(NH₃)₅H₂O³⁺, adds a proton to give the aqua complex which is then precipitated as the nitrate salt:

Co(NH₃)₅OH²⁺ + H⁺

Co(NH₃)₅H₂O³⁺

Co(NH₃)₅H₂O³⁺ + 3 NO₃^–

Co(NH₃)₅H₂O(NO₃)₃(s)

Prepare first a 1 M ammonia solution by diluting the 6 M solution available in the lab. You will need at least 20 mL of this solution per gram of crude product. (If you are uncertain how to make this dilution, check with your TA.) Weigh the sample of crude Co(NH₃)₅ (H₂O)(NO₃)₃ (molecular mass = 348.1 g/mol) on an electronic top-loading balance, transfer it to a small (30 mL or 50 mL, if crude weight is >1g) beaker, and dissolve it in 1 M ammonia, using a maximum of 20 mL of ammonia per gram of crude product. Place the solution in a hot water bath and stir with a clean glass rod until all the solid is dissolved. Keep the solution in the hot bath for a full five minutes. (The baths will be adjusted to be close to 80 °C. Note the actual temperature, but do not allow the temperature to rise above 85 °C.)

Next cool the solution in an ice bath (take care that your beaker does not tip over as the ice melts!) until the temperature is below 5 °C as measured by your thermometer (with the bulb fully immersed in the solution). When the solution reaches 5 °C, add 16 M nitric acid dropwise while the solution remains in the ice bath, in the hood. A precipitate should form when sufficient HNO₃ has been added to neutralize the amount of ammonia used to dissolve the crude product. Add a ~25% excess of nitric acid with an eye dropper, stir, and allow a few minutes for precipitation to proceed in the ice bath. Test a drop of the mixture with pH paper, to confirm a strongly acidic pH. The required amount of nitric acid is easily calculated. Suppose that 3 g of crude product have been dissolved in 60 mL of 1 M NH₃. The required volume of 16 M HNO₃ is given by the following calculation:

V_HNO₃ = (mmoles NH₃ present)/(conc. HNO₃) = (60 mL) (1 M)/16 M = 3.8 mL

For a 25% excess, (0.25)(3.8 mL) = 0.95 mL. Thus, about 5 mL would be required.

Precipitation may be aided by scratching the inside of the beaker with a stirring rod or by adding a "seed" crystal of impure material. Collect the solid using vacuum filtration. With the wet solid still on the filter paper in the funnel, rinse twice with 95% ethanol. The ethanol rinses remove residual solution that contains ammonium nitrate from the solid. Ammonium nitrate is reasonably soluble in ethanol and the complex ion salt is not soluble in ethanol. Ethanol is miscible with water and therefore serves to remove residual aqueous solution.

Allow air to pass through the solid on the filter for a few minutes, then carefully transfer the filter and solid, to a watchglass to dry. After you have cleaned up your glassware, return to your product and scrape the solid off the filter paper and onto a piece of folded weighing paper. Transfer the solid from the paper into a clean, tared glass vial. A good technique for storing a solid open to the air is to cover the vial with a kimwipe held in place with tape. This system allows air drying of the solid while preventing contamination. Label the vial with your name and lab section and save your product in your lab section's box, as you will analyze it in the following three weeks. It will dry completely by next week, and you can obtain your final weight of dry, recrystallized product at the start of the next lab.

¹ The addition of a huge excess of nitric acid is an application of the common ion effect to the precipitation of the complex. A nitrate salt is much less soluble in a solution with a large concentration of nitrate ion than it is in water. The addition of alcohol involves another common technique for precipitating salts from aqueous solutions. The complex ion nitrate salt is insoluble in 95% ethanol, but quite soluble in water. Since ethanol and water are miscible, the addition of an equal volume of ethanol produces a 50:50 water:ethanol solution in which the solubility of the complex ion salt is much less than that in pure water. Ammonium nitrate, the other salt present in the reaction mixture, is sufficiently soluble that it does not precipitate.