In this lab, you will combine nickel chloride hexahydrate and ethylenediamine in water to produce tris(ethylenediamine)nickel chloride. When nickel chloride hexahydrate dissolves in water, it rearranges to become the hexaaquanickel cation and two chloride anions.
Adding ethylenediamine results in three ethylenediamine molecules exchanging with the six water molecules. This generally happens one ethylenediamine molecule at a time. So, the nickel complex will pass through the light blue tetraaquaethylenediamine nickel and dark blue diaquabisethylenediamine nickel intermediates before forming the purple tris(ethylenediamine) nickel chloride product.
Based on this special process, you might guess that nickel chloride hexahydrate will be your limiting reactant. You'll test this hypothesis by using the amounts of the reactants that you added during the reaction to determine the theoretical yield. You'll also precipitate the product from solution and measure the mass. Once you know the limiting reactant and the final product mass, you can calculate the recovered yield.
Before starting the experiment, make a table in your lab notebook of the amount of ethylenediamine solution added and the color of your reaction solution. Include a line for the starting color of the solution. You will add 20 mL of 25% by volume ethylenediamine, 1 mL at a time. This is how you will monitor the progress of your reaction.
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Put on a lab coat, splash-proof safety glasses, and a pair of disposable gloves. Note: Nickel chloride is toxic and an irritant, so use caution when handling it. Ethylenediamine is toxic, so don't uncover it until you've placed it in the fume hood.
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Bring a 100-mL beaker to an analytical balance to measure nickel chloride hexahydrate.
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Tare a weighing boat on the analytical balance and then use the spatula to measure 4.5 – 5 g of nickel chloride hexahydrate. Note: This compound is hygroscopic, meaning that it can absorb a large amount of water from the atmosphere. Work quickly to get an accurate measurement and then close the container tightly.
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Once you have between 4.5 and 5 g, quickly record the mass in your notebook.
Table 2: Determining the Number of Moles of Reactants
| NiCl2•6H20 |
| Mass (g) |
|
| H2O added (mL) |
|
| Molar mass (g/mol) |
237.69 |
| Number of moles |
|
|
|
| C2H8N2 |
| Added volume of 25% v/v C2H8N2 (mL) |
|
| Actual volume of C2H8N2 (mL) |
|
| Density (g/mL) |
0.8995 |
| Mass (g) |
|
| Molar mass (g/mol) |
60.0989 |
| Number of moles (mol) |
|
Click Here to download Table 2
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Pour the nickel chloride hexahydrate into your 100-mL beaker. Clean the spatula with a lab wipe for the next person and dispose of the weighing boat and the lab wipe according to your institution's policies for nickel waste disposal.
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Measure 5 mL of deionized water, and pour it into the beaker. Stir the mixture with the glass rod to dissolve the nickel chloride hexahydrate, thereby forming a clear green solution of hexaaquanickel chloride.
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If the compound does not completely dissolve, add 1 mL of deionized water and keep stirring. If after five minutes it is still not dissolved, add another mL of water. Note: Be careful not to add more water than you need, because excess water will make it harder to precipitate your product later.
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Once you have made your solution of hexaaquanickel chloride, record the amount of water that you used in your lab notebook and note the color of your solution before the addition of ethylenediamine.
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Obtain 20 mL of a solution of 25% by volume ethylenediamine in deionized water from your instructor. You will be given this solution in a covered beaker.
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With the disposable plastic pipette, add the ethylenediamine to the hexaaquanickel chloride solution, 1 mL at a time, stirring the solution with the glass rod after each addition. Record the changes in color of the solution. Note: The solution will change from green to blue, then to dark blue, and finally to purple. This reaction is fast, so once you have added all the ethylenediamine, you'll have a solution of your product tris(ethylenediamine)nickel chloride in water.
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Measure 10 mL of acetone with the graduated cylinder. Pour the acetone into the beaker of purple tris(ethylenediamine)nickel chloride solution and stir with the glass rod to thoroughly mix the acetone into the solution. Note: Tris(ethylenediamine)nickel chloride is soluble in water, but insoluble in acetone. Adding acetone to your aqueous product solution makes it more favorable for the product to precipitate.
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Add 50 mL of acetone total to the tris(ethylenediamine)nickel chloride solution in this way, 10 mL at a time. Note: If you don't see any solid once you've added 50 mL of acetone, use the glass rod to gently scratch the inner surface of the beaker. This will help precipitation because solids form more easily on rough surfaces.
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Once the solid begins forming, remove the glass rod and leave the beaker undisturbed in the hood for 5 min. Note: Stirring or agitating the solution at this point may encourage the solid to redissolve.
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During that time, scoop about 600 mL of crushed ice into an 800-mL beaker. Add just enough tap water or fresh water to the beaker to fill the spaces between the ice, but not enough that the ice starts to float on a layer of water. This ice bath provides more even cooling than ice alone.
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Once your beaker of precipitating product has sat undisturbed for 5 min, carefully place it in the ice bath. Note: Make sure that the ice bath is higher than the level of the purple solution but lower than the top of the 100-mL beaker.
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Allow this solution to sit in the ice bath for 15 – 20 min. Cooling the solution lowers the solubility of the product, encouraging even more precipitation.
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While you wait, bring a paper towel to the hood to mop up water when you remove the beaker from the ice bath.
