Stoichiometry, Product Yield, and Limiting Reactants

Source: Smaa Koraym at Johns Hopkins University, MD, USA

1. Synthesis of tris(ethylenediamine)nickel chlorideExpand

   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.

   Table 1: Color Changes in Reaction Solution with Ethylenediamine

   Volume 25% Ethylenediamine Added	Solution Color
   0 mL
   1 mL
   2 mL
   3 mL
   4 mL
   5 mL
   6 mL
   7 mL
   8 mL
   9 mL
   10 mL
   11 mL
   12 mL
   13 mL
   14 mL
   15 mL
   16 mL
   17 mL
   18 mL
   19 mL
   20 mL

   Click Here to download Table 1

   • 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.
   • Bring a 100-mL beaker to an analytical balance to measure nickel chloride hexahydrate.
   • 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.
   • 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

     NiCl26H20
     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
   • 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.
   • 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.
   • 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.
   • 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.
   • 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.
   • 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.
   • 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.
   • 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.
   • 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.
   • 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.
   • 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.
   • 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.
   • While you wait, bring a paper towel to the hood to mop up water when you remove the beaker from the ice bath.
2. Precipitating tris(ethylenediamine)nickel chlorideExpand
   • Weigh a filter paper in a clean, tared weigh boat and record the mass in your notebook.

     Table 3: Determining the Percent Yield

     [Ni(en)3]Cl2
     Massfilter paper (g)
     Mass(product + filter paper) (g)
     Massproduct (g)
     Molar mass (g/mol)	309.893
     Actual yield (mol)
     Theoretical yield (mol)
     Percent yield (recovered)

     Click Here to download Table 3
   • Label an 800-mL beaker as ‘organic waste that contains nickel and water’.
   • Attach silicone tubing to the house vacuum in the fume hood and the sidearm of a Büchner flask. Clamp the flask upright in the hood and place a rubber filter adapter in the neck of the flask.
   • Set a Büchner funnel in the adapter and place the filter paper in the Büchner funnel. Wet the filter paper with just enough deionized water to seal it against the Büchner funnel surface.
   • Turn on the vacuum pump. Take the product mixture from the ice bath and pour the product into the funnel. Use a rubber policeman to transfer the precipitate to the funnel.
   • Once the last of the solution has been pulled into the flask, turn off the vacuum pump. Gently separate the Büchner funnel from the adapter to break the vacuum seal.
   • Lift the Büchner funnel and the adapter from the flask. Loosen the clamp and pour the solution from the flask into the waste beaker. Note: Be careful not to let any solution into the vacuum tubing.
   • Re-clamp the flask and settle the adapter and funnel in the flask.
   • Use the rubber policeman to gently break up the caked product without tearing the filter paper. Center the product in the funnel as much as you can.
   • Measure about 5 mL of acetone with the 10-mL graduated cylinder. Pour the acetone along the sides of the beaker and then pour the rinse into the funnel. Scrape any remaining solid from the beaker into the funnel with the rubber policeman. Repeat as needed if solid remains in the beaker.
   • Measure out another 5 mL of acetone, pour it into the funnel, and let the mixture sit for 5 min. This allows excess water and ethylenediamine stuck to the product to dissolve in the acetone.
   • After 5 min, briefly turn on the vacuum to pull the acetone into the flask. Break the vacuum and repeat soaking the solid product in 5 mL of acetone for 5 min in this way, twice more. Note: It is important to break up the product so that it is thoroughly soaked by acetone. Washing your product well is critical for an accurate yield calculation.
   • After pulling the final acetone wash into the flask, place a rubber stopper on top of the funnel and leave the apparatus under vacuum for 5 min to dry the product as much as possible.
   • While the product dries, label a 400-mL beaker with the chemical formula of the product. After 5 min, turn off the vacuum and gently remove the stopper from the funnel and the funnel from the flask to break the vacuum seal. Carefully transfer the funnel with the solid product to the 400-mL beaker.
   • Leave the product in the fume hood to dry for 2 – 3 days.
   • Remember to clean up the workspace before you leave. Dispose of the plastic pipette in the lab trash.
   • Clean labware that did not contact nickel complexes, such as graduated cylinders, according to your lab standard procedures. Note: Nickel waste may need to be disposed of separately from standard organic or aqueous waste, depending on your institution's requirements. If so, discard the filtrate and any other nickel organic waste in a designated container.
   • Once the product has dried for 2 – 3 days, bring it to an analytical balance. Tare a weighing boat, then transfer the filter paper containing the product into it.
   • Scrape any remaining product from the funnel using a rubber policeman and measure the mass of the final product. Record the mass in your lab notebook.
   • Dispose of the solid product in the appropriately marked container. Discard the filter paper and the weighing boat in the approved trash. Clean the beaker using the appropriate procedure for glassware used with a nickel compound.
3. ResultsExpand
   • Balance the chemical equation for this reaction. Consider ethylenediamine and water as units, because those molecules are unchanged in this type of reaction.
   • Calculate the moles of each reactant that you used and apply these stoichiometric relationships to find the theoretical yield for each starting amount. The reactant that produces a lower theoretical yield is the limiting reactant.

     Table 4: Determining the Limiting Reactant

     	Limiting reactant NiCl26H20	Limiting reactant C2H8N2
     Molar mass of product (g/mol)	309.893	309.893
     Reactant (mol)
     Mole ratio
     Theoretical yield (mol)

     Click Here to download Table 4
     
     Nickel chloride hexahydrate was the limiting reactant in this reaction. While you only needed a 3:1 molar ratio of ethylenediamine to nickel chloride hexahydrate to make tris(ethylenediamine)nickel chloride, the actual ratio was about 3.6:1 to 4:1, depending on how much nickel chloride hexahydrate was added.
     
     The table of color changes provide some qualitative evidence for this. The solution turned light blue around a 1:1 molar ratio of ethylenediamine to nickel, dark blue around a 2:1 ratio, and purple around a 3:1 ratio. This corresponds to nickel complexes with 1, 2, and 3 ethylenediamine molecules, respectively. Thus, you wouldn't clearly see that purple color if you had much more nickel than you needed. In this special type of reaction, nickel needs to be the limiting reactant to be sure of making the desired product.
   • Calculate the recovered yield, which is the ratio of the actual yield to the theoretical yield, based on the amount of limiting reactants that you added.
     
     If your percent yield is below 95% or above 100%, consider how you performed the experiment. Adding too much water or not letting the product chill for at least 15 min makes it harder to recover all the product you made. Insufficient washing in acetone will leave extra water and ethylenediamine in your product. Accurately recording your actions in your lab notebook as you performed the experiment can help you identify techniques to improve for the next experiment.