1  

Experiment Make-up Chemical Reactions and Stoichiometry: Which Method of Recovering the Product from a Precipitation Reaction Gives a More Accurate Result?1 ______________________________________________ INTRODUCTION Precipitation Reactions:

In many chemical investigations, scientists are interested in knowing the stoichiometry of the reaction and the yield of the reaction product. The precipitation reaction is a double displacement reaction where a cation and an anion in one reactant switch partners with the other reactant to produce a solid product. Stoichiometry is defined as the molar ratio of reactants and products in a balanced chemical equation. This ratio is very important for a scientist determining the yield of a chemical reaction. Products can continue to form only if there are enough reagents to continue the reaction. The reactant that controls how much product is formed is called the limiting reagent.

Consider the following reaction used to produce AgCl(s): AgNO3(aq) + HCl(aq) à AgCl(s) + HNO3(aq) This is an example of a precipitation reaction where the cation of one reactant bonds to the anion of the other reactant to form a solid product (AgCl). In the preceding reaction, the H+ and NO3

– ions are left floating in solution. They do not take part in the reaction, and are present as free ions on both sides of the equation. Therefore, they are called spectator ions. Eliminating the spectators leaves the net ionic equation: Ag+(aq) + NO3

–(aq) + H+(aq) + Cl-(aq) à AgCl(s) + H+(aq) + NO3-(aq)

Net ionic equation: Ag+(aq) + Cl-(aq) à AgCl(s) Now turning our attention to the stoichiometry of the reaction: If 3.43g of AgNO3 and 4.25mL of 2.99 M HCl is used, the limiting reagent can be determined using the 1:1 molar ratio of each reactant to product.

AgClmolesHClmolAgClmol

HClLmol

HClmLHClLHClmL

AgClmolesAgNOmolAgClmol

gAgNOmolAgNOg

0127.011

199.2

1000125.4

0202.011

87.1691

43.33

33

=⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛

=⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛

Since all of the HCl will be used up when 0.0127 moles of AgCl is produced, there will not be any HCl available to react with the excess AgNO3. Therefore, HCl is the limiting reagent.

The theoretical yield of a reaction is the amount of product that forms if the reaction runs fully to completion. It is based on the stoichiometry of the reaction and ideal conditions in which the starting material is completely consumed, undesired side reactions do not occur, the reverse reaction does not occur, and there are no losses in the experimental procedures. For the above reaction, theoretically it is possible to produce 0.0127 moles of AgCl after all of the HCl is used up. However, if only 0.00974 moles of AgCl is produced, the actual yield will not match the                                                                                                                1  This new lab was developed by Kristina Clara and Li-Qiong Wang with the help of Ning Hou and Ayse Bozkurt. Edited by Allison Lawman and Muge.  

  2  

theoretical yield. To calculate the percent error, subtract the actual yield from the theoretical yield, divide that difference by the theoretical yield, and then multiply by 100%.

Ex. Theoretical yield – Actual yield = Difference ( 0.0127– 0.00974) moles AgCl = 0.003

Difference/ Theoretical x 100% = % error 0.003/0.0127 = 0.223 x 100% = 23 % error

This is a rather large error margin, thus indicating a low yield of product. . Assuming that losses of product are only from the experimental procedure, what kinds of errors were present in the experiment? Is there a more precise technique for recovering the product? These are the types of questions that will be answered in this experiment.

In this experiment, two different methods of recovering the reaction products will be tested. After testing both in the lab, use error analysis to determine which technique produces the higher product yield.

Technique 1: Part A utilizes the technique of vacuum filtration. The reactants are 0.50M CaCl2 and 0.40M Na2CO3(see reaction below). The CaCO3 precipitate will be separated from the solution using a vacuum filter. Use the weight of the recovered precipitate to determine the percent error.

CaCl2(aq) + Na2CO3(aq) à CaCo3(s) + 2NaCl

Technique 2: Part B involves a different precipitation recovery technique called centrifuge. 0.50M CaCl2 with 0.40M Na2CO3 again serve as the reactants, and CaCO3 is the solid product to be recovered. Following the first recovery, add methanol to the mixture (methanol is very volatile and will evaporate quickly), centrifuge again, and use a nitrogen line to dry the precipitate. The solid product will then be reacted with HCl. The equation for this reaction is shown below:

CaCO3(s) + 2HCl(aq) à CO2(g) + H2O(l) + CaCl2(aq) (Eq. 1)

The percent yield can be determined using the mass of product recovered. Additionally, the weight of the precipitate, coupled with the stoichiometry of the reaction of CaCO3 with HCl, can be used to determine another value: the molar mass of CaCO3.

Error analysis will determine which method (centrifuge or vacuum filtration) is more accurate.

Error Analysis

“Results! Why man, I have gotten a lot of results.

I know several thousand things that won't work." - Thomas Edison

Even for the best of scientists, mistakes are common. In fact, erring is a certain and necessary part of research. Being able to identify the sources of error is important. By figuring out what went wrong in an experiment, it is possible to learn from this mistake and avoid making it again in the future. This can also lead to the exploration and development of new and better ways to perform the experiment.

By analyzing the stoichiometry of the reaction of CaCl2 with Na2CO3, the theoretical yield will be compared to the actual yields. Then determine and compare the magnitude and sources of error in the centrifuge and vacuum filtration methods. See Table 1.1 to view some common types of error and examples of each.

  3  

Table 1.1 Classification of Experimental Error:

 

 

Type Definition Examples Random Error (or Indeterminate Error)

-Small in magnitude -Always present; can be lessened but not eradicated -This type of error is from unknown and unpredictable variation in the experimental situation.

-You and your partner record different volumes of your reactant -A lab report asks you to monitor the color of a solution. You cannot tell if a solution is light blue or dark blue because you have no basis of comparison. -You cannot tell the exact start and end time of the experiment because you might have stopped your watch a few seconds too early or too late. -An instrument malfunctions

Systematic Error (or Determinate Error)

-Errors associated with particular instrument or techniques. Improperly calibrated instruments or techniques are one of the sources of this error. -An error that skews the results the same way every time -You should be able to understand and correct for systematic errors when the source is identified

-The balance is not set properly and is consistently reading 1 gram higher than it should. Account for this detail in your data, and you can find the more accurate weight (subtract 1g). -The automatic pipette is not calibrated correctly and delivers 1.01mL as opposed to 2.00mL (deliver 2 pumps).

Human Error (Mistakes)

-Mistake that causes a measurement to be much farther from the mean measurement than other measurements. -Any error that can be traced directly back to the mistakes that a person made

-You missed a portion of the procedure -Overshooting an endpoint -Using contaminated glassware -Someone mislabeled a bottle -The molarity of a certain reagent was miscalculated -Reagents were spilled

  4  

PROCEDURE (WORK IN PAIRS):

SAFETY PRECAUTIONS: Hydrochloric acid is corrosive. If you spill it or any other reagents on your skin, wash them off immediately. Wear safety goggles at all times. Wash your hands thoroughly at the conclusion of the experiment.

Materials:

General Equipment: Special Equipment: Reagents: 2 test tubes pH paper 0.50M CaCl2 1 glass stirring rod 2 Filter flasks 0.40M Na2CO3 2x 50 mL beakers Buchner funnel 0.50M HCl Weighing paper A rubber gasket Methanol 2 10 mL graduated cylinder Water trap attached Large clamp with a vacuum hose 1 small spatula 2 filter papers, size 5 1 Tweezer Centrifuge/Stands for test tubes Watch glass 1 Pasteur (glass) pipette 3 Plastic pipettes 1 vacuum hose 1 surgical tubing that can be attached to a Pasteur (glass) pipette Important Notes:

This lab consists of two separate experiments (Part A and B). In Part A, vacuum filtration is used to recover the reaction product (precipitates), while in Part B a centrifuge technique is used. Since precipitates take time to form and vacuum filtration requires a relatively long wait time, you and your partner need to design a new procedure based on the procedures given below for Parts A and B in order to minimize the amount of down time. Please draw a simple flow chart showing the new procedures in your lab notebook. It is important that you and your partner work together for this experiment. It is recommended that you use one workstation for Part A and the other workstation for Part B.

Also, to avoid confusion as you switch back and forth between Parts A and B, divide the lab notebook page in half for each part. In the beginning of both parts, prepare the mixture and then use either filtration (Part A) to purify, dry and weigh, or centrifuge and drying with N2 before weighing (Part B). In this experiment both techniques are repeated twice (for solution 1 and 2.) To avoid cumbersome repetition while still practicing both techniques, keep this strategy in mind: Each student should use the first prepared mixture to carry out either Part A or B. Then, for the second set of solutions, switch workstations. If you first worked on Part A (solution 1) now complete Part B for solution 2 and vice versa. A table is indicated at the bottom to clarify. Consult your TA with any questions about the rotation.

  5  

A basic flow chart for both of the experiments is shown below:

PART A

PART B

  6  

9. Weigh and record the mass of the filter paper. Place the filter paperinside the funnel.

10. Attach a vacuum hose between the side arms of two flasks. Be carefulnot to force it. Instead, gently turn the tubing to insert the sidearm ofthe flask.

11. Place the water trap with a vacuum hose into a second filter flask. Thenattach the end of hose to the vacuum line in the hood.

12. Turn on the vacuum and, using a water bottle, slightly wet the filterpaper in order to seal it inside the funnel.

13. Check the notebook if you wait for more than 25 minutes. Normallyafter 25 minutes, most precipitate has formed and settled on the bottomof the beaker.

14. Slowly pour the solution with the precipitate onto the middle of thepaper to avoid losing the solid underneath the edge of the paper. Youmay use a glass rod to help remove the precipitate from the beaker andto place it onto the filter. Just be sure to rinse the stirring rod with somewater to flush the crystals onto the filter paper. We want to collect asmuch precipitate as we can.

15. Allow the solid to dry on the filter paper by running the vacuum forabout 25–30 minutes if the filtered liquid is relatively clear.

16. After all of the liquid has passed through the filter paper and if theliquid in the flask is milky, you may turn off the vacuum, disconnectthe hose, and pour the liquid out of filter flask into the original beaker.Filter the liquid again by re-connecting the vacuum hose, turn on thevacuum, and pour the liquid on the filter paper. While you are waitingfor 25–30 minutes, you may go to Part B.

130 Chemistry 0330 Laboratory Manual n Brown University

!DepartmentofChemistry,

BrownUniversity

Figure 1. Vacuum Filtration Set-up

  7  

Part A: Precipitation and Vacuum Filtration

1. Dispense 25mL of 0.50M CaCl2 into a 50 mL beaker, and dispense 25mL of 0.40M Na2CO3 into another 50 mL beaker (This is going to be your stock solution for parts A and B).

2. Obtain a clean, dry graduated cylinder. Using the plastic pipette designated for 0.5 M CaCl2, rinse the graduated cylinder with 1-2 mL of the CaCl2 solution. Then, measure 4mL and pour it into a clean, dry plastic beaker. Record the exact concentration and volume used. NOTE: Do not place any solution extracted from the beaker back in!

3. Clean and dry the graduated cylinder and use the plastic pipette designated for 0.40 M Na2CO3 to rinse the graduated cylinder with 1-2 ml Na2CO3 solution. Then, measure out 3.5 mL and pour it into the beaker containing the calcium chloride. Record the exact concentration, volume and the starting time for precipitation.

4. Stir the solution with a stirring rod. Avoid losing precipitate by using some water to flush the crystals that stick to the stirring rod back into the beaker. Label the first beaker.

5. Repeat steps 1-3 to prepare a solution of 3.0 mL CaCl2 and 4.5 mL Na2CO3 in the second beaker. Please label the second beaker.

6. Let the precipitate set for about 25-30 minutes. You may go on to Part B. 7. Using Figure 1 along with the following guidelines, set up the glassware for the vacuum filtration

(observe as your TA demonstrates how to set up vacuum filtration) 8. Clean and clamp both vacuum filtration flasks in place. 9. Place the Buchner funnel on a rubber gasket and seat the gasket in the opening of the flask. 10. Weigh and record the mass of the filter paper and the watch glass. Place the filter paper inside the

funnel. 11. Attach a vacuum hose between the side arms of two flasks. Be careful not to force it Instead, gently

turn the tubing to insert the sidearm of the flask. 12. Place the water trap with a vacuum hose into a second filter flask. Then attach the end of hose to the

vacuum line in the hood. 13. Turn on the vacuum and using a water bottle, slightly wet the filter paper in order to seal it inside the

funnel. 14. Check the notebook to see if you have waited for more than 25 minutes. Normally after 25 minutes,

most of the precipitate has formed and settled on the bottom of the beaker. 15. Slowly pour the solution with the precipitate onto the middle of the paper to avoid losing the solid

underneath the edge of the paper. Use a glass rod to help remove the precipitate from the beaker and place it on the filter. Be sure to rinse the stirring rod with some water to flush the crystals onto the filter paper. Remember the goal is to collect as much precipitate as possible.

16. Allow the solid to dry on the filter paper by running the vacuum for about 25-30 minutes if the filtered liquid is relatively clear.

17. After all of the liquid has passed through the filter paper and the liquid in the flask is milky, turn off the vacuum, disconnect the hose, and pour the liquid out of the filter flask into the original beaker. Filter the liquid again by re-connecting the vacuum hose, turn on the vacuum, and pour the liquid on the filter paper. If the liquid in the Erlenmeyer flask is full, the vacuum will not work as effectively. Make sure you do not have excess liquid in the flask. While you are waiting for 25-30 minutes, you may go to Part B.

18. After the solid is dried, stop the vacuum by turning the knob in your workstation. Gently remove the filter paper from the funnel with a pair of tweezers. Place it in the watch glass.

19. Weigh the filter paper and the watch glass with the precipitate and record the mass. Subtract the mass of the filter paper and the watch glass.

20. Repeat steps 9-18 using the second piece of filter paper (weigh and record the mass) for the solution of 3.0mL of CaCl2

and 4.5mL of Na2CO3. 21. Dispose of the solids, used solutions, and unused solutions in the waste container at the back of the lab.

All Pasteur pipettes should be placed into the (red) sharps disposal container and plastic pipettes should be put in the (black) medical waste container.

  8  

Part B: Precipitation and Centrifuge

1. Label two clean and dry test tubes (test tube #1 and test tube #2). Mark your initials on both tubes, weigh them and record the mass of each.

2. Using a clean, dry plastic pipette, rinse a clean graduated cylinder with 1-2 mL CaCl2 (you might want to mark this plastic pipette, as it will be used to deliver CaCl2 from your beaker for the entire experiment).

3. Use a graduated cylinder to measure about 4mL of the 0.50M CaCl2 solution and place it in test tube #1. Never place unused CaCl2 solution back into your beaker containing the fresh CaCl2. Record the exact concentration and volume of CaCl2 delivered. Note that 10 mL graduated cylinders can be read to two decimal places.

4. Measure out about 3.0mL of the 0.50M CaCl2 solution and place it in test tube #2. Record the exact concentration and volume delivered.

5. Following the procedure outlined in steps 3 and 4, measure 3.5mL of 0.40M Na2CO3 and place it into test tube #1. Again, record the exact concentration and volume delivered.

6. Then, measure a 4.5mL of 0.40M Na2CO3 solution, and place it in test tube #2. 7. Record your observations of the reaction mixtures in both test tubes and the starting time for the

precipitation. Let them sit for about 25-30 minutes or until the precipitate has settled to the bottom of the test tube. While you are letting the two test tubes set, return to Part A.

8. Once the majority of the solid has collected at the bottom, ensure that the two test tubes have about the same amount of liquid in them. Add distilled water if necessary to bring the liquid up to the same level in both test tubes. Make sure the centrifuge is balanced (meaning that the test tubes are across from one another and each has about the same amount of liquid).

9. Ask your TA for assistance with the centrifuge. Your TA will centrifuge the mixtures (the cap of the centrifuge MUST BE LOCKED when running!) in test tubes #1 and #2 for 5 minutes. If the centrifuge device is busy, you may have to leave your test tubes on the stand next to the centrifuge. Once the TA finishes the centrifuge, your tubes will be put back in the stand for pick up (Note: Sometimes the tubes can break in the centrifuge, if only one tube breaks you do not need to start from the beginning. Continue the steps for your other sample and ask your TA to give you the data from other group for your lab report. However, if both of your samples break (very unlikely) – prepare them again in order to proceed with the next steps).

10. Use a clean plastic pipette to gently remove the supernatant from the test tube by pushing the air out of a plastic pipette before putting it into the supernatant. Make sure not to blow the air into the tube to disturb the newly formed precipitates. Be gentle. Try to keep your pipette as close to the surface as possible in order not to disturb the precipitate. Repeat several times, until all supernatant is gone.

11. Add about 1.5 mL methanol to the test tube. The concentration and volume of methanol is insignificant, as it does not participate in the reactions. The volatile methanol used here is to make the solid products dry faster.

12. Centrifuge the tubes for another 5 minutes (again, make sure the centrifuge is balanced by adding a small amount of methanol to balance the mass).

13. After the centrifuge, follow the same procedure to remove the methanol. 14. Hook the plastic tubing to the valve marked “N” in your hood. (This is the nitrogen line.) To the other

end, attach a Pasteur (glass) pipette (Note: Pasteur pipettes are very fragile. Also, make sure you maintain the space between the pipette and the sample you want to dry.).

15. Begin the flow of nitrogen by turning the nozzle. Use a very gentle stream of nitrogen to evaporate the liquid (test the flow by blowing it on your hand first). If too much nitrogen gas is blown through the tube, the glass pipette may launch out of the tubing and shatter. Also, be careful not to blow the precipitate out of test tube #1.

16. After the precipitate has dried for about 5 minutes, reweigh each test tube and record your findings (read up to four decimal places)

17. Obtain 10 mL of 0.50M HCl in a clean, dry graduated cylinder. Record the concentration and exact volume dispensed. Weigh the graduated cylinder and record the data.

18. Using a clean and dry plastic pipette (that has been rinsed with a bit of HCl), dispense about 1mL from the graduated cylinder into test tube #1.

19. Record your observations.

  9  

20. Continue adding HCl slowly (drop wise) until all or most (there may be a few stubborn flecks) of the precipitate has dissolved and the solution stops bubbling. Use a clean spatula or a clean glass rod to stir the solution each time after you add HCl.

21. NOTE: If there is ever any HCl left in your pipette, place it BACK into the graduated cylinder. You are using the graduated cylinder to determine how much HCl you used. None should be placed into the waste beaker until the end of the experiment. This is a method of back titration. Normally, you would not place unused solution back into the graduated cylinder (to avoid contamination), but we are using this method as a crude and quick way to measure the amount of acid necessary for the back titration. (A buret would have been a more accurate device for the titration).

22. Record the amount of HCl used to complete the reaction (by volume in mL, reading up to 0.01mL). 23. Record the weight of the graduated cylinder and HCl after the reaction. 24. Test the pH of the solution in test tube #1 with pH paper. 25. Repeat steps 14-21 with test tube #2. 26. Pour the contents of the test tubes into your waste beaker. Wash all glassware.