Supernatant: The liquid layer lying above the solid layer after a precipitation reaction occurs.

Transcription

1 Limiting Reagent Introduction The quantities of substances involved in a chemical reaction represented by a balanced equation are often referred to as stoichiometric amounts. Solution stoichiometry is often used in the laboratory setting to determine the quantities of each reactant and how they will consume one another to produce the desired product. This is calculated using mole-to-mole ratios. Often one reagent is consumed completely leaving the other reagent in excess. This concept is referred to as the limiting reagent concept. This week s lab will use solution stoichiometry to determine the limiting reagent from the exact quantities dispensed. The supernatant from the resulting precipitation reactions will be studied to ensure the correct limiting reagent and excess reagent was identified from the stoichiometric calculations. Supernatant: The liquid layer lying above the solid layer after a precipitation reaction occurs. For more information: Chemistry: Atom s First by OpenStax sections 7.3 Reaction Stoichiometry and 7.4 Reaction Yields Equations to use for the calculations: VTotal = VFinal - VInitial Mass of precipitate = mass of filter paper with precipitate mass of filter paper Molarity = moles solute L solution % Yield = Actual Yield X 100 Theoretical Yield Materials: 9 test tubes test tube rack 2 25 ml burets 3 plastic funnels vortex mixer 3 small ring clamps 3 50 ml beakers 0.5M CaCl2 0.5M Na2CO3 ring stand 3 watch glasses filter paper hot plate plastic pipets DI water calculator Procedure 1. Label 3 clean test tubes A1, A2, and A3. 2. Condition a ml buret with a few ml of the 0.5M calcium chloride solution, then fill the buret with the CaCl2 solution. See technique Using a buret to deliver solution. Drain a small amount of the CaCl2 solution into your waste beaker so it fills the buret tip (with no air bubbles present). Record the exact molarity of the CaCl2 solution on your report sheet in the data section. 3. Add approximately 4 ml of 0.5M CaCl2 solution to the three test tubes A1-A3. Record initial and final buret readings on your report sheet. GCC CHM 151LL: Limiting Reagent GCC, 2017 page 1 of 9

2 4. Condition a ml buret with a few ml of the 0.5M sodium carbonate solution, then fill the buret with the Na2CO3 solution. See technique Using a buret to deliver solution. Drain a small amount of the Na2CO3 solution into your waste beaker so it fills the buret tip (with no air bubbles present). Record the exact molarity of the Na2CO3 solution on your report sheet in the data section. 5. Add approximately 1 ml of 0.5M Na2CO3 solution to test tubes A1 only. Record initial and final buret readings on your report sheet. 6. Add approximately 4 ml of 0.5M Na2CO3 solution to test tubes A2 only. Record initial and final buret readings on your report sheet. 7. Add approximately 6 ml of 0.5M Na2CO3 solution to test tubes A3 only. Record initial and final buret readings on your report sheet. 8. Place each test tube on the vortex mixer (set to Auto/Touch; adjust the dial to a low-medium setting (a 4-5 setting on the vortex) for about seconds. Once all six solutions have been thoroughly mixed, place them in the test tube rack and allow them to settle for 10 minutes. Record the start and stop time under observations. 9. Under the observations section, draw representations of test tubes A1, A2, and A3 showing liquid levels and solid levels. 10. Obtain three medium-flow filter papers and label them in pencil on their edges with a small A1, A2 and A3. Weigh each filter paper and record their masses on your report sheet. Obtain three 50 ml beakers and label them A1, A2 and A Set up a filtration apparatus with a plastic funnel in a ring stand. Fold and insert the filter paper A1 into the funnel and place the 50mL beaker A1 beneath the funnel to catch the filtrate. 12. Pour the contents of test tube A1 into the funnel, using a D.I. wash bottle to wash any remaining precipitate out of the test tube into the filter paper. When the liquid had passed through filter, wash it with about 3mL of D.I. water. When this liquid had passed through the filter, repeated the wash with another 3mL of D.I. water. SAVE the filtrate for each trial to use in the supernatant test. 13. Remove the filter paper and carefully open it up, placing it on a watch glass. Place the watch glass on a hot plate (set to heat setting 4, or 90 C) to dry. Record the time drying started and ended in your notebook under observations. While the filter paper A1 is drying, repeat steps 2-4 above with A2. While the filter paper A2 is drying, repeat steps 2-4 with A3. (You may filter all three simultaneously with three ring clamps, three funnels, and three beakers if you wish. Please dry each on a separate hot plate, though.) 14. Complete all of your calculations while waiting for the solids to dry. 15. Weigh each filter paper when dry and record its new mass on your report sheet. 16. Test the ions dissolved in the filtrate solutions: Supernatant Test a. Transfer a small amount of the supernatant solution from trial A1 to two different clean test tubes with a disposable pipette. Use new disposable plastic pipettes for each test to prevent cross-contamination. b. Add about 1 ml CaCl2 solution to the first new test tube and about 1 ml Na2CO3 solution to the second. Record your observations. c. Repeat steps a-b for the filtrates from A2 and A3. Remember: use new pipettes for each test. GCC CHM 151LL: Limiting Reagent GCC, 2017 page 2 of 9

3 Clean-Up: Rinse everything extremely well with vinegar solution first then soapy tap water followed by a quick DI water rinse. Clean your benchtop. Put all equipment back exactly where you found it. GCC CHM 151LL: Limiting Reagent GCC, 2017 page 3 of 9

4 Name: Limiting Reagent Pre-lab assignment Purpose: Summary of procedure: Drawing of apparatus used: Instructor signature: Pre-lab points: Date: GCC CHM 151LL: Limiting Reagent GCC, 2017 page 4 of 9

5 Name: Limiting Reagent Pre-Lab Questions and Calculations 1. (1 pt) Describe the concept of limiting reagent using an everyday analogy. 2. (1 pt) Complete the beaker drawings for the two reagents used in this week s Limiting Reagent Experiment. Formula Reagent 1: Formula Reagent 2: 3. (1 pt) Write the balanced chemical reaction for the reagents used in this week s Limiting Reagent Experiment. Molecular Equation: Total Ionic Equation: Net Ionic Equation: 4. (2 pts) A student places 4.50 ml of 0.25M potassium sulfide, K2S, solution into a test tube. The student then adds 6.10 ml of 0.35M aluminum nitrate, Al(NO3)3, solution to the same test tube and observes the formation of a solid precipitate. Calculate the theoretical mass of precipitate formed. Balanced Equation: 3 K2S (aq) + 2 Al(NO3)3 (aq) 6 KNO3 (aq) + Al2S3 (s) GCC CHM 151LL: Limiting Reagent GCC, 2017 page 5 of 9

6 Name: Partners: Limiting Reagent Lab Report Data: Molarity of CaCl2: Molarity of Na2CO3: Data Table 1: Buret Readings for CaCl2 and Na2CO3 Trial A1 A2 A3 Initial Volume CaCl2 Final Volume CaCl2 Initial Volume Na2CO3 Final Volume Na2CO3 Data Table 2: Mass Recordings for Precipitates Trial A1 A2 A3 Mass of filter paper Mass of filter paper with precipitate Observations: Balanced Chemical Reaction: Test Tube Drawings: GCC CHM 151LL: Limiting Reagent GCC, 2017 page 6 of 9

7 Supernatant Test: Trial A1 A2 A3 Predicted reagent in excess Observation after adding CaCl2 (aq) Observation after adding Na2CO3 (aq) Calculations for Data Table 1: (Show one example set of calculations for CaCl2 and one for Na2CO3) CaCl2: Na2CO3: Calculations for Data Table 2: GCC CHM 151LL: Limiting Reagent GCC, 2017 page 7 of 9

8 Results: Results Table: Trial A1 A2 A3 Volume CaCl2 (VTotal) Volume Na2CO3 (VTotal) Theoretical mass of precipitate Limiting reagent formula Excess reagent formula Actual mass precipitate Percent yield Discussion Questions: 1. (5 pts) Aspirin, C9H8O4, can be produced in the laboratory by combining salicylic acid, C7H6O3, with acetic anhydride, C4H6O3. If you were working for a pharmaceutical company and you reacted 10.0 g of salicylic acid with 10.0 g of acetic anhydride and found you only achieved an 86.4% yield, what mass of aspirin did you actually produce? 2 C 7 H 6 O 3 (s) + C 4 H 6 O 3 (l) 2 C 9 H 8 O 4 (s) salicylic acid acetic anhydride aspirin GCC CHM 151LL: Limiting Reagent GCC, 2017 page 8 of 9

9 2. (9 pts) In the beakers below, using your calculation results, draw what the filtrate solutions contained for each trial. Show the precipitate and the excess reagents in the beakers. Trial A1 Trial A2 Trial A3 3. (4 pts) If sodium carbonate is the limiting reagent and calcium chloride is in excess: a. Which ions are present in the supernatant? b. What would happen if you added CaCl2 to the supernatant? c. What would happen if you added Na2CO3 to the supernatant? 4. (4 pts) If calcium chloride is the limiting reagent and sodium carbonate is in excess: a. Which ions are present in the supernatant? b. What would happen if you added CaCl2 to the supernatant? c. What would happen if you added Na2CO3 to the supernatant? Conclusion: Summarize the results for determining the limiting reagent and percent yield in this week s lab. Be sure to include data and three sources of error. GCC CHM 151LL: Limiting Reagent GCC, 2017 page 9 of 9