Activity 1: Limiting Reagent for a Chemical
        Reaction in Solution

        LABORATORY
        ACTIVITY:
        TEACHER
        NOTES                                                                   .
         Major Chemical Concept:
            The major chemical concept is the stoichiometry and the determination of the limiting reactant in a chemical reaction

        Level
            Honor students.
         

                    Expected Student Background
                            Students should be capable of accurately weighing items on whatever balance is being used. A balance such as a CentigramÔ balance should
                            suffice for this experiment. The correct method of using a centrifuge should be demonstrated (or reviewed if used before). Related concepts
                            include balancing equations, molarity, and stoichiometric calculations and the use of limiting reactant in such calculations.
     
              Time
                    Because of a bottleneck that can occur with using the centrifuge, and the need to allow about 20 min for drying precipitates in the drying oven
                    and 5-10 min needed for cooling, about 80 min is required to complete this experiment. The experiment can be conveniently broken into two
                    parts with Steps 1 through 10 being conducted one day and the experiment being completed the second day. If you intend to complete the
                    experiment in one day you should plan to conduct the pre-laboratory discussion the day before the experiment. Teacher/technician
                    preparation time for the experiment is about 45 min, mainly devoted to making the two required solutions and calibrating the BeralÔ pipets.

              Safety
                    Read the Safety Considerations in the Student Version. Barium ion in solution, or in a soluble compound, is highly toxic if ingested; therefore,
                    students should wash their hands thoroughly before leaving the laboratory. Barium compounds must not be discarded down the drain. There
                    should be a waste container for these salts and solutions. After all students have completed the activity, you can convert the sample to very
                    insoluble barium sulfate by adding excess sodium sulfate. The barium sulfate waste can be filtered, dried, and stored for proper disposal
                    Safety precautions related to using a centrifuge should be stressed—be sure to place a second tube containing the same amount of material in
                    a tube holder opposite the holder containing the sample. Because students may be in a hurry at the time when they are using the centrifuge,
                    stress that the centrifuge should not be manually stopped by the insertion of fingers or other objects. If safety considerations don’t impress
                    students you may point out that quickly stopping a centrifuge tends to bring the solid back up into the liquid, thereby undoing what centrifuging
                    has just accomplished. Also warn students to keep ties and long hair away from the spinning centrifuge. Occasionally a test-tube breaks while
                    being centrifuged. This usually does not result in flying glass but students should be given instructions about cleanup procedures.  Probably the
                    most common accident involves finger burns when placing glassware into or removing it from the drying oven or handling glassware that has
                    not sufficiently cooled. Proper techniques for handling hot glassware should be reviewed before the experiment.
       

      Materials (For 24 students working in pairs)

      Balances, CentigramÔ or equivalent
      Centrifuge
      Drying oven

      Nonconsumables
      48 Test-tubes, 13 x 100-mm or larger to fit available centrifuge
      24 Beakers, 50-mL
      24 Beakers, 250-mL
      36 Plastic BeralÔ pipets, stems calibrated to 1.0 mL
      12 Stirring rods
      12 Wash bottles filled with distilled water

      Consumables
      1.0 M Barium chloride, BaCl 2 ·2H 2 O, 200 mL (48.8 g BaCl 2 ·2H 2 O per 200 mL)
      0.50 M Sodium sulfate, Na 2 SO 4 ·10H 2 O, 250 mL (40.3 g Na 2 SO 4 ·10H 2 O per 250 mL)
       

      Advance Preparation

      Solutions should be prepared as above. If the Na 2 SO 4 in your stockroom is in the anhydrous form, 17.8 g per 250 mL yields the desired 0.50 M
      solution.  All glassware should be clean and dry. If students have to clean and dry the glassware, particularly the test-tubes, before beginning they will
      not have sufficient time to complete the activity. If the plastic BeralÔ pipets are to be reused they should also be carefully washed to avoid
      cross-contamination during reuse and all visible traces of moisture removed.  The BeralÔ pipet stems may be calibrated for 1 mL by filling a 10-mL
      graduated cylinder to the 10 mL mark with distilled water and drawing sufficient water into the pipet to bring the water in the cylinder to the 9 mL mark. The water level in the pipet stem should then be marked with a waterproof felt marker.  The drying oven should be turned on at the beginning of the
      class so that it comes to drying temperature (120-130 °C) by the time it is to be used. The thermostat settings should be determined the day before the
      laboratory.  Always try demonstrations before doing them for a class. Wear safety goggles when doing any chemical demonstration. Concentrated ammonia is an eye, nose, and throat irritant. Be sure you have adequate ventilation. Dry ice is extremely cold and can cause burns. Handle with care. Use gloves.
       

            Pre-Laboratory Discussion

               The pre-laboratory discussion should take place the day before the activity is to be performed if it is to be completed in one 80-min period.

            1. Review safety considerations and disposal methods involved with the handling of the chemical substances being used.

            2. Emphasize the necessity not to mix the BeralÔ pipets during the activity-one should be used for the barium chloride solution and another used for the
                sodium sulfate solution, and the third for removal of the supernatant liquid.

            3. Make sure that students understand the difference between amount and concentration of a solution.

            4. Review mole and molarity calculations as well as reaction stoichiometry calculations. The net-ionic equation for the reaction in this experiment is:

                                                                            Ba 2+ (aq) + SO 4 2– (aq) ® BaSO 4 (s)

                The overall reaction is:

BaCl 2 (aq) + Na 2 SO 4 (aq) ® BaSO 4 (s) + Na 2 SO 4 (aq)

                Therefore, one mole of BaCl 2 reacts with one mole of Na 2 SO 4 . You should be able to elicit this information from your students if they have studied
                chemical stoichiometry.

            5. Review the procedure used and safety considerations in operating the centrifuge and the drying oven.

            6. Explain that students can use the calibration marks on the beakers to measure the 10- and 15-mL volumes needed in Step 2 as only approximate
                volumes are necessary.

            7. Because the test-tubes are to be put in the oven, labeling in Step 3 should not be made with a wax marking crayon or with paper labels.
 

            Teacher-Student Interaction

            One problem that arises is the fineness of the barium sulfate precipitate. Even after extended centrifuging the precipitate may not settle; some precipitate
          may adhere to the sides of the test-tube. The adherence to the test-tube is not a problem, since the precipitate will remain in the test-tube and be weighed.
          The precipitate that does not settle is a very small fraction of the total precipitate and, if removed with the supernatant liquid, will cause a small percent error.
          However, you may want to tell students to add a small quantity of soap (not detergent) solution by dipping the stirrer in a soap solution, mixing the solution
          with the stirrer, and then recentrifuging.  This will usually aid the settling of the precipitate.  Monitor student use of the centrifuge. Any large vibration usually
          signals that the centrifuge is not balanced. When in a hurry, students tend to disregard the admonition against using fingers or other objects to stop the
          centrifuge.  Monitor the use of the drying oven. Students may need help in removing the hot beaker and test-tubes. It is best if all students place their
          beakers into the oven within minutes of each other. This strategy permits the oven to heat up to the required drying temperature and remain there, speeding
          drying.  Circulate around the room while students are doing their weighings. It is important  that the weighings be done properly and masses recorded to
          0.01g.

            Anticipated Student Results


 

               * The actual yield should be close to the theoretical yield. The factor 10 4 means that the quantities in columns 7-11 have been multiplied by that
              amount to yield numbers in the tens range. The actual quantity is obtained by reversing the process and dividing by 10 4 ; e.g., 8.6 = 8.6 x 10 –4
              (in Column 7; see Introductory Module for use of exponents).
 

            Answers to Implications and Applications
        1. No. Sodium sulfate was the solution of lower concentration, but the results in Tube 4 show that barium chloride was the limiting reactant.

        2. No. There were 2.0 mL barium chloride solution and 3.0 mL sodium sulfate solution in Tube 2 and sodium sulfate was the limiting reactant.

        3. You must know the stoichiometric ratio from the balanced equation, the concentrations of the reactants and the volumes of solutions mixed together
            in order to determine the limiting reactant and consequently the maximum amount of product to be expected.

        4. Brand A costs $ 0.092 per ounce of corn sweetener. Brand B costs $ 0.088 per ounce of corn sweetener. Brand B (17.8 ounces) has to be used to
            obtain 8.0 ounces of corn sweetener.

        5. Barium sulfate is so insoluble that a negligible amount of barium ions go into solution. The barium sulfate ingested in “liquid” during a gastrointestinal
            X-ray is really a slurry of barium sulfate in water similar to the slurry obtained in this activity when the barium sulfate precipitate is washed with distilled water.
 

             Possible Extensions
              Depending upon the students’ ability, the topics covered prior to the Solutions module and the time allotted,
              the following topics might be discussed:
              1. The overwhelming number of chemical processes that take place in solution.

              2. Introduction to the use of stoichiometric ratios in quantitative analysis.

              3. The need to introduce excessive concentrations of reactants to shift equilibrium.
 

        Assessing Laboratory Learning
        1. Laboratory Practical. The use of the centrifuge may be tested as part of a laboratory practical. Also, an “unknown”
            concentration of one of the solutions in this experiment may be presented and students asked how they would go about
            determining the concentration of that solution—or asked to actually do it.

        2. Written Examination. Problems involving the use of solutions and limiting reactant determination could be presented. Students
            could be asked to construct a “pictures in the mind” illustration of the limiting reactant for the BaCl 2 -Na 2 SO 4 reaction
            similar to the H 2 -O 2 reaction illustrated in the Stoichiometry module.


TABLE OF CONTENTS TOPIC OVERVIEW CONCCEPT/SKILLS DEVELOPMENT LINKS/CONNECTIONS REFERENCES