Materials Included In Kit

Additional Materials Required

Safety Precautions

Barium chloride solution is toxic by ingestion. Sodium bisulfite solution is a severe skin and tissue irritant and is slightly toxic; it has a slight sulfur odor; keep tightly capped. Avoid contact with all body tissues. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please consult current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Soluble barium salts must be disposed of by precipitating all barium ions with sodium sulfate to form the insoluble barium sulfate. Sodium sulfate is included in this kit for this purpose. Or barium salts can be disposed of according to Flinn Suggested Disposal Method #27h. The insoluble barium sulfate that is produced may be then be disposed of according to Flinn Suggested Disposal Method #26a. Excess potassium iodate solution can be disposed of according to Flinn Suggested Disposal Method #12a, or saved for further laboratory use.

Lab Hints

• The kit includes 75 reusable test tubes—5 per student group—each 16 x 100 mm in order to hold 15 mL of solution. Each test tube should have a labeled pipet in order to extract samples of supernatant liquid; 75 pipets are provided in the kit. Pipets can be rinsed and reused or disposed of after the lab.
• Also included are 30 reusable 10-mL syringes—2 per student group—in order for students to add accurate volumes of the solutions to the tubes.
• Measurement of the heights of the precipitates is the largest source of error in this lab. This is due to different settling rates, the rounded bottoms of the tubes, etc. If data are to be used for anything more quantitative than a visual reference, we recommend averaging the class height data.
• Be sure to have students label each test tube and pipet in order to avoid cross-contamination when the liquid supernatant sample is drawn from the test tube.
• Reaction plates, 24-well, (Catalog No. AP1447) are most convenient for the chemical testing and allow the results to be saved for the entire stage. If reaction plates are not available, small test tubes or transparency sheets can be used.
• Provide a disposal container labeled “Barium Waste” for any waste solutions generated in this lab. See the Disposal section for further information.

Teacher Tips

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. This laboratory activity can reasonably be completed in two 50-minute lab periods. Data analysis can be completed out of class or can be done at the end of lab on each day.

• The lab is best done in three stages, as shown in the student data sheet. If possible, stages 1 and 2 should be done on Day 1. Stopping to discuss the results and calculations after stage 2 is useful before moving on to stage 3, which can be done on Day 2. This suggestion may or may not be feasible depending on your allotted lab time.
• Stage 1—In the first stage, increasing amounts of barium solution are added to a constant amount of iodate solution. The calculations are relatively straightforward and the pattern of volume to millimoles added will most likely be easily understood (from the balanced equation). Students see that once the volume of barium solution is greater than that of the iodate, the barium appears in excess (as one would expect). Tube B may show excess Ba²⁺ or excess IO₃^– depending on the precision of the volume measurements. Since different groups will see different results, this is an excellent opportunity in group discussion to reinforce the need for care in measurement before the next, more challenging sets are attempted.
• Stage 2—In stage 2, increasing volumes of iodate solution are added to a constant amount of barium solution. This time, barium remains in excess until the volume of iodate is twice that of barium. This result forces students to consider the mole ratio from the balanced chemical equation in order to explain the results. Class discussion will reinforce the concept.
• Stage 3—The third stage of five combinations uses a constant total volume but challenges students to explain the increasing and then decreasing heights of precipitate and to rationalize their measurements with the amounts of each reactant in each tube. The calculation of moles follows a now familiar pattern so this set serves as an effective test of their grasp of the underlying concepts.

Sample Data

Answers to Questions

Calculations for Data Analysis Section

1. Note: Sample calculations for #1 are shown for stage 1 only. Using the volume and concentration (0.2 M) of each starting material, calculate the number of starting millimoles (mmol)of Ba²⁺ and IO₃^– that were combined. Record this value in the appropriate data chart.

   A. 1 mL Ba²⁺ x 0.2 M = 0.2 mmol Ba²⁺ 
   B. 3 mL Ba²⁺ x 0.2 M = 0.6 mmol Ba²⁺
   C. 5 mL Ba²⁺ x 0.2 M = 1.0 mmol Ba²⁺
   D. 7 mL Ba²⁺ x 0.2 M = 1.4 mmol Ba²⁺
   E. 9 mL Ba²⁺ x 0.2 M = 1.8 mmol Ba²⁺
   A–E. 3 mL IO₃^– x 0.2 M = 0.6 mmol IO₃^–

2. Use the balanced chemical equation for the reaction of barium ions with iodate ions from the Background section. What is the mole ratio of each reactant and product?

   Ba²⁺(aq) + 2IO₃^–(aq) → Ba(IO₃)₂(s)
   
   The Ba²⁺:IO₃^–:Ba(IO₃)₂ mole ratio is 1:2:1. If Ba²⁺ is the limiting reagent, then for every one mole of Ba²⁺, one mole of Ba(IO₃)₂ is produced. If IO₃^– is the limiting reagent, then for every one mole of IO₃^–, one-half of a mole of Ba(IO₃)₂ is produced.

3. Use the equation to calculate the number of millimoles of solid Ba(IO₃)₂ precipitate that are expected to form in each tube (A–E). Note which material is in excess and which is the limiting reagent (LR). Record the expected millimoles of precipitate in the data chart. Also record which material is expected to be in excess and the amount of excess in mmol. Record the limiting reagent for each tube. [Hint: Review your data chart to determine which reagent was completely used up—the limiting reagent. Start with mmoles of that limiting reagent (from #1) to calculate the mmoles of precipitate expected from the balanced equation.]

   Stage 1 (Tube A)—IO₃^– was determined to be in excess. This indicates that Ba²⁺ was the limiting reagent. Therefore, the expected amount of product produced must be calculated from the Ba²⁺. The mole ratio from the balanced equation shows that 1 mole Ba²⁺ produces 1 mole Ba(IO₃)₂. So 0.2 mmol Ba²⁺ will produce 0.2 mmol precipitate. To determine how much excess IO₃^– remains, we know from the balanced equation that 1 mole Ba²⁺ reacts with 2 moles of IO₃^–; therefore, 0.2 mmol Ba²⁺ reacts with 0.4 mmol IO₃^–. Starting with 0.6 mmol IO₃^– minus 0.4 mmol IO₃^– leaves 0.2 mmol in excess.

4. Compare the calculated number of mmol of precipitate expected (from #3) with the height ratios found in the tubes. Account for any patterns that are observed. Determine the point at which each chemical becomes the limiting reagent forthe reaction.

   Student answers will vary as the height ratios will vary. In general, students should be able to qualitatively observe the general patterns of amount of precipitate that formed in each case.

Discussion

Many different combinations of chemicals have been proposed to illustrate the principle of limiting reagents. Most involve a precipitation reaction with subsequent measurement of the height of precipitate as an indicator of the amount of product derived from the available reagents. To be an effective experiment, however, the precipitate should be dense and regular in texture at all combinations of concentrations in order to give fast-settling precipitates and consistent measurements. (i.e., PbI₂ crystal size is different with excess Pb²⁺ or I^– and Cu(OH)₂ changes texture when the concentrations of OH^– vary.) The ratio should not be 1:1 to show dependence on reacting mole ratios and not on simple logic.

The barium iodate reaction in this kit meets these criteria and includes an easy, visual chemical test to show the presence of the ion in excess. This allows students to see that one ion is no longer available in solution (limiting reagent) while the other is still unreacted (in excess). Students grasp the idea of the process, but often have trouble working through the calculations to prove the result quantitatively. The kit also contains a set of data sheets and suggested sequence for doing the calculations that some students find confusing.

References

Special thanks to Doug De La Matter, chemistry teacher (retired), Madawaska Valley H.S. Barry’s Bay, ON, Canada for providing Flinn Scientific with the instructions for this laboratory activity.

Introduction

If you have 12 hamburger buns, 20 pieces of cheese, and 3 hamburger patties, how many cheeseburgers can you make? This question may be simple, but it can help us learn about an important idea in chemistry—limiting reagents. Let’s investigate the concept of limiting reagents with this lab activity.

Concepts

• Stoichiometry
• Limiting reagent
• Balanced chemical equations

Background

Most companies manufacture products made of other components. Having the correct quantity of each component is important to ensure efficient production. Ordering too many of one component creates unnecessary costs. If too few are available, production stops when the supply of that one component runs out. For example, no matter how many buns a fast food restaurant may have in stock, if the burgers run out, production stops. The burgers in this case could be called the limiting reagent.

Chemists often face similar situations. It is impossible for a chemist to make a certain amount of a desired compound if there is an insufficient quantity of any one of the required reactants. The balanced chemical equation is the chemist’s “recipe.” The coefficients in a balanced chemical equation tell us the whole number of moles of each reactant needed to complete the reaction in the correct proportions. In the example given, 1 hamburger bun + 1 hamburger patty + 1 piece of cheese → 1 cheeseburger. The whole numbers written in the equation are called coefficients. In chemical reactions, to ensure that the limiting reactant is completely used up (and the reaction “goes to completion”), most recipes call for more than the minimum amount of the other reactants. These reactants are referred to as being present “in excess.”

In this experiment, insoluble barium iodate Ba(IO₃)₂ will be prepared by mixing solutions of barium chloride and potassium iodate, according to Equation 1.

The solutions will be mixed in test tubes of uniform size, so that the amounts of Ba(IO₃₎₂ precipitate produced can be compared by measuring the heights of the solids in the tubes. Different amounts of each stock solution will be assigned and class results will be analyzed. Using stock solutions of known concentrations of barium chloride and potassium iodate allows small amounts of each chemical to be measured out quickly and accurately, in varying ratios.

After the amount of precipitate has been measured, you will determine which of the reactants—barium chloride or potassium iodate—is still present in the tube in unreacted form (the excess reagent) and which has been completely used up (the limiting reagent). To do this, the precipitate is allowed to fully settle. Some of the clear liquid above the precipitate (called the supernatant liquid) is then extracted with a pipet. This liquid can be tested for unreacted starting material using two different chemical tests.

Barium ions (Ba²⁺) react readily with sulfate ions (SO₄^2–) to make insoluble BaSO₄. When drops of the clear supernatant liquid are added to a sample of sodium sulfate solution, a dense white precipitate of BaSO₄ forms. This indicates that not all of the barium ions have been used up. In other words, barium ions are in excess and iodate is the limiting reagent.

Iodate ions (IO₃^–) are reduced to free iodine (I₂) by bisulfite ions (HSO₃^–). The resulting iodine then reacts with dissolved starch to form a dark blue complex. If a blue color appears when the clear supernatant liquid is added to bisulfite/starch solution, this indicates that not all of the iodate ions have been used up. In other words, iodate ions are in excess and barium is the limiting reagent.

Materials

Prelab Questions

Activity 

1. Obtain two 10-mL syringes—label one syringe as BaCl₂ and the other as KIO₃.
2. Label five test tubes A–E. (Note: All five test tubes should be the exact same height and diameter.)
3. Label five Beral-type pipets A–E.
4. Obtain five wood sticks for stirring.

Safety Precautions

Barium chloride solution is toxic by ingestion. Sodium bisulfite solution is a severe skin and tissue irritant and is slightly toxic; it has a slight sulfur odor; keep tightly capped. Avoid contact with all body tissues. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

STAGE 1. Constant Iodate Volume

Part 1. Formation of Barium Iodate

1. Fill the appropriately-labeled 10-mL syringe with 0.2 M barium chloride solution. Fill the other labeled syringe with 0.2 M potassium iodate solution. Prepare the syringes to deliver accurate volumes by removing any air from the tips.
2. Place the five labeled test tubes in a test tube rack.
3. Following the STAGE 1 Data Chart, add the assigned volumes of BaCl₂ and KIO₃ to test tubes A–E. (Note: Add the larger volume of solution second to ensure efficient mixing. For example, if 1 mL of BaCl₂ solution is to be combined with 3 mL of KIO₃ solution, add the KIO₃ last.) Stir the contents of each test tube with a separate wood stick.
4. Once the reagents have been thoroughly mixed, allow the solid precipitates to settle for about 3–5 minutes. Tap the tubes very gently so the precipitate settles evenly.
5. After the precipitates in each of the five tubes have settled undisturbed, use a metric ruler to measure the height in millimeters of solid in each tube. Record the height of each precipitate in the STAGE 1 Data Chart.

Part 2. Testing the Supernatant Liquid for the Excess Reagent

6. Obtain a 24-well reaction plate.
7. Use a Beral-type pipet labeled A to extract a small amount of the clear liquid from the top of tube A. (Note: Be careful to avoid drawing up any precipitate into the dropper.)
8. Add 3 drops of the liquid from pipet A to wells 1A and 1B in the well plate.
9. Test for Excess Ba²⁺: Add 3 drops of 0.1 M sodium sulfate solution to well 1A and make observations. A cloudy white precipitate indicates that there are still barium ions in the solution. Determine whether or not there is excess Ba²⁺ and record this result in the STAGE 1 Data Chart.
10. Test for Excess IO₃^–: Add 5 drops of sodium bisulfite/starch indictor solution to well 1B and make observations. An initial yellow followed by a dark blue color indicates that there are still iodate ions in the solution. Determine whether or not there is excess IO₃^– and record this result in the STAGE 2 Data Chart.
11. Repeat both tests (steps 9 and 10) using pipet B and the liquid from the top of tube B. Use wells 2A and 2B.
12. Repeat both tests (steps 9 and 10) for tubes C–E.
13. Clean out the test tubes, well plates, and pipets by combining all rinse solutions in a barium waste container as provided by your instructor. Do not dump any solutions down the drain.
14. Discuss class results before going on to STAGE 2.

STAGE 2. Constant Barium Volume

15. Perform steps 1–13 for STAGE 2 of the lab using the STAGE 2 Data Chart. Discuss class results before going on to STAGE 3.

STAGE 3. Changing Volumes of Iodate and Barium Ions

16. Perform steps 1–13 for STAGE 3 of the lab, using the STAGE 3 Data Chart. Discuss class results.

Data Analysis

Complete the following for each of the stages 1–3. Show all work on a separate sheet of paper.

1. Using the volume and concentration (0.2 M) of each starting material, calculate the number of starting millimoles (mmol) of Ba²⁺ and IO₃^– that were combined. Record this value in the appropriate Data Chart.
2. Use the balanced chemical equation for the reaction of barium ions with iodate ions from the Background section. What is the mole ratio of each reactant and product?
3. Use the equation to calculate the number of millimoles of solid Ba(IO₃)₂ precipitate that are expected to form in each tube (A–E). Note which material is in excess and which is the limiting reagent (LR). Record the expected millimoles of precipitate in the Data Chart. Also record which material is expected to be in excess and the amount of excess in mmol. Record the limiting reagent for each tube. [Hint: Review your data chart to determine which reagent was completely used up—the limiting reagent. Start with mmoles of that limiting reagent (from stage 1) to calculate the mmoles of precipitate expected from the balanced equation.]
4. Compare the calculated number of mmoles of precipitate expected (from stage 3) with the height ratios found in the tubes. Account for any patterns that are observed. Determine the point at which each chemical becomes the limiting reagent for the reaction.

Student Worksheet PDF

11998_Student1.pdf