Materials Included In Kit

Additional Materials Required

Prelab Preparation

Solution B: Prepare a 0.05 M sodium meta-bisulfite solution by first weighing out 1.9 g of powdered sodium meta-bisulfite. Add to a clean, dry 500-mL stoppered bottle. Add 200 mL of distilled or deionized water directly to the bottle containing the 1.9 g of powdered sodium meta-bisulfite. Shake well to dissolve the solid. The sodium meta-bisulfite solution has a poor shelf life. Prepare this solution fresh (within one week of performing the lab).

Starch Solution: Using a spray starch, generously spray the starch into 50 mL of room temperature distilled or deionized water then stir to mix. An alternative preparation is to mix 5 mL of liquid starch with 50 mL of distilled or deionized water.

Safety Precautions

Sulfuric acid is severely corrosive to eyes, skin and other tissue. Sodium meta-bisulfite is a skin and tissue irritant and is slightly toxic by ingestion. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review 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. Sulfuric acid should be neutralized according to Flinn Suggested Disposal Method #24b. Potassium iodate should be reduced according to Flinn Suggested Disposal Method #12a. Sodium bisulfite should be oxidized according to Flinn Suggested Disposal Method #12b.

Teacher Tips

• Enough materials are provided in this Super Value Kit for 5 classes of 30 students each, working in pairs (75 total student groups). The micro strips and polystyrene cups are completely reusable.
• Use distilled water only.
• Be sure to use a fresh starch solution, oth­er­wise the dark-blue color complex may not be formed.
• Be sure to have the students stir the liquid in each well with a plastic toothpick, when there is more than one liquid present in the well.
• Better results are obtained when the module con­tain­ing the larger amount of liquid (solution A) is placed on the top (upside down).
• In the temperature-effect portion of the ex­peri­ment, a beaker of ice water can be used. For a hot water trial, the water must be very hot (90+ degrees C) to have any effect. Water at such a high tem­pera­ture is too hot to handle in the Beral pipet and not suggested in the experiment.
• For the catalyst, use a 0.05 M of sulfuric acid. Remember to inform the students that at this part of the experiment, they will replace 1 drop of water with 1 drop of the acid.
• When the catalyst is present, the reaction will take place in about 2 seconds! One catalyst trial is suf­fi­cient to observe the effect.
• Use of a cotton swab to dry the inside of the wells of the micro strip works great.
• Remember to have the student perform each trial twice.
• The manner in which the data is collected with respect to average time is called approximate average time. Instead of reproducing the reaction eight separate times and calculating the average, one can run the eight reactions simultaneously by noting the time of the first reaction observed and the time of the last reaction observed and averaging the two tim­ings. This will give you the approximate average time.
• Each trial is done twice, to ensure some sort of precision. If the two approximate averages are ex­tremely different, a third trial should be done, then average the data of the two that are closest to each other.
• If the reaction is too fast, dilute solution A. If the reaction is too slow, add a small amount of sulfuric acid to solution B.
• The ice water may be stored in a polystyrene cup to keep the water cold for a longer period of time.

Sample Data

Part I. The Effects of Concentration

Answers to Questions

1. What effect does concentration have on the rate of a chemical reaction? Why?

   The rate of a chemical reaction increases as the concentration increases. This is due to a greater number of particles present in solution which increases the probability of a collision to take place and thus increases the probability of a chemical reaction to take place.

2. What effect does temperature have on the rate of a chemical reaction? Why?

   Decreasing the temperature of a system decreases the rate of a chemical reaction. Since the decrease in temperature is a decrease in the average kinetic energy of the particles, there is a decrease in the probability for a collision to take place between particles and thus decreases the probability of a chemical reaction to take place.

3. What effect does a catalyst have on the rate of a chemical reaction? Why?

   The addition of a catalyst increases the rate of a chemical reaction. This is due to the fact that a catalyst presents alternate pathways for a chemical reaction to take place.

Introduction

In this experiment, three factors that affect chemical reactions will be investigated and determined.

Concepts

• Reaction rates
• Temperature
• Catalysts
• Concentrations

Background

In order for chemical reactions to take place, there must be collisions among atoms, ions, or molecules with sufficient energy and proper orientation. The rate at which these collisions take place depends upon several factors, such as concentration of the reactants, temperature, and inhibiting or accelerating catalysts. The effects of these three factors will be demonstrated in this experiment.

The two principal solutions that will be used in this experiment are sodium meta-bisulfite (Na₂S₂O₅) which will ionize in water to produce acid (H⁺) and potassium iodate (KIO₃) which will be a source of iodide ions (I^–). The sodium meta-bisulfite solution will be referred to as solution B, and potassium iodate solution as solution A in this experiment. The starch solution will be the indicator, while the 0.05 M sulfuric acid will be used as the catalyst.

The chemical pathway is not completely understood; however, the following simplified version may help to understand the concept.

1. For solution A

   H₂O + KIO₃ → IO₃^– + K⁺

   For solution B
   H₂O + Na₂S₂O₅ → 2HSO₃^– + 2Na⁺

2. The IO₃^– reacts with the HSO₃^– to produce I^–

   IO₃^– + 3HSO₃^– → I^– + 3H⁺ + 3SO₄^2–

3. In the presence of acid, the I^– reacts with additional IO₃^– to produce I₂.

   6H⁺ + 5I^– + IO₃^– → 3I₂ + 3H₂O

4. The iodine (I₂) immediately reacts with any HSO₃^– present in the system to form iodide ions

   I₂ + HSO₃^– + H₂O → 2I^– + SO₄^2– + 3H⁺

5. When all of the hydrogen sulfite ions (HSO₃^–) are used up, then the iodine (I₂) begins to accumulate. Only then will the iodine be able to react with the starch to form the dark-blue color complex

   I₂ + starch → dark-blue color complex

Materials

Safety Precautions

Sulfuric acid is severely corrosive to eyes, skin and other tissue. Sodium meta-bisulfite is a skin and tissue irritant and is slightly toxic by ingestion. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Procedure

Part I. The Effects of Concentration

1. Using a clean, dry 1 x 8-well micro strip (Module 1), place 5 drops of solution A into each of the 8 wells. Set the module aside.
2. Using another clean, dry 1 x 8-well micro strip (Module 2), see Figure 1, place 1 drop of solution B and 1 drop of starch solution into each of these 8 wells. Using a toothpick, mix the solution in each well.
   {12149_Procedure_Figure_1}
3. Placing Module 1 on top and upside down, hold both modules together so that the wells of both modules are aligned (see Figure 2). When both modules are aligned, hold the modules so that a snap of the wrist will force the liquid in the upper module to move into the lower module. Simultaneously begin timing the reaction, too.
   {12149_Procedure_Figure_2}
4. Note the time (seconds) when the first color change is observed in any well, and note the time when the last color change is observed in a well. Average these two values and record the result.
5. Now rinse the two micro strips with warm water and dry. Use a cotton swab to dry the inside of the wells.
6. Repeat the process once again. Now calculate the average of the two average times and record the result.
7. Repeat steps 1 through 6 for each of the combinations as indicated in the following table.
   {12149_Procedure_Table_1}
   Remember to stir the solution in each well before reacting them.

8. Using a clean, dry 1 x 8-well micro strip (Module 1), place 1 drop of solution A into each of these 8 wells. Now place 4 drops of ice water into each of the wells and mix with a toothpick. Set the module aside.
9. Using another clean, dry 1 x 8-well micro strip (Module 2), place 1 drop of solution B and 1 drop of starch solution into each of these 8 wells. Using a toothpick, mix the solution in each well.
10. Placing Module 1 on top and upside down, hold both modules together so that the wells of both modules are aligned (see Figure 2 in Part I). When both modules are aligned, hold the modules so that a snap of the wrist will force the liquid in the upper module to move into the lower module. Simultaneously begin timing the reaction, too.
11. Note the time (seconds) when the first color change is observed in any well, and note the time when the last color change is observed in a well. Average these two values and record the result.
12. Now rinse the two micro strips with warm water and dry. Use a cotton swab to dry the inside of the wells.
13. Repeat the process once again. Now calculate the average of the two average times and record the result.

14. Using a clean, dry 1 x 8-well micro strip (Module 1), place 1 drop of solution A, 1 drop of sulfuric acid (0.05 M), and 3 drops of water into each of the 8 wells. Now mix the solution with a toothpick. Set the module aside.
15. Using another clean, dry 1 x 8-well micro strip (Module 2), place 1 drop of solution B and 1 drop of starch solution into each of these 8 wells. Using a toothpick, mix the solution in each well.
16. Placing Module 1 on top and upside down, hold both modules together so that the wells of both modules are aligned (see Figure 2 in Part I). When both modules are aligned, hold the modules so that a snap of the wrist will force the liquid in the upper module to move into the lower module. Simultaneously, begin timing the reaction.
17. Note the time (seconds) when the first color change is observed in any well, and note the time when the last color change is observed in a well. Average these two values and record the result.
18. Now rinse the two micro strips with warm water and dry. Use a cotton swab to dry the inside of the wells.
19. Repeat the process and calculate the average of the two average times. Record the result.

Student Worksheet PDF

12149_Student1.pdf