Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Prepare a 0.05 M sodium meta-bisulfite solution by adding 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).
2. Prepare 200 mL of starch solution by making a smooth paste of 4 g soluble starch and 20 mL of distilled or deionized water. Pour the paste into 180 mL of boiling water while stirring. Cool to room temperature before using. Starch solution has a poor shelf life and will develop mold if kept for too long. Prepare fresh for use.

Safety Precautions

Potassium iodate is an oxidizer. It is moderately toxic by ingestion and a body tissue irritant. Sodium meta-bisulfite is a skin and tissue irritant and moderately toxic by ingestion. Sulfuric acid solution is corrosive to eyes, skin and other tissues. 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. Dispose of all products down the drain according to Flinn Suggested Disposal Method #26b.

Teacher Tips

• It is important to empty the syringe with a reasonable amount of force so that good mixing occurs between the solutions in the wells. But, if the syringe is emptied with too much force, the solutions will spill out of the desired wells. Therefore, urge students to find an emptying rate that ensures good mixing without spillage.
• As the beakers containing the solutions become emptied, it will be harder to fill the syringes without introducing air bubbles into them. If this occurs, have students tilt the beakers so that the liquid can be drawn up into the syringe without air bubbles. Each beaker contains two extra mL of solution to avoid this problem.
• Prepare a stock beaker containing the sulfuric acid solution. Have students bring their syringes to this stock solution and fill them directly from the stock solution.
• You may want to have stock “hot” and “cold” potassium iodate solutions for the students to use rather than have each of them prepare their own. In this case, students can bring their syringes to these stock solutions and fill directly from the stock container. This will reduce the need for two beakers per lab group.
• If six-well microplates are not available, small beakers, clear plastic cups, or other small clear containers may be used instead.
• If the calculations in Question 1 are difficult for your students, you may want to provide an example calculation that they can follow.
• A 0.1 N sulfuric acid solution is a 0.05 molar equivalent.

Sample Data

Answers to Questions

1. Fill in any remaining cells in the data table. Calculate the initial concentrations of the potassium iodate and the sodium meta-bisulfite solutions for each reaction both before and after combining the reactants. (Remember that these will not be the same because combining the reactants dilutes all of the solutions.)

   See Sample Data table. Before combining reactants, the concentrations of the potassium iodate solutions are simply calculated using M₁V₁ = M₂V₂. For example, in column 2, the initial solution, which is 0.05 M, is diluted by mixing 1 mL of water with 4 mL of the 0.05 M solution. Plugging into the equation gives

   (0.05 M)(4 mL) = (x M)(5 mL)
   x = 0.04 M

   Thus, the concentration after diluting, but before combining reactants, is 0.04 M.
   
   To calculate the sodium meta-bisulfite concentration before combining reactants, simply divide the stock concentration of 0.05 M by 2 since it is mixed with an equal volume of starch solution.
   
   To calculate the concentrations after combining reactants, again, use the M₁V₁ = M₂V₂ equation. For example, in column 2, once the potassium iodate solution is mixed with the sodium meta-bisulfite/starch solution, the total volume is 7 mL. Plugging into the equations gives 0.03 M.

   (0.04 M)(5 mL) = (x M)(7 mL)
   x = 0.03M

   Similarly, for all of the sodium meta-bisulfite solutions, the concentration after combining reactants is 0.007 M.

   (0.025 M)(2 mL) = (x M)(7 mL)
   x = 0.007 M

2. What effect does concentration have on the reaction rate (compare columns 1–5)? Explain.

   The rate increases as the concentration increases. This is because more reactant molecules are present in more concentrated solutions, increasing the probability of a collision.

3. What would you predict for the rate if 2.5 mL of water were mixed with 2.5 mL of the potassium iodate solution, then 2 mL of the sodium meta-bisulfite/starch solution were added? Explain how you arrived at your answer.

   The rate would fall between the rates determined for columns 3 and 4—probably about 12 seconds.

4. What effect does temperature have on the reaction rate (compare columns 6 and 7 with column 1)? Explain.

   The rate increases as the temperature is increased and decreases as the temperature is decreased. As the temperature is increased, the average kinetic energy of the sample is increased providing a sample with more molecules that possess enough kinetic energy to reach and overcome the activation energy barrier. As the temperature is decreased, the average kinetic energy of the sample is decreased which decreases the number of molecules that have enough kinetic energy to reach and overcome the activation energy barrier.

5. What would you predict for the rate if 5 mL of the potassium iodate solution was heated to 100 °C and then mixed with 2 mL of the sodium meta-bisulfite/starch solution? Explain how you arrived at your answer.

   The rate would be faster than that of column 6—probably about 1 second.

6. What effect does a catalyst have on the reaction rate (compare column 8 with column 5)? Explain.

   The addition of a catalyst increases the rate of a chemical reaction. In general, a catalyst provides a modified or new mechanism for the reaction that is faster than the original mechanism. The rate of the catalyzed reaction is faster because the activated complex in the catalyzed mechanism is of lower energy than the activated complex in the original mechanism. Hence the barrier to products is lower in the catalyzed reaction. A greater percentage of molecules will possess the needed energy to reach and overcome the activation energy resulting in a greater number of successful collisions between reactant molecules.

7. Predict how the reaction rate would change if the concentration of the sodium meta-bisulfite solution were changed instead of the potassium iodate concentration?

   The rate cannot be predicted. The rate depends on the concentration of each reactant individually according to the rate law. Just because the rate increases with the concentration of one reactant does not mean that the other reactants will have the same effect on the rate.

References

Cardillo, C.; Micro Action Chemistry; Flinn Scientific: Batavia, IL, 1998; Vol. 1, pp 85–87.

Introduction

What factors affect the rate of a chemical reaction? In this laboratory experiment, the effects of concentration, temperature and a catalyst on the reaction rate will be investigated.

Concepts

• Rates of reaction
• Catalysts
• Kinetics
• Clock reactions

Background

Collision Theory

In general, the effect of concentration, temperature and a catalyst on the reaction rate can be understood by looking at the energy profile for a given reaction.

In an energy profile diagram, the left side of the diagram represents the energy level of the reactants, while the right side represents the energy level of the products. In the energy profile diagram shown in Figure 1, the products are lower in energy than the reactants. In terms of thermodynamics, this reaction is exothermic (releases heat) and should occur spontaneously. However, not all exothermic reactions are spontaneous because not all collisions between reactants will produce products. The collision energy for a particular collision must exceed a critical energy for products to be formed. This critical energy is called the activation energy and is represented by the hump in the energy profile diagram.

Why must reactant molecules overcome this activation energy, or get over the hump, to reach products? As the reactant molecules approach each other, their atoms interact causing distortion in the bonds of both molecules. This distortion reaches a maximum as the reactants form an activated complex, or transition state. The activated complex is a hybrid species formed as the reactant molecules come together and interact to form products. Only those colliding molecules that have enough kinetic energy to reach this distorted intermediate will produce products. As is evident from the energy profile diagram in Figure 1, the potential energy of this distorted transition state determines the activation energy, or height of the barrier, for a particular reaction. If the barrier is low, almost all colliding molecules will have sufficient energy to reach and overcome the barrier. These reactions will occur spontaneously. If the barrier is high, only a small percentage of collisions will occur with sufficient energy to reach and overcome the barrier and go on to form products. These reactions occur much more slowly than those with a low barrier. In general, as the height of the barrier increases, the rate of the reaction decreases. Therefore, the rate of a reaction depends on the height of the barrier, or the activation energy.

The above description of the energy profile assumes the reaction occurs in a single step. This theory can be applied to multistep mechanisms by assuming that one of the steps in the mechanism is much slower than the other steps. This step then determines the rate of the reaction and is called the rate-determining step. It is generally a good approximation to say that the energy profile of a reaction describes the energy profile of the rate-determining step.

How to Increase the Rate of a Reaction

To increase the rate of a reaction, one of two things must occur: (1) more molecules with sufficient kinetic energy to overcome the barrier must be involved in the reaction to produce a higher number of successful collisions, or (2) the activation energy must be decreased.

One way to obtain a higher number of successful collisions is to increase the concentration of reactant molecules. Increasing the concentration of a reactant means more reactant molecules are present. The same fraction of collisions will produce products, but because more reactant molecules are present, more successful reactions will occur. Therefore, the probability that a successful reaction will occur increases.

Another way to obtain more molecules with sufficient energy to overcome the barrier is to increase the temperature. The strong temperature dependence of reaction rates can be understood by looking at the relationship between temperature and energy. The average kinetic energy (or motion of molecules) of a sample is directly proportional to the temperature. As the temperature is increased, the average kinetic energy of the sample is increased, providing a sample with more molecules that possess enough kinetic energy to reach and overcome the barrier.

To lower the activation energy, a catalyst may be added to the reaction mixture. A catalyst is a substance that, when added to a reaction mixture, participates in the reaction and speeds it up, but is not itself consumed in the reaction. In general, a catalyst provides a modified or new mechanism for the reaction that is faster than the original mechanism. The rate of the catalyzed reaction is faster because the activated complex in the catalyzed mechanism is of lower energy than the activated complex in the original mechanism. Hence the barrier to products is lower in the catalyzed reaction. A greater percentage of reactant molecules will possess the needed energy to successfully collide and overcome the barrier. Therefore, the rate of the reaction is increased.

Iodine Clock Reaction

In this lab, the reaction between potassium iodate and sodium meta-bisulfite to form iodine will be studied. This reaction is called the Iodine Clock Reaction. A starch solution serves as an indicator of the end of the reaction, forming a dark-blue colored starch–iodine complex in the presence of iodine. The chemical pathway for the formation of iodine is complicated and not completely understood, but the following mechanism serves as an outline.

Step 1: The sodium meta-bisulfite, Na₂S₂O₅, and potassium iodate, KIO₃, solutions contribute hydrogen sulfite ions, HSO₃^–, and iodate ions, IO₃^–, to the solution.

Step 2: The iodate ions react with the hydrogen sulfite ions to produce iodide ions, I^–.

Step 3: In the presence of hydrogen ions, H⁺, the iodide ions react with excess iodate ions to produce iodine, I₂.

Step 4: Before the iodine can react with the starch to produce a dark-blue colored complex, it immediately reacts with any hydrogen sulfite ions still present to form iodide ions.

Step 5: Once all of the hydrogen sulfite ions have reacted, the iodine is then free to react with the starch to form the familiar dark-blue colored complex.

The dark-blue color of the complex is due to the presence of the pentaiodide anion, I₅^–(aq). By itself, the pentaiodide ion is unstable; however, in this reaction it is stabilized by forming a complex with the starch. The appearance of the dark-blue color in solution indicates that all of the reactants have been used up and the reaction has gone to completion. Therefore, the rate of reaction can be measured by recording the time to the appearance of the dark-blue color.

Materials

Safety Precautions

Potassium iodate is an oxidizer. It is moderately toxic by ingestion and a body tissue irritant. Sodium meta-bisulfite is a skin and tissue irritant and moderately toxic by ingestion. Sulfuric acid solution is corrosive to eyes, skin and other tissues. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron.

Procedure

Part A. Preparation

1. Label one small beaker “Na₂S₂O₅/starch.” Add 10 mL of 0.05 M sodium meta-bisulfite solution plus 10 mL of starch solution to this beaker. Stir the contents with a stirring rod.
2. Label three small beakers “KIO₃,” “cold KIO₃” and “hot KIO₃.” Pour 18 mL of 0.05 M potassium iodate solution into the “KIO₃” beaker. Pour 5 mL of the 0.05 M potassium iodate solution into both the “cold KIO₃” and “hot KIO₃” beakers.
3. Set the “hot KIO₃” beaker on a hot plate. Allow it to warm to about 60 °C. Do not allow it to come to boiling.
4. Fill a 250-mL beaker about half-full with ice water. Immerse the “cold KIO₃” beaker in the ice water and allow it to cool to about 10 °C.
5. Label a fifth small beaker “water.” Pour 15 mL of distilled or deionized water into this beaker.
6. Place a paper towel or white piece of paper underneath the six-well microplate so that any color changes may be easily observed.

Part B. Effect of Concentration on the Reaction Rate

7. Measure the temperature of the room temperature potassium iodate solution (in the “KIO₃” beaker). Record this temperature in columns 1–5 in the data table.
8. Fill the 5-mL syringe to the 1-mL mark with distilled water by submerging the syringe in the “water” beaker and drawing water into the syringe until the plunger is at the 1-mL mark. (If the syringe has a tip cover, remove it before filling the syringe.)
9. Empty the syringe into well 2 of the six-well microplate (see Figure 2).
   {11795_Procedure_Figure_2}
10. Fill wells 3–5 with the amount of water indicated in Figure 2.
11. Now submerge the syringe in the “KIO₃” beaker and draw 5 mL of potassium iodate solution into the syringe so that the plunger sits at the 5-mL mark. Empty the syringe into well 1.
12. Fill wells 2–5 with the amount of potassium iodate solution indicated in Figure 2.
13. Stir each well with a stirring rod making sure that the stirring rod is rinsed and dried between each well to avoid contamination between wells.
14. Fill the 3-mL syringe to the 2-mL mark with the sodium meta-bisulfite/starch solution. Quickly, but carefully, empty the syringe into well 1 and start the timer. Time the reaction with a stopwatch or timer by measuring the time from when the solution was added until the appearance of the blue color. Record the time in seconds in column 1 of the data table.
15. Repeat step 14 for wells 2–5, adding 2 mL of the sodium meta-bisulfite/starch solution to each well. Time each reaction with a stopwatch or timer by measuring the time until the appearance of the blue color. Record the time in seconds in columns 2–5 of the data table.

Part C. Effect of Temperature on the Reaction Rate

16. Empty the six-well microplate from Part B into the sink and rinse the wells with water. Dry each well with a paper towel.
17. Measure the temperature of the “hot KIO₃” solution and when it is about 60 °C, pour the contents of the “hot KIO₃” beaker into well 1.
18. Using the 3-mL syringe, quickly, but carefully, add 2 mL of the sodium meta-bisulfite/starch solution to well 1 and start the timer. Time the reaction with a stopwatch or timer by measuring the time until the appearance of the blue color. Record the temperature of the hot solution and the time in seconds in column 6 of the data table.
19. Measure the temperature of the “cold KIO₃” solution and when it is about 10 °C, remove it from the ice-water bath. Pour the contents of the “cold KIO₃” beaker into well 2.
20. Using the 3-mL syringe, quickly, but carefully, add 2 mL of the sodium meta-bisulfite/starch solution to well 2 and start the timer. Time the reaction with a stopwatch or timer by measuring the time until the appearance of the blue color. Record the temperature of the cold solution and the time in seconds in column 7 of the data table.

Part D. Effect of a Catalyst on the Reaction Rate

21. Fill the 5-mL syringe with 1 mL of the potassium iodate solution and empty the syringe into well 3.
22. Rinse the 5-mL syringe with water. Fill the 5-mL syringe with 3 mL of distilled water and empty the syringe into well 3.
23. Fill the 5-mL syringe with 1 mL of 0.1 N sulfuric acid solution (a catalyst) and empty the syringe into well 3. Stir with a clean, dry stirring rod.
24. Fill the 3-mL syringe to the 2-mL mark with the sodium meta-bisulfite/starch solution. Quickly, but carefully, empty the 3-mL syringe into well 3 and start the timer. Time the reaction with a stopwatch or timer by measuring the time until the appearance of the blue color. Record the time in seconds in column 8 of the data table.
25. Empty the six-well microplate into the sink and rinse the wells with water. Dry each well with a paper towel.

Student Worksheet PDF

11795_Student1.pdf