Materials Included In Kit

Additional Materials Required

Prelab Preparation

Vitamin C stock solution: Measure and add 90 mL of distilled or deionized water to a 125-mL Erlenmeyer flask. Add 3 grams of ascorbic acid to the flask. Swirl the contents of the flask until the ascorbic acid is dissolved. Label the flask, “Vitamin C Stock Solution.” Stopper the flask.

Starch solution: (Best prepared the day of the lab to prevent mold.) Heat 100 mL of distilled water to boiling in a 250-mL borosilicate glass beaker. Add 1 g of starch to a clean 125-mL flask. Add a very small amount of the hot water to the flask with the starch. Stir to make a paste. Continue to add the remaining hot water slowly to the flask. Stir the mixture until it appears uniform (homogeneous). Allow the colloidal starch solution to cool slowly to room temperature before using. Label the flask, “Starch Solution.”

Safety Precautions

Tincture of iodine contains ethyl alcohol, which is toxic by ingestion, inhalation and skin absorption and is a flammable liquid. Tincture of iodine will stain skin and clothing. While a 3% solution of hydrogen peroxide is very weak, it is an oxidizer and a skin and eye irritant. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. 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. Always check local regulations before using any disposal procedures. All liquid waste containing iodine that is left over when the clock reactions are “done” may be disposed of by reacting with sodium thiosulfate and then pouring down the drain with an excess of water according to Flinn Suggested Disposal Method #12a. (A more detailed procedure for this disposal recommendation appears below.) Leftover Vitamin C stock solution and starch solution may be disposed of down the drain with an excess of water according to Flinn Suggested Disposal Method #26b. Leftover tincture of iodine can be stored as a flammable liquid for future use, or if storage is not desired, then it can be added to the waste solution to be treated with sodium thiosulfate.

Disposal of Waste Containing Iodine
Iodine is an oxidizer. When it oxidizes other substances, considerable heat may be generated. It is for this reason that oxidizers are recommended to be chemically changed before disposal, so there is no possibility of the discarded oxidizing agent inadvertently reacting at some later time with a chemically combustible material. An inexpensive chemical that will destroy the oxidizing power of iodine is sodium thiosulfate. This reaction works best in weakly acidic solutions, which is already provided by the ascorbic acid in the waste solution.

Prepare a dilute solution of sodium thiosulfate.

1. Measure and add 450 mL of distilled water to a 500-mL beaker or 500-mL flask.
2. Add 45 g of sodium thiosulfate pentahydrate.
3. Stir until dissolved.

Pour all of the liquid waste iodine solution into a container that will hold over 2 liters. Fifteen groups of students performing the activity as written will produce 1.68 L of liquid waste. (If a large container is not available, the disposal can be carried out in two portions, using half the liquid waste and half of the dilute sodium thiosulfate solution each time.) Add 350 mL of the dilute sodium thiosulfate solution to the waste iodine solution and stir with a magnetic stirrer or stirring rod until the solution turns colorless. Wait several minutes to make sure the solution remains colorless. If the solution turns blue, add 10 mL more sodium thiosulfate solution and stir until colorless. Repeat the last step until the solution remains colorless for at least 30 minutes. Any leftover sodium thiosulfate solution may be added to the liquid waste or stored for future use. The treated solution may be disposed of down the drain with an excess of water according to Flinn Suggested Disposal Method #26b.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. Both experiments in this laboratory activity can reasonably be completed in one 45- or 50-minute class period. The pre-laboratory assignment should be completed before coming to lab.
• Vitamin C tablets may be used instead of ascorbic acid (1500 mg of vitamin C per 90 mL of water). These tablets contain binders which are insoluble in water, so the vitamin C stock solution would not be completely clear.
• It is important that the solution in beaker A be stirred until colorless. If it remains orange, then too much elemental iodine will be present in beaker C, and the “alarm” will appear instantly. If this is a problem, a few additional drops of vitamin C stock solution may be added to beaker A.
• Reaction times may vary among groups. This variability can occur in measuring liquid volumes. Review with the students how to use a graduated pipet and how to read a graduated cylinder to help increase the accuracy of their measurements.
• If not enough glassware is available for each group, groups may share graduated cylinders. Clear plastic cups, such as 10-oz disposable cups (available from Flinn Scientific, Catalog No. AP6542) may be used instead of beakers.

Teacher Tips

• Students should have an understanding of chemical reactions, reactants and products, and ions.
• The following is an explanation of the clock reaction. In beaker A, tincture of iodine reacts with ascorbic acid (C₆H₈O₆) to produce iodide ions (I^–). In beaker C, two reactions take place simultaneously.
  {12610_Tips_Equation_1}
  {12610_Tips_Equation_2}
  When the hydrogen peroxide from beaker B is added, (Equation 1), iodine is produced. This is a relatively slow reaction. But the ascorbic acid reacts with the iodine immediately, producing iodide ions (Equation 2), thus preventing the iodine from reacting with the starch. The net result, during the clock period, is a colorless mixture with excess iodide ions. Once the ascorbic acid is completely consumed, the alarm appears—because the elemental iodine is no longer being reduced, it can react with the starch to produce the blue starch–iodine complex. The faster the iodine is produced in Equation 1, the faster the vitamin C is used up in Equation 2, and the sooner the alarm appears.
• An interesting application of the reaction of iodine with vitamin C is found in backpacking. Many campers use iodine to disinfect their drinking water. Adding a little vitamin C “kills” the iodine taste. The iodine is reduced to iodide, which no longer has the iodine taste and is an essential mineral (the same form that is in iodized salt). At the same time, the vitamin C (ascorbic acid) is oxidized to dehydroascorbic acid, which is no different than ascorbic acid in effectiveness as a vitamin.
• The demonstration kit, Iodine Clock Reaction: Effect of Concentration, Temperature, and a Catalyst on Reaction Rate (AP 4601) can be used to further explore reaction rates.

Answers to Prelab Questions

1. Describe the safety hazards and precautions associated with using tincture of iodine.

   Tincture of iodine contains ethyl alcohol, which is toxic by ingestion, inhalation and skin absorption and is a flammable liquid. Avoid contact with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

2. What amounts of water will be added to beaker A and beaker B for each experiment? How will this affect the concentration of the reactants?

   15 mL of water will be added to each beaker in Experiment 1. Experiment 2 will have 30 mL of water in each beaker. Since the amounts of iodine, ascorbic acid and hydrogen peroxide are the same in Experiments 1 and 2, the reactants in Experiment 2 will be less concentrated than in Experiment 1.

3. Write a hypothesis predicting what will happen to the reaction rate from Experiment 1 to Experiment 2. Explain your hypothesis in terms of the collision theory.

   The reaction rate will decrease (the reaction will take more time) from Experiment 1 to Experiment 2 because with more water, the concentrations of the reactants will be less and the likelihood of a collision will be lower.

Sample Data

Data Table 1

Data Table 2

Answers to Questions

1. In this activity, two forms of iodine are present—the element form, iodine (I₂), and the ion form, iodide (I^–). In step 5, Vitamin C reacts with the iodine initially present in beaker A to produce iodide ions. According to your observations, what color is this product?

   Iodide is colorless.

2. In the presence of starch, elemental iodine (I₂) will change color while iodide ions will be colorless. Which form of iodine—I₂ or I–—reacts with starch to produce the the final product (the alarm) in beaker C? How do you know?

   The element form of iodine (I₂) reacts with starch to produce the final product because the final solution is bluish-black.

3. Write a conclusion regarding the effect of concentration on reaction rate based on your results. Indicate whether or not the results support your hypothesis from Prelab Question 3.

   The greater the concentration of reactants, the less time the reaction takes, therefore the reaction rate is greater. This supports the hypothesis which stated that the reaction rate would decrease when the concentration of reactants was decreased.

References

“Iodine and Vitamin C,” Ask a Scientist. http://www.newton.dep.anl.gov/askasci/chem00/chem00880.htm (accessed July 2018).

Wright, Stephen W., “Tick Tock, a Vitamin C Clock,” Journal of Chemical Education, Vol. 79, No. 1, January 2002, pp. 40A–40B.

Introduction

Mix two colorless solutions and measure the time until they undergo a dramatic change! Using common household chemicals, investigate the effect of concentration on the rate of a reaction.

Concepts

• Clock reaction
• Collision theory
• Reaction rate

Background

The reaction rate of a chemical reaction is the speed at which a product forms or a reactant disappears. Iron reacting with oxygen in the air to produce iron oxide, known as rust, is an example of a reaction that can take a long time. Sodium and chlorine react very rapidly to form sodium chloride, known as table salt. The greater the rate of a chemical reaction, the less time is needed for a specific amount of reactants to be converted to products. This is analogous to “the greater the rate or speed of a car, the less time is needed to reach a destination.”

The collision theory explains what factors affect the reaction rate. The collision theory states that the rate of reaction depends on the number of collisions between molecules, the average energy of the collisions, and the effectiveness of the collisions. The collision theory can be illustrated by the following example.

Imagine that two people each have a small Velcro^®-covered ball. They stand at opposite ends of a room and throw the balls toward the center of the room. What might happen? Most likely the balls will miss each other. They could also collide, but may or may not stick together. Two balls that stick together would represent a new product in a reaction. What would increase the chance of two balls colliding and sticking together? New “products” would form at a faster rate if there were more people throwing more balls (reactants) at one another. Doing this would increase the concentration of balls and therefore increase the number of collisions. Throwing the balls harder (increasing their energy) would increase the likelihood that more would stick together when they collide. Throwing the balls more accurately would increase the effectiveness of any collisions, which represents the likelihood of a reaction between two reactants. Increasing any of these factors will increase the reaction rate.

A clock reaction is one way to measure the rate of a reaction. A clock reaction is a reaction characterized by an initial period with no noticeable change, followed by a sudden change, commonly in the color of the solution, indicating the formation of a new product. The time period during which no noticeable change occurs is called the clock period, and the sudden change is called the alarm. The alarm marks the point or time when at least one of the reactants has completely reacted. The rate of a clock reaction can be measured by the amount of time elapsed from the beginning of the clock period to the appearance of the alarm.

Experiment Overview

The purpose of this experiment is to determine the effect of the concentration of reactants on the rate of a chemical reaction with common household chemicals—Vitamin C (ascorbic acid), hydrogen peroxide and iodine. The reaction rate will be measured by timing how long it takes for one of the reactants to be used up. This will be indicated by an abrupt change in appearance—the alarm!

Materials

Prelab Questions

Read through the lab and answer the following questions on a separate sheet of paper.

1. Describe the safety hazards and precautions associated with using tincture of iodine.
2. What amounts of water will be added to beaker A and beaker B for each experiment? How will this affect the concentration of the reactants?
3. Write a hypothesis predicting what will happen to the reaction rate from Experiment 1 to Experiment 2. Explain your hypothesis in terms of the collision theory.

Safety Precautions

Tincture of iodine contains ethyl alcohol, which is toxic by ingestion, inhalation and skin absorption and is a flammable liquid. Tincture of iodine will stain skin and clothing. While a 3% solution of hydrogen peroxide is very weak, it is an oxidizer and a skin and eye irritant. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Follow all laboratory safety guidelines.

Procedure

1. Obtain three 150-mL beakers. Using a wax pencil, label one beaker A, another B and the third C.
2. Obtain three graduated pipets and label them iodine, starch and vitamin C.
3. Measure 15 mL of distilled or deionized water using a 100-mL graduated cylinder and add to beaker A.
4. Using the graduated pipet labeled Vitamin C, measure and add 2 mL of vitamin C stock solution to beaker A. Record the color and appearance of solution A in Data Table 1 for beaker A in the row labeled Water + Vitamin C Solution on the Vitamin C Clock Reaction Worksheet.
5. Using the graduated pipet for iodine, measure and add 1 mL of tincture of iodine to beaker A. Record the color and appearance of the solution in Data Table 1 in the row labeled Addition of Iodine.
6. Stir the solution with a stirring rod for several seconds until a change is observed. Record observations in Data Table 1 in the row labeled After Stirring. Save this solution for use in step 11.
7. Measure and add 15 mL of distilled water to beaker B.
8. Using a 10-mL graduated cylinder, measure and add 7 mL of 3% hydrogen peroxide solution to beaker B. Record the color and appearance of solution B in Data Table 1 for beaker B in the row labeled Water + Hydrogen Peroxide.
9. Using the graduated pipet for starch, measure 1 mL of starch solution and add to beaker B.
10. Stir the solution in beaker B with a clean stirring rod. Record the color and appearance of the solution in Data Table 1 in the row labeled Addition of Starch Solution. Save this solution for use in step 12.
11. Pour all of the contents of beaker A into beaker C.
12. Add the contents of beaker B all at once to beaker C. Start the stopwatch or note the time. Record the initial color and appearance of the solution in Data Table 1 for beaker C in the row labeled Start of Clock Period.
13. Stir the solution for 5 seconds.
14. Continue timing, watching the solution until an abrupt change is observed. Stop timing. Record the final color and appearance of the solution in Data Table 1 in the row labeled Alarm.
15. Record the amount of time elapsed from when solution B was added to beaker C until the alarm was observed for Experiment 1 in Data Table 2.
16. Empty the contents of beaker C into the designated waste container. Rinse the beaker thoroughly with water.
17. Using the same 150-mL beakers, make a new solution A and solution B by repeating steps 3–10, except add 30 mL of water to beakers A and B instead of 15 mL. Be sure to use the same graduated pipets as before for each chemical—do not interchange them! Continue with steps 11–16. Record the the time elapsed under Experiment 2 in Data Table 2. It is not necessary to record observations of color and appearance of the solutions for Experiment 2.

Student Worksheet PDF

12610_Student1.pdf