Materials Included In Kit

Additional Materials Required

Prelab Preparation

Starch Solution, 2%: Prepare 1500 mL of 2% starch solution by making a smooth paste of 30 g soluble starch and 230 mL of distilled or deionized water. Pour the paste into 1270 mL of boiling water while stirring. Cool to room temperature before using. Starch solution has a poor shelf life and will form mold if kept for too long. Prepare this solution fresh.

Sodium Bisulfite Solution, 0.2 M: This solution must be freshly prepared within 1–2 days of use. To prepare 500 mL of 0.2 M sodium metabisulfite solution (enough for a class of 30 students working in pairs), add 19 grams of Na₂S₂O₅ to 400 mL of distilled or deionized water and stir to dissolve. Dilute the solution to 500 mL with DI water and mix well before dispensing.

Potassium Iodate Solution, 0.1 M: Dilute 500 mL of the 0.2 M KIO₃ solution to 1 liter with distilled or deionized water. Mix well. A class of 30 students will require 1300 mL of solution.

Demonstration Procedure

1. Label three 500-mL beakers 1, 2, and 3. Using a 100- and 250-mL graduated cylinder, respectively, measure and add the following amounts of 0.1 M potassium iodate solution and distilled water to each beaker. These are Solution A for each experiment.
   {12840_Preparation_Table_2}
2. Obtain three 100-mL beakers. Using a separate graduated cylinder for each solution, measure and add 8 mL of 0.2 M sodium metabisulfite solution, 24 mL of starch solution and 32 mL of distilled water to each 100-mL beaker. These are Solution B for each experiment. Stir each solution.
3. Pour Solution B into Solution A in Beaker 1 and immediately start timing. Measure and record the time from when the two solutions are mixed until the appearance of the blue color. 
4. Repeat step 3 two more times with Beakers 2 and 3.

Safety Precautions

Potassium iodate solution is moderately toxic by ingestion and a body tissue irritant. Sodium meta-bisulfite is also irritating to skin, eyes and other body tissues. Avoid contact of all chemicals with eyes and skin. 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. The final solutions may be reduced with sodium thiosulfate solution according to Flinn Suggested Disposal Method #12a. Add just enough reducing agent to decolorize the blue color of the starch–iodine complex.

Teacher Tips

• Enough supplies are included for two demonstration and for each student group to do three trials: 1 liter of 0.2 Molar potassium iodate, 25 grams of sodium metabisulfite, 30 grams of soluble starch and 36 plastic cups.
• Thorough student preparation is essential for success in a student-directed inquiry activity. In this inquiry “challenge,” the students should follow the procedure used by the teacher in the demonstration. To ensure a safe lab environment, the teacher should review the students’ understanding of the procedure and the safety precautions before students work in the lab.
• Starch solutions have a poor shelf life. Prepare fresh within one week of use.
• Sodium bisulfite and sodium metabisulfite are interconverted in the presence of water (Na₂S₂O₅ + H₂O → 2NaHSO₃). In aqueous solution, the equilibrium overwhelmingly favors sodium bisulfite. Both sodium bisulfite and sodium metabisulfite thus produce a solution of bisulfite ions (HSO₃^–) when dissolved in water. Sodium metabisulfite, also known as anhydrous sodium bisulfite or sodium pyrosulfite, is the preferred source of bisulfite ions for this demonstration.
• While iodine clock demonstrations are a popular and effective means of teaching kinetics concepts, the overall reaction mechanisms are actually quite complicated and frequently confusing to students. (See the sequence of reactions in Post-Lab Question 3.) The slow steps in the overall reaction are assumed to be the formation of iodine (Equations 1 and 2). Iodine formed in the slow step is quickly consumed by a very fast reaction with bisulfite ions (Equation 3). The blue color does not appear, therefore, until all of the bisulfite ions have been consumed. Bisulfite ions are the limiting reactant and the rate of the overall reaction is first order in potassium iodate. The details of this experimental design are omitted in the student section of this writeup so students can focus on the kinetics concepts.

Answers to Prelab Questions

Demonstration

1. Observe the clock reactions and record the reaction times.
   {12840_Answers_Table_3}
2. Calculate the concentration of potassium iodate (KIO₃) in Solution A for each Experiment 1–3.

   Use the dilution equation, M₁V₁ = M₂V₂. The concentrations are 0.025 M, 0.05 M and 0.013 M in Experiments 1–3, respectively.

3. (a) How did the reaction time change as the KIO₃ concentration was changed? (b) How is the rate of the reaction related to the reaction time?

   (a) Using Experiment 2 as a “control,” the reaction time increased when the concentration was decreased (compare Experiments 1 and 2), and the reaction time decreased when the concentration was increased (compare Experiments 2 and 3). (b) The rate of a reaction is inversely proportional to the reaction time. The faster the rate of a reaction, the less time is needed for reactants to be converted to products. Note: This is a frequent source of student misconceptions.

4. Write a general statement describing the effect of reactant concentration on the rate of a reaction.

   In general, the rate of a reaction increases when the concentration of reactant(s) increases.

Sample Data

Answers to Questions

1. For the final test run, how close did the actual reaction time come to the predicted time of 25 seconds? Discuss possible sources of error in the experiment and whether they would have led to longer or shorter reaction times. Student answers will vary. They should point to too much DI water leads to low concetrations and longer time; too little DI water, leads to higher concetrations and shorter times. Graphing errors would lead to both high and low times.
2. Based on the results of the demonstration and your trial runs, predict the reaction time in seconds if 125 mL of KIO₃ solution were used in Solution A. The extrapolation of the volume of KIO₃ solution vs. 1/t graph yields a 1/t value of 0.16. t =1/0.16 or about 6 seconds.
3. Explain the effect of concentration on reaction rate in terms of collisions between molecules: When the concentration of reactants increases, the reaction time decreases, because increasing the concentration of molecules or ions in solution increases the rate of collisions between them.
4. The overall formation of the starch–iodine complex in the iodine clock reaction occurs in a series of steps. Balance Equations 1–3 and identify the oxidizing and reducing agents in each.
   {12840_Procedure_Equation_1}
   Iodate ions are the oxidizing agent; bisulfite ions are the reducing agent.
   {12840_Answers_Equation_4}
   {12840_Procedure_Equation_2}
   Iodate ions are the oxidizing agent; iodide ions are the reducing agent.
   {12840_Answers_Equation_5}
   {12840_Procedure_Equation_3}
   Iodine is the oxidizing agent; bisulfite ions are the reducing agent.
   {12840_Answers_Equation_6}

References

For a complete explanation of the principles of iodine clock reactions, please see “The Order of Reaction” experiment in Kinetics, Volume 14 in the Flinn ChemTopic™ Labs series; Cesa, I., Editor; Flinn Scientific: Batavia, IL (2003).

Introduction

The demonstration of an “iodine clock” involves a chemical reaction that suddenly turns blue due to the formation of the familiar iodine–starch complex. The color change occurs abruptly, like an alarm clock ringing! Can you predict the time it will take for the iodine clock to ring?

Concepts

• Kinetics
• Rate of reaction
• Concentration
• Collision theory

Background

Kinetics is the study of the rates of chemical reactions. As reactants are transformed into products in a chemical reaction, the amount of reactants will decrease and the amount of products will increase. The rate of the reaction describes how fast the reaction occurs. The greater the rate of the reaction, the less time is needed for a specific amount of reactants to be converted to products. Some of the factors that may affect the rate of a chemical reaction include temperature, the nature of the reactants, their concentrations, and the presence of a catalyst.

Experiment Overview

The purpose of this guided-inquiry activity is to observe the iodine clock reaction, determine how the concentration of potassium iodate influences the rate of the reaction, and predict the amount of potassium iodate needed to make the clock “ring” in 25 seconds.

Materials

Prelab Questions

Demonstration

The iodine clock demonstration involves mixing two colorless solutions and measuring the time required for the blue color to suddenly appear. Solution A contains different amounts of 0.1 M KIO₃ and water, while Solution B is a standard solution containing 8 mL of 0.2 M Na₂S₂O₅, 24 mL of starch solution, and 32 mL water.

1. Pay close attention to the procedure used by the teacher in the iodine clock demonstration. Start timing as soon as Solutions A and B are mixed and stop when the blue color appears. Observe the reactions and record the reaction times.
   {12840_PreLab_Table_1}

   *Distilled or deionized water.

2. Calculate the concentration of potassium iodate (KIO₃) in Solution A for each Experiment 1–3.
3. (a) How did the reaction time change as the KIO₃ concentration was changed? (b) How is the rate of the reaction related to the reaction time?
4. Write a general statement describing the effect of reactant concentration on the rate of a reaction.

Safety Precautions

Potassium iodate solution is moderately toxic by ingestion and a body tissue irritant. Sodium metabisulfite is also irritating to skin, eyes and other body tissues. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron.

Procedure

Inquiry Design

1. Form a working group with two other students and brainstorm the following questions.
   • Why was the total volume of Solution A kept constant in the iodine clock demonstration?
   • In order to investigate the effect of KIO₃ concentration on the rate of the iodine clock reaction, the composition and amount of Solution B was not changed. Explain why.
   • Prepare graphs of (a) reaction time and (b) 1/time versus the volume of KIO₃ solution in Solution A. Explain the shape of each graph.
   • Which graph would probably give a more accurate prediction of the amount of KIO₃ solution needed to make the iodine clock “ring” in 25 seconds? Explain.
2. Read the list of Materials that may be provided along with the Safety Precautions for their use. Each group will have enough materials to run three trials of the iodine clock reaction—two trial runs to gather additional data for the graph(s) and a final “challenge” run that must ring in 25 seconds. Write a detailed step-by-step procedure for the group challenge.
3. Carry out the trial runs, record data and graph the results. Ask the teacher to measure the reaction time for the final test run.

1. For the final test run, how close did the actual reaction time come to the predicted time of 25 seconds? Discuss possible sources of error in the experiment and whether they would have led to longer or shorter reaction times.
2. Based on the results of the demonstration and your trial runs, predict the reaction time in seconds if 100 mL of KIO₃ solution were used in Solution A.
3. Explain the effect of concentration on reaction rate in terms of collisions between molecules: When the concentration of reactants increases, the reaction time _______________, because increasing the _______________ of molecules or ions in solution increases the rate of _______________ between them.
4. The overall formation of the starch–iodine complex in the iodine clock reaction occurs in a series of steps. Balance Equations 1–3 and identify the oxidizing and reducing agents in each.
   {12840_Procedure_Equation_1}
   {12840_Procedure_Equation_2}
   {12840_Procedure_Equation_3}

   I₂(aq) + Starch → Dark-blue colored complex