Materials Included In Kit

Additional Materials Required

Prelab Preparation

• Colloidal starch solution, 2%: To make 1500 mL of 2% starch solution, first prepare a smooth paste containing 30 g soluble starch and 250 mL of distilled or deionized water. Pour the paste into 1250 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 within one week of use. The colloidal solution will be cloudy and translucent.
• Potassium iodate solution, 0.1 M: Add 53.5 g of potassium iodate (KIO₃) to about 1 liter of distilled or deionized water in a beaker or flask. Stir to dissolve and then dilute to 2.5 liters with water. Mix well before dispensing.
• Sodium meta-bisulfite solution, 0.04 M: Prepare fresh the day of lab. For ease of preparation, measure the amount of sodium meta-bisulfite into an appropriately labeled bottle or flask in advance, and then add the required amount of water to the pre-weighed solid prior to the start of lab. To prepare 2.5 L of 0.040 M sodium meta-bisulfite solution, add 2.5 liters of distilled or deionized water to 19.0 grams of Na₂S₂O₅. Mix well using a magnetic stirrer before dispensing.

Safety Precautions

The iodine clock reaction generates elemental iodine. It is harmful if inhaled or in contact with skin and causes severe skin burns and eye damage. Perform this experiment in a fume hood or well-ventilated lab. Some individuals may be allergic to iodine. Sodium meta-bisulfite solution may be harmful if swallowed. It is irritating to skin and eyes and may cause eye damage. Potassium iodate solution is generally considered nonhazardous. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a lab coat or chemical-resistant apron. Remind students to wash their hands thoroughly with soap and water before leaving the lab. 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 iodine clock solutions contain iodine and should be reduced with excess sodium thiosulfate solution according to Flinn Suggested Disposal Method #12a.

Lab Hints

• Student preparation is essential for success in a guided-inquiry activity. To ensure a safe lab environment, the instructor 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. For best results, use recently purchased starch solution or prepare fresh within one week of use.
• Sodium bisulfite and sodium meta-bisulfite are interconverted in water (2NaHSO₃ → Na₂S₂O₅ + H₂O). In aqueous solution, the equilibrium overwhelmingly favors sodium bisulfite. Both sodium bisulfite and sodium meta-bisulfite thus produce a solution of bisulfite ions (HSO₃^–) when dissolved in water. Sodium meta-bisulfite, also known as anhydrous sodium bisulfite or sodium pyrosulfite, is the preferred source of bisulfite ions for this experiment.
• While iodine clock activities are a popular and effective means of teaching kinetics concepts, the overall reaction mechanism is actually quite complicated and frequently confusing to students. The slow step in the overall reaction is assumed to be the formation of iodine (Equations 1 and 2). Iodine formed in the slow step is quickly consumed by a 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 analysis of experimental results so that students can focus on the kinetics concepts.
• For convenience, have each student group reduce their combined iodine clock reaction mixtures. Pour the combined contents of five trials into a 2-L beaker and add 75 mL of 4% sodium thiosulfate solution. Stir to mix—the blue iodine solution will turn colorless within 30–60 seconds and may then be rinsed down the drain with running water. Dissolve 40 g of sodium thiosulfate pentahydrate (Na₂S₂O₃•5H₂O) in one liter of water.

Answers to Prelab Questions

1. Calculate the concentration values of potassium iodate [KIO₃] in Solution A for trials 1–3. Enter the concentration values in the data table on the Lab Report.

   Use the dilution equation, M₁V₁ = M₂V₂. The concentrations are 0.020 M, 0.04 M and 0.010 M in trials 1–3, respectively.

2. The purpose of this experiment is to investigate the effect of KIO₃ on the rate of the iodine clock reaction. Why are the total volumes of Solutions A and B both kept constant in the baseline experiments?

   Keeping the total volumes of Solution A and Solution B constant means that the concentration of KIO₃ in the final reaction mixtures will be directly proportional to the volume (amount) of KIO₃ used in Solution A. This simplifies the analysis of the experimental results. Plotting either the reaction time or the reaction rate versus the volume of KIO₃ will give the same general curves (graphs) as plotting either variable versus the concentration of KIO₃.

3. Write a general statement predicting the effect of KIO₃ concentration on the rate of the iodine clock reaction.

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

4. See Equations 1–3 in the Background section. Identify the oxidizing agent and reducing agent in each.
   {14037_PreLabAnswers_Equation_1}
   Iodate ions are the oxidizing agent; bisulfite ions are the reducing agent.
   {14037_PreLabAnswers_Equation_2}
   Iodate ions are the oxidizing agent; iodide ions are the reducing agent.
   {14037_PreLabAnswers_Equation_3}
   Iodine is the oxidizing agent; bisulfite ions are the reducing agent.

Sample Data

Iodine Clock Challenge

*Trials 4 and 5 are shown here for information purposes. Students will carry out one supporting trial and the final challenge trial.

Answers to Questions

1. Plot the data from trials 1−3 to show how changing the volume of KIO₃ affects the reaction time. Draw a trendline through the data points and explain the shape of the graph.
   {14037_Answers_Figure_1}
   The reaction time decreases as the volume of KIO₃ in Solution A increases. The graph is a power function (exponentially decreasing).
2. Explain the effect of concentration on reaction rate in terms of collision theory: When the concentration of reactants increases, the reaction time decreases, because increasing the number of molecules or ions in solution increases the frequency of collisions between them.
3. Calculate 1/time in sec⁻¹ for each trial. This is the reaction rate—enter the results in the table.

   Sample calculation: The reaction rate for trial 1 is 1/15 sec = 0.067 sec^–1. See the data table for additional results.

4. Plot the reaction rate versus the volume of KIO₃ solution in Solution A for each trial. Draw a best-fit straight line through the data points and explain the shape of the graph.
   {14037_Answers_Figure_2}
   The reaction rate (1/time) is directly proportional to the volume (concentration) of KIO₃ in each trial. As the concentration of KIO₃ increases, the reaction rate increases in a linear manner.
5. Which graph will probably give a more accurate prediction of the amount of KIO₃ solution needed to make the iodine clock “ring” in 25 seconds? Explain.

   The effect of volume (concentration) of KIO₃ on the reaction rate (Question 4) produces a straight line. Since we have to extrapolate the results to estimate how much KIO₃ to use in the challenge experiment, it is easier to use a straight-line graph rather than the “curved” or exponential graph in Question 1.

6. For the final challenge trial, how close did the actual reaction time come to the target time of 25 seconds? Discuss possible sources of error in the experiment and whether they would have led to longer or shorter reaction times.

   The reaction time for the challenge run matched the predicted value! Using too much KIO₃ would have led to a shorter reaction time, while too little KIO₃ would have given a longer reaction time. Graphing and extrapolation errors could produce either shorter or longer reaction times.

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 amount of time it will take for the iodine clock to ring?

Concepts

• Kinetics
• Rate of reaction
• Oxidation and reduction
• Concentration
• Collision theory
• Balancing redox equations

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.

The rate of a reaction can be determined by measuring the concentration of reactants or products as a function of time. In some cases, it is possible to use a simple visual clue to determine a reaction rate. Thus, if the reactants are colorless but one of the products is colored, the rate of the reaction can be followed by measuring the time it takes for the color to develop. The iodine clock reaction in this experiment involves mixing two colorless solutions A and B and measuring the time required for the blue color to suddenly appear. Solution A contains different amounts of 0.1 M potassium iodate solution (KIO₃) and water; Solution B is prepared from 0.04 M sodium meta-bisulfite (Na₂S₂O₅) and starch.

Sodium meta-bisulfite dissolves in water to produce bisulfite ions (HSO₃^–), which undergo a slow reaction with iodate ions to generate iodide ions (I^–) and sulfate ions (Equation 1).

When iodate ions are present in excess, subsequent reaction of iodate and iodide ions produces elemental iodine (I₂). Although this reaction (Equation 2) is fast, excess iodine does not accumulate because of a competing reaction with bisulfite ions (Equation 3). This reaction is also fast. The familiar dark blue color due to the starch iodine complex (Equation 4) appears suddenly when all of the bisulfite ions have been fully consumed. The time needed for the blue “iodine clock” color to appear is therefore a proxy for the rate of the first slow reaction (Equation 1). Studying how the reaction time changes as the volume of potassium iodate is varied allows one to determine the effect of this reactant concentration on the rate of reaction 1.

Experiment Overview

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

Materials

Prelab Questions

1. Calculate the concentration values of potassium iodate [KIO₃] in Solution A for trials 1–3. Enter the concentration values in the data table on the Lab Report.
2. The purpose of this experiment is to investigate the effect of KIO₃ on the rate of the iodine clock reaction. Why are the total volumes of Solutions A and B both kept constant in the baseline experiments?
3. Write a general statement predicting the effect of KIO₃ concentration on the rate of the iodine clock reaction.
4. See Equations 1–3 in the Background section. Identify the oxidizing agent and reducing agent in each.

Safety Precautions

The iodine clock reaction generates elemental iodine. It is harmful if inhaled or in contact with skin and causes severe skin burns and eye damage. Perform this experiment in a fume hood or well-ventilated lab. Some individuals may be allergic to iodine. Dispose of the final iodine clock mixtures as directed by the instructor. Sodium meta-bisulfite solution may be harmful if swallowed. It is irritating to skin and eyes and may cause eye damage. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a lab coat or chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the lab.

Procedure

Baseline Experiments

1. Label three 400-mL beakers 1−3. Using a clean, 100-mL graduated cylinder, measure 40 mL of 0.1 M KIO₃ solution and pour it into beaker 1.
2. Using the same graduated cylinder, measure and pour the appropriate amounts of 0.1 M KIO₃ solution into beakers 2 and 3, as shown in the table.
   {14037_Procedure_Table_1}
3. Using a clean 250-mL graduated cylinder, measure and pour the appropriate amounts of water into beakers 1–3, as shown in the table.
4. Obtain three plastic cups and prepare Solution B for each trial. Using a separate, clean 50-mL graduated cylinder for each solution, measure and pour 40 mL of 0.04 M Na₂S₂O₅ solution and 20 mL of starch into each cup.
5. Pour Solution B from one cup into Solution A in beaker 1 (trial 1) and immediately start timing. Measure and record the time from when the solutions are mixed until the sudden appearance of the blue color.
6. Repeat step 5 two more times with beakers 2 and 3. For each trial, start timing as soon as Solutions A and B are mixed and stop when the blue color appears. Record the reaction time for each trial.

Guided-Inquiry Challenge

7. Answer Questions 1−5 on the Lab Report. Based on your answer to Question 5, choose a suitable graph and estimate volumes of KIO₃ needed to gather additional data for the final challenge—getting the iodine clock to “ring” in 25 seconds! Each group has enough materials to run one supporting trial and one final trial.
8. Choose a volume of KIO₃ and carry out the supporting trial. Analyze the results and make any necessary adjustments to the volume of KIO₃.
9. Conduct the final challenge trial with the instructor measuring the reaction time. Return to Question 6 in the Lab Report to analyze your results.

Student Worksheet PDF

14037_Student1.pdf