Expt 051 -- Iodine Clock Kinetics

Iodate and metabisulfite are dispensed at concentrations where the clock reaction requires about 10 seconds when the equal volumes of solutions are mixed. Students alter the conditions to make the color change in exactly 30 seconds. Students with minimal knowledge about kinetics usually succeed by applying scientific reasoning.

Objective

Alter the conditions to make the color change in exactly 30 seconds.

Safety

The chemicals studied are toxic. Wear goggles and apron. Wash hands after the experiment. The iodate is a strong oxidizing agent.

Procedure

Shakedown Technique

1. Practice the 'shake-down' technique. Fill the first well of each of 2 12-well strips with 5 drops of distilled water. The instructor demonstrates the "shake-down" method of mixing. Stack one strip atop the other. When one strip is inverted and stacked on the second strip, capillary action keeps the liquid in the upper wells. Holding the wells firmly practice an up and down motion slowly. No mixing takes place.

   !!!Click here to See Movie.

   !!!Click here to See Movie.

2. At the lab station you find solutions X, Y and W in a kit. W is water. Measure out 5 drops of X in the first well of a 12-well plate. Measure 5 drops of Y into the first well of the second 12 well plate. Prepare to time the reactions with a clock or stop watch. Holding the plates firmly together, shake them once vigorously in a downward motion as practiced. As soon as the strips are mixed, start the timer.

   !!!Click here to See Movie.

Your Challenge

• Determine what proportions of X, Y and W cause the final solution to change colors precisely 30.0 seconds after mixing. Make any dilutions you wish including X or Y. Record all attempts -- even dead ends.

  !!!Click here to See Movie.

• Credit is awarded based on how close you come as timed by the instructor. You may practice as many times as you like, but you are only allowed three attempts in front of the instructor. Only your last attempt counts.

Grading scale

Questions

• Record the steps you used to find the mixture that turns blue in 30 seconds. Describe your dead-end trials also.
• Carefully record the procedure for the reaction that turns blue in 30 seconds.

Handout Makeup

Name ___________________________ Class ________

Teacher __________________________

BeckerDemos 051 Iodine Clock Kinetics

Watch the movies and use the data below to answer the questions. The data below were collected during the actual taping.

Sample Data:

HQ1. The slowest sample is still a little too fast. Suggest 3 different approaches which may change the time to exactly 30 seconds.

Curriculum-

This experiment is used when rates of reaction are discussed. The reaction also can be studied with redox reactions or the chemistry of iodine. The activity is NOT appropriate for studying rate laws quantitatively.

Activity-

• Laboratory or Demonstration
• The experiment works well as a discovery activity in anticipation of studying kinetics. This clock reaction has complex kinetics so it is a poor choice for a straight rate law study. A hydrogen peroxide-iodide clock is much better for rate law studies.
• I use this as a part of the final exam. My students do not cover kinetics in their first year of chemistry, so this entire activity is completely new and open-ended to them. If they have learned the principles of the scientific method though, and if they approach the problem in a systematic manner, they usually succeed.

Safety-

The chemicals studied are toxic. Wear goggles and apron. Wash hands after the experiment.

Time-

Teacher Preparation: 15 minutes

Class Time: 20 minutes

Materials-

(Per Group)

• 5 mL Starch bisulfite solution X (Dissolve 1 starch packing peanuts, 0.2 g sodium metabisulfite (Na₂S₂O₅), and 1 mL of 1 M HCl in approx. 200 of water fill to the top. Let it set for a few minutes (Prepare iodate.), then suction debris off top.) (Prepare fresh)
• 5 mL of 0.006M KIO₃ solution Y (Dissolve 0.25 g of potassium iodate (KIO₃) in approx. 100 mL of tap water, add another 100 mL of water.) Test the solutions (See Lab Hints.)

Place the solutions in labeled plastic transfer pipets. Use small pipets with constricted tips. (Students should have uniform drop size to insure success.)

• 3 pipets for solutions
• 2 12-well strips
• 1 cassette box to hold solutions
• distilled water
• clock with sweep second hand, digital watch with seconds display, or stopwatch

Optional:

• Ice

Disposal-

The materials used in this experiment may be disposed of safely at the sink.

Lab Hints-

• Refer to "Procedure" for the method of shaking down solutions. Demonstrate this 'shake-down' procedure. During the demonstration impress upon them the necessity of holding the wells gently but firmly. The wells are mixed all at once with a snapping motion (there is no shaking up and down, as this would cause leakage).
• You may use test tubes for the experiment instead of 12-well plates.

• The starch - metabisulfite solution X must be prepared within 24 hours of use. You may prepare the metabisulfite a few days ahead and add acid and starch just before use. Test a sample. Dilute the iodate (Y) solution if equal volumes of X and Y change in less than 8 seconds. The Y solution shown in the movies is too fast for most students to work with. It needs to be diluted to half this strength.
• The reaction rate is easily changed with temperature, but temperature control in the 12-well plates is horrible. Consequently, the rate is difficult to reproduce --the objective of this particular exercise. You might wish to make ice available to students on demand.
• As a follow up activity, use the same solutions to demonstrate the temperature dependence of the reaction for your students. Do not heat solutions above 40ºC, because the starch-iodide complex is unstable.

Observations-

In complex systems, several reactions may take place in parallel competing with one another. Changing conditions of a reaction may actually change the pathway and products of the reaction in these complex systems.

The reactions to be studied in this experiment are:

S₂O₅^2- + H₂O --> 2 HSO₃^-

IO₃^- + 3 HSO₃^- --> 3 SO₄^2- + I^- + 3 H⁺ (slow)

IO₃^- + 8 I^- + 6 H⁺ --> 3 I₃^- + 3 H₂O (slow)

I₃^- + HSO₃^- + H₂O --> 3 I^- + SO₄^2- + 3 H⁺ (fast)

2 I₃^- + starch --> blue complex + I^- (fast)

The endpoint color appears after all of the bisulfite is consumed. Note: the iodide is not all consumed by the iodate reaction when the color changes. The combination of two slow steps with different rates complicates the kinetics of this reaction.

The rate law has been studied and found to be complex. (T is the temperature in ºC.)

time(sec) = (906.05-23.01 T + 0.188 T²) / (C_1^0.904C_2^1.642)

C₁ = concentration of H₂SO₃ in moles/meter³

C₂ = concentration of HIO₃ in moles/meter³

moles/meter³ = moles/10 liters

The temperature dependence makes a nice demonstration. See Lab Hints above.

Data Table-

Sample Data:

Answers-

Q1. Record the steps you used to find the mixture that turns blue in 30 seconds. Describe your dead-end trials also.

A1. Reagent Y was added serially to the 12 well strip. The first well contains 5 drops the last well 1 drop of x. The wells were diluted to a total volume 5 drops with water. 5 drops of reagent X was added to each well. The solutions were mixed with a quick shakedown and the time was measured. The first attempt resulted in a 27 sec time for one well. The iodate solution (Y) was diluted with one drop of water. A Mixture of 1 drop of the diluted iodate Y, 5 drops of X and 4 drops of water required 30 seconds.

Q2. Carefully, record the procedure for the reaction that turns blue in 30 seconds.

A2. 5 drops of x and 1 drop of Y and 4 drops of water reacted in 29 seconds. The Y solution was diluted with one drop of water.

Makeup Ans.-

HQ1. The slowest sample is still a little too fast. Suggest 3 different approaches which may change the time to exactly 30 seconds.

HA1. Several answers are possible. 4 are given below.

Dilute X one drop at a time.

Dilute Y one drop at a time.

Chill the solutions.

Rinse a well before mixing the reactants. (The rinse water dilutes the reactants.)

References-

Shakhashiri, B. Z., in his "Chemical Demonstrations", Volume 4 (p 22., University of Wisconsin Press, 1985), discusses the reactions used in this experiment.

Clifford, A.F., in "Inorganic Chemistry of Qualitative Analysis"(p.404, Prentice-Hall, 1961) discusses the kinetics of this reaction and reports the rate equation .

Key Words 1-

reaction rate, oxidation-reduction, redox, kinetics