MicroScale Iodine Clock Reaction

Description

A kinetics experiment is performed in such a way that a fixed amount of a reagent is consumed by a product of a slow chemical reaction. The time required for a fixed amount of thiosulfate to be used up is measured and related to the concentration of one of the reactants, hydrogen peroxide. From this information, the order of hydrogen peroxide in the reaction is determined.

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• The rate of a reaction is governed by the following 'rate law':

  rate = k [A]^a[B]^b[C]^c

• The quantities in brackets are read as moles/liter and are raised to an appropriate power. Multiplied together with the constant k, they give the rate of the reaction.
• The numerical values of a, b, and c must be determined by experiment. These numbers determine the 'order' of each reactant. It is the purpose of this experiment to determine the order of H₂O₂ in an iodine clock reaction.
• The reaction to be studied is:

  3 I^- + H₂O₂ + 2 H⁺ --> I₃^- + 2 H₂O

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Hydrogen peroxide will damage tissue bleach clothing. Iodine will stain clothing.

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Wash spills immediately. Do not ingest chemicals. Wear old clothing and aprons.

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1.  Place 4 drops of water in each well of two 12-well strips.
2. Invert one of the strips. Carefully place that strip on top of the other strip so that one well is above another.
3. Surface tension will hold one solution above the other.
4. Carefully lift the strips holding them in contact. Use a brisk downward motion to mix the solutions.
5. Check to be certain that the well contents are mixed. Practice until mixing is consistent.
6. Prepare the reaction mixtures in two 12-well strips as follows:
   • Strip 1: Solution A and Water
   • Strip 2: Solutions B and C

   

7. Invert one of the strips. Carefully place that strip on top of the other strip so that one well is above another. Capillary action will hold one solution above the other.
8. Carefully lift the strips holding them in contact. Use a brisk downward motion to mix the solutions. Record the time at the moment of mixing.
9. As the result of variations in drop sizes, not all wells in a group of three will change color at the same moment. Record the average time required for each group of wells to change color.

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• The rate of reaction can be represented by the following equation:
  • Rate = k[H₂O₂ ]^a[I^-]^b[H⁺]^c
• The concentrations of I^- and H⁺ are held constant in the procedure; all wells in row 2 were filled with the same amount of solutions B and C. We may write the rate:
  • Rate = k'[H₂O₂ ]^a
  • Where [I^-]^b[H⁺]^c has been absorbed into the pseudo-rate constant, k'.
• This experiment has been set up in a clever fashion. The end point color appears after all of the thiosulfate is used up. The amounts of reactant used up in causing this to take place are small, so the reactant concentrations remain essentially constant throughout the time of reaction.
• The expression for the rate is:
  • (Δ[H₂O₂ ] / Δt)
• But the amount of reactant used up at the time of the endpoint is a constant because the amount (moles) of thiosulfate present is constant for each well:
  • 3I^- + H₂O₂ + 2H⁺ --> I₃^- + 2 H₂O
  • I₃^- + 2 S₂O₃^2- --> 3 I^- + S₄O₆^2-
• Therefore, the rate is related to a constant divided by the time it takes to reach the end point. Plotting (1 / Δt) is the same as plotting a constant times the reaction rate.
• But the rate is equal to k'[H₂O₂ ]^a. Therefore, a plot of the rate versus [H₂O₂ ] gives an indication of the exponent, a. If the slope equals zero, a = 0. If there is a straight line through the origin, a = 1. If there is a parabola, a = 2.

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Name ___________________________ Class ________

Teacher__________________________

DoChem 114 MicroScale Iodine Clock Reaction

1. Plot a graph of the results. Use the x-axis for number of drops and the y-axis for the reciprocal of the reaction time. Draw the best fitting curve to this plot.
2. Based upon the graphs, determine the order of the reaction with respect to H₂O₂.
3. Briefly describe how to use variations of this experiment design to determine the order for the I^- and H⁺. Having determined the order for the three reactants, how is the value of k determined?

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Name ___________________________ Class ________

Teacher__________________________

DoChem 114 MicroScale Iodine Clock Reaction

Watch the movie. Use this sample data to plot the graph and answer the questions.

1. Plot a graph of the results. Use the x-axis for number of drops and the y-axis for the reciprocal of the reaction time. Draw the best fitting curve to this plot.
2. Based upon the graphs, determine the order of the reaction with respect to H₂O₂.
3. Briefly describe how to use variations of this experiment design to determine the order for the I^- and H⁺. Having determined the order for the three reactants, how is the value of k determined?

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Purpose

To determine the order of a reactant in the rate expression for a chemical reaction.

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(per 10 students working in pairs):

• Solution A: 15 mL of 3% peroxide and 80 mL of water.
• Solution B: Starch solution with 0.005M sodium thiosulfate (Na₂S₂O₃): Bring 100 mL of distilled water to a boil and spray in laundry starch from a spray can until a faint bluish translucence is noticeable. Cool. Add 0.124 g of Na₂S₂O₃•5H₂O in 100 mL of solution.
• Solution C: 1.74 g of KI, 1.4 g of NaC₂H₃O₂, and 3 mL of 6M acetic acid in a total of 200 mL of water.
  • Place the solutions in labeled Beral pipets. (Students should have uniform drop size to insure success.)
• 10 single row tissue culture plates (1 x 12 well [FB] strip TiterTek cat. No. 78-591-99)
• distilled water
• 60 Beral pipets
• 15 Beral pipet holders (cassette tape cases)
• clock with sweep second hand or stopwatches

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• A teacher demonstration is necessary to show the students the proper method of shaking the solution. 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). The "averaging" technique works well visually and the time increments of 20-30 seconds allow recording of data. Set up the required solutions in Beral pipets. One pipet holds enough solution for a class of 30 to run four trials.
• The solutions will turn blue in sequence from left to right in a total time of under three minutes. The times will vary from run to run but the graphs of the lines will still reliably show the order.

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Teacher preparation: 20 minutes

Class time: 20-30 minutes

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

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An early version of the microscale clock reaction was prepared at the 1987 Woodrow Wilson/Dreyfus workshop by Bruce Clark and Richard Perry.

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• kinetics
• first order
• clock reaction
• reaction rates

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