The rate of a reaction is governed by the following relationship:
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 experimentation. These numbers determine the order of the reaction. Added together they give the over-all order of the reaction. It is the purpose of this experiment to determine the order of an iodine clock reaction with respect to H2O2.
The reaction to be studied in this experiment is the acid buffered oxidation of iodide to triiodide by hydrogen peroxide:
3I- + H2O2 + 2H+ --> I3- + 2 H2O
Your assignment is to study the rate of this reaction at various concentrations of H2O2.
Work with a partner.
Practice the shake-down technique:
Fill the first 5 wells of each of 2 12-well strips with 3 drops of distilled water. Your instructor will demonstrate the "shake-down" method of mixing. Stack one strip atop the other, as in the Figure.
Holding the plates firmly together shake them once vigorously in a downward motion. This is done by flicking your hand down as fast as you can and stopping abruptly. There is no upward motion so this is a shakedown method.
Begin the experiment:
Arrange two 12-well strips so the numbers are read from left to right. Place drops of solutions into the two strips according to the following table:
Row 1: Solution A
| Cell #: | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
| Drops Soln A: | 4 | 4 | 4 | 3 | 3 | 3 | 2 | 2 | 2 | 1 | 1 | 1 |
| Drops Water: | 0 | 0 | 0 | 1 | 1 | 1 | 2 | 2 | 2 | 3 | 3 | 3 |
Row 2: Solutions B and C
| Cell # | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
| Drops Soln B | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Drops Soln C | 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 |
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, your partner should start the timer.
The cells may not change color at the same instant. Record the "blue-time" when the second color change occurs in each group of three cells.
Repeat if necessary. This experiment does not take much time so if you miss a blue-time you can easily repeat the procedure.
The materials used in this experiment may be disposed of safely at the sink.
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 rate is:
[H2O2 ]/Δt)
But the concentration change at the time of the endpoint is a constant:
3I- + H2O2 + 2H+ --> I3- + 2 H2O
I3- + 2 S2O32- --> 3 I- + S4O62-
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 does not change, a = 0. If there is a straight line through the origin, a = 1. If there is a parabola, a = 2.
Plot a graph of your 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.
Based upon the graphs, determine the order of the reaction with respect to H2O2.
Briefly describe how you would use variations of this lab to determine the order for the I- and H+.
Having determined the order for the three reactants, how would you determine the value of k?
If the students follow instructions 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.
The reaction rate is measured by determining the time required for the reaction to consume a small amount of thiosulfate which is initially present in solution. Thiosulfate is relatively inert toward hydrogen peroxide, but is very rapidly oxidized by triiodide:
I3- + 2S2O32- --> 3I-- + S4O62-
No appreciable amount of triiodide forms until the thiosulfate has been completely consumed. At this point, the starch reacts with the triiodide suddenly turning dark blue (starch iodine complex). During the process of this reaction only a negligible amount of the peroxide reacts so its initial value is essentially unchanged. The H+ concentration is buffered and remains appreciably unchanged. The I- oxidized by the peroxide is immediately regenerated by the reaction of triiodide with thiosulfate so the I- concentration is unchanged during the measured time interval.
Students should be able to determine that this reaction is first order with respect to H2O2.
Data Combined From 11 Experiments
|
Drops H2O2 |
Time (seconds) |
loge (drops) |
1/drops |
| 4 | 15.0 | 1.386 | 0.250 |
| 3 | 21.2 | 1.099 | 0.333 |
| 2 | 32.1 | 0.693 | 0.500 |
| 1 | 64.4 | 0.000 | 1.000 |
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 Woodrow Wilson microscale version of this lesson was developed by:
Bruce Clark
Buena H. S.
Sierra Vista, AZ 85635
Richard Perry
Cottage Grove H. S.
Cottage Grove, OR 97424