Exploring Kinetics with the Iodine Clock Reaction

Roles: Leader will guide discussion, make sure all aspects of assignment are completed.

Experimenter is responsible for gathering and returning materials, cleaning up spills, making sure correct procedure is followed.

Recorder is responsible for recording group answers to questions.

Procedure: Obtain the following supplies:
1. One well plate with five reagents (0.2% starch, 0.12M Na2S2O3, 0.20M KI, 0.20M (NH4)2S2O8 as illustrated below.

Use care! Potassium peroxydisulfate is a strong irritant and oxidizing agent---Wipe up all spills thoroughly!

A1 A2 A3 A4
Starch 0.012M Na2S2O3 0.20M KI 0.20M (NH4)2S2O8

B1 B2 B3 B4

C1 C2 C3 C4

2. Five 1mL graduated pipets
3. Small glass stirring rod
4. 1-2 paper towels
5. One pair of safety glasses per person

Background:
In this experiment we will study the reaction between iodide, I-, and peroxydisulfate, S2O82-. The equation for this reaction, which is balanced with respect to charge and all elements except oxygen, is:

3I-(aq) + 2 S2O82-(aq) → I3-(aq) + 2S2O42-(aq)

The first product of this reaction, I3-(triiodide ion), may be detected by strong purpleblack complex it forms with starch. To each of our reaction mixtures, we will add a constant amount of Na2S2O3, sodium thiosulfate, which reacts with I3- according to the equation:

I3-(aq) + 2 S2O32-(aq) → 3I-(aq) + S4O62-(aq)

Thus, in each reaction you do, the same amount of triiodide will be consumed before excess triiodide accumulates in solution, giving a purple color. The time for the purple color to appear is therefore a measure of the reaction rate between I- and S2O82-. We will determine the approximate rate law for this reaction by conducting a simple experiment. Important note: the rate law for any reaction must be determined experimentally, and it cannot be predicted simply by an examination of the stoichiometric equation.

Procedure:
A. Varying the concentration of I-
In