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8. Add distilled water to Flask B to make a total volume of 1 L.
9. Allow Flask B to return to room temperature.
10. Pour 100 mL of Solution A in a 400 mL beaker (Beaker A).
11. Pour 100 mL of Solution B in a second 400 mL beaker (Beaker B).
12. Pour the solution in Beaker B into Beaker A, and mix.
13. Start the timer (optional). The ability to record the time at which the blue complex appears allows the rate
of reaction to be determined accurately with a timer.
Expected Results:
Upon mixing the two reactants, the colorless solution turns dark blue after a set amount of time.
Teaching Notes:
• Time the reaction just before the demonstration to say a "magic word" to activate the reaction.
• Prepare the solution no earlier than the day before you plan to do the demonstration.
• A white background will heighten the impact of the sudden and spectacular color change. There is no warning when the blue
color is about to appear.
• This demonstration is a captivating way to show the classic characteristics of a clock reaction. The reaction is also an excellent
way for students to learn about kinetics—the effects of concentration, temperature, and the presence of a catalyst on the rate
of reaction.
Follow up/Extension:
Student Questions:
1. What is the role of the starch in this reaction?
2. What conclusions would you make from this activity regarding the effect of temperature or the effect of concentration
of a reagent on the rate of a chemical reaction?
• Allow students to investigate the effect of concentration on the rate of reaction by diluting a known amount of
Solution B with a known amount of water.
• Allow students to investigate the effect of temperature on the rate of reaction by heating and cooling the contents
of Beaker A and Beaker B.
• Replace starch with cellulose paper. This provides the reaction with a white solid surface in which color change can be
clearly observed and reduces reagent amounts required to 540 μL per group. Ask students to calculate reaction orders
and the reaction constant k to determine the rate equation for this reaction.
Disposal/Clean-Up:
The solutions can be disposed of down the drain. Consult local regulations before any disposal activity.
Iodine Clock Reaction (continued)
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