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Iodine Clock Reaction
Recommended Grade Level(s):
Appropriate for: Middle school and High school
Time Requirements:
Activity Time: 10 minutes
Teaching Topics & Concepts:
To illustrate a time-delayed reaction, which leads to the exploration of several factors which affect reaction rate.
• Kinetics, Analyzing and interpreting data, Clock reactions, Indicators, Structure and properties of matter, Chemical reactions.
Background:
A chemical clock is a mixture of chemicals that show an observable change after a short time. When one of the reagents has a
visible color, crossing a concentration threshold can lead to an abrupt color change. Teachers can use this demonstration to
introduce concepts about kinetics. For example, students can learn about the effect of concentration, temperature, and a catalyst
on the rate of reaction. Chemical kinetics has applications across many disciplines, including cosmology, geology, biology,
engineering, and even psychology.
Materials:
• Soluble starch
• Potassium iodate, KlO3
• Sodium hydrogen sulfite (sodium bisulfite), NaH5O3
• Sulfuric acid, H2SO4 (1.0 M)
• (2) 1 L Erlenmeyer flasks
• (2) 400 mL beakers
• 1 each 25 mL and 100 mL graduated cylinders
• Distilled water
• Stirring rod
• Electronic balance
• Timer (optional)
Procedure:
1. Prepare Solution A in a 1 L Erlenmeyer flask (Flask A) containing 0.9 L of water.
2. Add 2 g of potassium iodate to Flask A. Stir.
3. Add distilled water to Flask A to make a total volume of 1 L.
4. Prepare Solution B in a 1 L Erlenmeyer flask (Flask B) containing 0.9 L of boiling water.
5. While stirring, slowly add 4.0 g of soluble starch to Flask B.
6. Add 0.8 g of sodium bisulfite to Flask B. Stir.
7. Add 10.0 mL of 1.0 M sulfuric acid to Flask B. Stir.
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Safety
• Read the SDS sheets for all chemicals before using them.
• Wear safety glasses, gloves, and a lab coat.
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