Iodine Clock Reaction

A timed reaction that suddenly turns dark blue after a predictable delay

Classroom Demonstrations Intermediate (High School) 45 min ~$10.00

Objective

Demonstrate reaction kinetics and the effect of concentration and temperature on reaction rate using the iodine clock reaction.

Background

The iodine clock reaction is a classic demonstration of chemical kinetics. Two colorless solutions are mixed and after a predictable delay, the solution suddenly turns dark blue. The delay occurs because bisulfite (HSO₃⁻) rapidly reduces any I₂ produced back to I⁻ (the clock mechanism). When all the bisulfite is consumed, I₂ accumulates and reacts with starch to produce the dramatic blue-black color. By varying concentration or temperature, students can explore factors affecting reaction rate.

Safety Warnings

  • H₂SO₄ is corrosive
  • NaHSO₃ releases SO₂ gas if over-acidified — work in ventilated area
  • KIO₃ is an oxidizer
  • Hot plate surfaces are a burn hazard

Required PPE

goggles gloves lab_coat

Materials

  • Potassium iodate (KIO₃) (2 g)
    Solution A
  • Sodium bisulfite (NaHSO₃) (1 g)
    Solution B
  • Starch solution (1%) (10 mL)
    Indicator in Solution B
  • Sulfuric acid (0.5M H₂SO₄) (20 mL)
    Solution A acidification
  • Distilled water (500 mL)

Equipment

Beakers (250 mL, 4) Magnetic stirrer Stopwatch Thermometer Graduated cylinders Hot plate Ice bath

Procedure

1

Prepare Solution A: Dissolve 2 g KIO₃ in 200 mL water, add 20 mL of 0.5M H₂SO₄.

5 min
2

Prepare Solution B: Dissolve 1 g NaHSO₃ in 200 mL water, add 10 mL of 1% starch solution.

5 min
3

Trial 1 (baseline): Mix 50 mL of A with 50 mL of B and start the stopwatch immediately. Record the time until the solution turns blue.

5 min
4

Trial 2 (concentration effect): Mix 25 mL of A + 25 mL water with 50 mL of B. Record the time. It should take approximately twice as long.

7 min
5

Trial 3 (temperature effect): Warm both solutions to 40°C. Mix 50 mL of each and record the time. It should be significantly shorter.

8 min Handle warm solutions carefully
6

Trial 4 (cold): Cool both solutions to 10°C in an ice bath. Mix and record the time.

8 min
7

Plot reaction time vs. concentration and temperature. Discuss rate law, activation energy, and the Arrhenius equation.

7 min

Expected Results

The blue color appears suddenly after a delay of 5–60 seconds depending on conditions. Doubling concentration roughly halves the time. Increasing temperature by 10°C roughly halves the time (consistent with the Arrhenius equation).

Cleanup

Pour reacted solutions down the drain with water. Rinse all glassware. Clean the magnetic stirrer.

Details

Category
Classroom Demonstrations
Difficulty
Intermediate (High School)
Duration
45 min
Est. Cost
$10.00
Steps
7
Materials
5

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Frequently Asked Questions

What is the objective of Iodine Clock Reaction?
Demonstrate reaction kinetics and the effect of concentration and temperature on reaction rate using the iodine clock reaction.
How difficult is Iodine Clock Reaction?
This experiment is rated as Intermediate (High School). It takes approximately 45 minutes to complete.
What safety precautions are needed for Iodine Clock Reaction?
Key safety precautions include: H₂SO₄ is corrosive; NaHSO₃ releases SO₂ gas if over-acidified — work in ventilated area; KIO₃ is an oxidizer.
What materials are needed for Iodine Clock Reaction?
The main materials required are: Potassium iodate (KIO₃), Sodium bisulfite (NaHSO₃), Starch solution (1%), Sulfuric acid (0.5M H₂SO₄), Distilled water.
What results should I expect from Iodine Clock Reaction?
The blue color appears suddenly after a delay of 5–60 seconds depending on conditions. Doubling concentration roughly halves the time. Increasing temperature by 10°C roughly halves the time (consistent with the Arrhenius equation).