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The Iodine Clock Reaction

Materials Needed

• Distilled water
• Beakers and graduated cylinders
• Stirring rod

Reactants:

• Potassium Iodate, KIO₃, 0.20 M, 325 mL
• Sodium metabisulfite, Na₂S₂O₅, 0.20 M, 60 mL
• Starch solution, 180 mL

Setup

• Reaction 1
• Beaker A: 100 mL KIO₃, 100 mL water
• Beaker B: 10 mL Na₂S₂O₅, 30 mL starch solution, 40 mL water
• Reaction 2
• Beaker A: 50 mL KIO₃, 150 mL water
• Beaker B: 10 mL Na₂S₂O₅, 30 mL starch solution, 40 mL water
• Reaction 3
• Beaker A: 25 mL KIO₃, 175 mL water
• Beaker B: 10 mL Na₂S₂O₅, 30 mL starch solution, 40 mL water

The proportions for Beaker A (potassium iodate to water) is what changes resulting in different reaction times.

Steps of the Reaction

1. Formation of iodate ions (IO₃-) and hydrosulfite ions (HSO₃-) in solution
1. In Solution A: H₂O(l) + NaS₂O₅(s) → 2HSO₃^- (aq) + 2Na⁺(aq)
2. In Solution B: KIO₃(aq) → IO₃^–(aq) + K⁺(aq)

2.)When the solutions mix, iodate ions react with hydrogen sulfite ions to produce iodide ions.

a) IO₃^–(aq) + 3HSO₃^–(aq) → I^–(aq) + 3H⁺(aq) + 3SO₄^2-(aq)

3.)Iodide ions react with excess iodate ions in the presence of hydrogen to produce iodine.

a) 6H⁺(aq) + 5I^–(aq) + IO₃^–(aq) → 3I₂(aq) + 3H₂O(l)

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Steps of The Reaction

• Iodine immediately reacts with any hydrogen sulfite ions still present to form iodide ions.
• I2(aq) + HSO3–(aq) + H2O(l) → 2I–(aq) + SO42–(aq) + 3H+(aq)
• Once all of the hydrogen sulfite ions have reacted, the iodine reacts with the starch, resulting in a dark-blue color.

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Collision Theory

• States that in order for a reaction to occur, reactant molecules must collide first
• Collision energy must exceed the activation energy
• Low activation energy — almost all molecules will have enough energy to exceed the energy barrier
• High activation energy — a small fraction of the molecules will have enough energy to exceed the energy barrier
• Changing the concentration or the temperature of reactants influences collision rate between molecules

Effect of Concentration and Temperature

• Increasing concentration of reactants increase the rate of collisions between molecules
• This increases the total number of effective collisions
• Increasing temperature would increase the average kinetic energy of the colliding molecules
• Kinetic energy is proportional to the absolute temperature

Applications (Circadian Clocks)

• A biochemical clock present in virtually all light-sensitive organisms
• Regulates our sleep-wake cycle, hormonal secretion, and many other physiological functions
• Similar to how the iodine clock can experience changes from temperature, so can circadian rhythms
• Heat exposure increases wakefulness and decreases slow wave sleep and rapid eye movement sleep

Applications in Medicine

• The ability of iodide ions to form a complex with a variety of polymers is important in medicine.
• Topical antiseptics (Providone-iodine)
• Providone: synthetic polymer
• Commonly used for skin cleansing, as a prophylaxis or treatment for wound infection
• Radiopaque materials
• Iodide-containing copolymers
• Excellent absorbers of x-rays so they are used for diagnostic purposes

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