Chemical Kinetics (Clock Reaction)

Formula / Core Reaction:

$$\text{H}_2\text{O}_2 + 2\text{I}^- + 2\text{H}^+ \rightarrow \text{I}_2 + 2\text{H}_2\text{O} \quad \text{(Slow Rate-Determining Step)}$$

$$\text{I}_2 + 2\text{S}_2\text{O}_3^{2-} \rightarrow 2\text{I}^- + \text{S}_4\text{O}_6^{2-} \quad \text{(Fast Reaction)}$$

Concept: In the iodine clock reaction, liberated $$\text{I}_2$$ is instantaneously consumed by a fixed amount of sodium thiosulfate ($$\text{Na}_2\text{S}_2\text{O}_3$$). As soon as all thiosulfate is completely consumed, excess $$\text{I}_2$$ immediately reacts with starch indicator to produce a sudden blue-black color, allowing precise timing of the reaction rate.

Notation / Key Observation:

• Starch Indicator: Turns blue-black upon appearance of free $$\text{I}_2$$;
• $$\text{Na}_2\text{S}_2\text{O}_3$$: Sodium thiosulfate (Consumes $$\text{I}_2$$ to create a timed delay proportional to rate).