The Chemical Clock Reaction Lab Report
Course: SCH4U – Grade 12 Chemistry (University Preparation)
Experiment: Chemical Kinetics & Organic Chemistry Lab
Topic: The Iodine–Vitamin C Chemical Clock Reaction
Introduction
A chemical clock reaction is a reaction that undergoes observable periodic
or sudden changes due to competing reaction pathways. In this experiment,
the iodine–vitamin C clock reaction was investigated to examine concepts
related to chemical kinetics, oxidation–reduction reactions, catalysis, and
collision theory. The reaction involved vitamin C (ascorbic acid), iodine,
hydrogen peroxide, and starch. As long as vitamin C was present, iodine was
rapidly reduced back to iodide ions. Once the vitamin C was consumed,
iodine accumulated and formed a dark blue-black complex with starch,
signaling the endpoint of the reaction.
Purpose
To observe and analyze the iodine–vitamin C chemical clock reaction and
investigate how competing reaction pathways and catalytic behavior
produce a sudden colour change.
Hypothesis
If vitamin C reduces iodine faster than hydrogen peroxide produces it, then
the solution will remain colourless initially. Once the vitamin C is depleted,
iodine will accumulate and react with starch, causing the solution to
suddenly turn dark blue-black.
Background Theory
The experiment demonstrates an oscillating chemical system involving
oxidation–reduction reactions. Vitamin C acts as a reducing agent, while
iodine and hydrogen peroxide act as oxidizing agents. The starch functions
as an indicator because it forms a blue-black complex in the presence of
iodine.
Main reaction:
I₂ + Ascorbic Acid → 2I⁻ + Dehydroascorbic Acid
Slow catalytic reaction:
2H₂O₂ → 2H₂O + O₂
The iodide ion acts as a catalyst by lowering the activation energy of
hydrogen peroxide decomposition. According to collision theory, increasing
the number of effective collisions increases the reaction rate.
