Formaldehyde Clock Reaction Demonstration Sheet

22.16 Formaldehyde Clock Reaction-- Red/Green Race

Chemical Concepts Demonstrated: Concentration effects on reaction rate, addition reactions across a C=O double bond

Demonstration:

The first set of beakers contains varying amounts of solution A and water. The second set of beakers contains varying amounts of solution B and water. One set has green indicator, the other red.

Solution A: Formalin (37% formaldehyde) diluted.
Solution B: NaHSO₃, Na₂SO₃, and disodium EDTA dissolved in water and diluted.
Green Indicator: Thymolphthalein mixed with p-nitrophenol solution.
Red Indicator: Phenolphthalein mixed with p-nitrophenol solution.

Observations:

The clock reaction shown proceeds in a linear fasion. The beakers on the left change first while the beakers on the right change last.

Explanation:

The beakers change color at different rates because of the different concentrations of starting materials, particularly the materials involved in the rate-limiting step, the production of formaldehyde:

CH₂(OH)₂--> HCHO + H₂O

The faster the formaldehyde is produced, the faster the beakers change color.