A Reversible Blue-and-Gold Reaction

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Page ID: 2967

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Chemical Concept Demonstrated

Reversible oxidation-reduction reactions

Demonstration

Rochelle salt (KNaC₄H₄O₆) is dissolved in water.
3% H₂O₂ is added while stirring the solution.
CuSO₄ is also added.

Observations

Within about 20 seconds, a blue solution forms, which undergoes a vigorous reaction that liberates gas and raises the temperature of the solution. Soon, an opaque orange-gold precipitate of Cu₂O forms. Additional H₂O₂ is added to the soltuion, and the metal is then oxidized back to Cu (II) oxidation state. The solution turns from orange to blue as the precipitate disappears. With time, the orange precipiate of Cu₂O forms once again.

Explanations (including important chemical equations)

The Cu ²⁺ tartrate ions react to form a blue complex ion, which catalyzes the decompostion of the initial aliquot of H₂O₂. This raises the temperature of the solution to the point that the Cu²⁺ ion oxidizes the tartrate ion and is thereby reduced to Cu⁺, which precipitates from solution as Cu₂O. The aliquot of H₂O₂then oxidizes the copper (I) oxide back to Cu ²⁺, which forms a blue complex with the tartrate ion that remains is solution. Most of the gas evolved in this demonstration is oxygen, with a small amount of CO₂. Only about 2-3% of the tartrate is oxidized to CO₂ in each cycle of the reaction.

Contributors

Dr. George Bodner (Perdue University)