A Visible Activated Complex
This is a classic chemistry demonstration, performed by students in my upper 6 chemistry class. Hydrogen peroxide oxidises potassium sodium tartrate (Rochelle salt) to carbon dioxide. The reaction is catalysed by cobalt(II) chloride. When solutions of hydrogen peroxide and Rochelle salt are mixed, carbon dioxide is slowly evolved. The addition of cobalt(II) chloride causes the reaction to froth, indicating a large increase in the reaction rate. At the same time the colour of the cobalt(II) chloride turns from pink to green (an activated complex), returning to pink again within a few seconds as the reaction dies down. This indicates that catalysts actually take part in the reaction and are returned unchanged when the reaction is complete.
The basic reaction appears to be:
5H2O2(aq) + C4H4O62–(aq) → 4CO2(g) + 2OH–(aq) + 6H2O(l)
The equation may also be written in two parts:
3H2O2(aq) + C4H4O62–(aq) → 2CO2(g) + 2HCOO–(aq) + 4H2O(l)
2HCOO–(aq) + 2H2O2(aq) → 2CO2(g) + 2H2O(l) + 2OH–(aq)
The reaction is catalysed by pink Co2+ ions which are first oxidised to green Co3+ ions (complexed by tartrate ions) and then reduced back to Co2+. While the majority of the gas evolved is carbon dioxide, oxygen will also be produced from the decomposition of some of the hydrogen peroxide. The gas mixture will turn limewater milky, but does not extinguish a glowing splint.
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