Exhibition chemistry

Traffic lights

By 2005-07-01T00:00:00

Beakers with red, yellow and green liquid

Source: Skodadad/Getty Images

An exciting gateway to a full-scale kinetic investigation

In this simple demonstration, potassium sodium 2,3-dihydroxybutanedioate or potassium sodium tartrate (Rochelle salt) is oxidised by hydrogen peroxide in the presence of cobalt(II) ions. You simply mix the hot reactants and add the catalyst.

C4H4O62-(aq) + 3H2O2(aq) → 2CO2(g) + 2HCO2-(aq)+ 4H2O(l)

The interesting feature of this reaction is the colour change which occurs as the reaction proceeds. The initial colour is pink which changes to dark green and then back to pink again - hence the traffic light reaction. When you see the coloured intermediate, you can try to stabilise it by cooling the mixture rapidly. The nature of this intermediate can lead to some stimulating discussion about colour, thermodynamic and kinetic stability.

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