This is a dramatic demonstration of the effect of adding a catalyst to a sluggish reaction. The resulting accelerated reaction is vigorous and the survival of the catalyst is evidenced by the color of the cobalt ion in solution.

300 mL Potassium sodium tartrate (83.3 g/1000 mL water) solution (hot)
100 mL Hydrogen peroxide, 6% aqueous
25 mL Cobalt(II) chloride hexahydrate, 4% in water
"University of Texas, Austin" test tube and holder

This demonstration should accompany a discussion of kinetics and the idea of catalysts, namely, substances that increase the rate of a reaction without being consumed themselves. In this case the reaction is a decarboxylation of tartrate ion, the evidence for which is production of lots of bubbles of carbon dioxide gas. This reaction is very slow, even when the reactants are hot.

Set out a Texas-size test tube for the reaction. Heat 300 mL of the tartrate solution to simmer on a hot plate and pour it into the test tube. Pour in 100 mL of the hydrogen peroxide solution. Nothing in particular will happen. Now add 25 mL of the cobalt(II) chloride solution. Call the initial pink color to the attention of the class.

When the cobalt chloride catalyst is added to the hot colorless reactant solutions a vigorous reaction starts within a few seconds. Lots of gas is formed and the solution turns dark green for a few seconds. After about one minute, the reaction subsides, the green color disappears, and the pink color of the cobalt chloride catalyst remains.

For a reaction like

A + B product (very slow)

The effect of a catalyst can be shown in the following mechanistic steps:

A + cat Acat (moderate)

Acat + B product + cat (fast)

Notice that when the two steps are combined, one obtains the same overall equation, but at a much faster rate. One can make a rough parallel between this mechanistic scheme and the observed reaction. The color of the cobalt(II) catalyst is pink, which can be seen at the start of the reaction. One can associate the dark green color with the intermediate Acat, which forms and then rapidly disappears.

The actual mechanism of this particular reaction is probably more complex, but the simple two step process and the observed color changes illustrate the concept of a catalyst very well.

One could also set out a blank reaction in which the tartrate and the hydrogen peroxide solutions are combined without the cobalt catalyst. This can stand in comparison to the fast catalyzed reaction.

Exercise appropriate care when handling hot solutions.

The catalyzed reaction can be very vigorous and can froth over the top of the test tube. It is best to set the test tube in its holder. If you have enough nerve, you can hold the test tube while the reaction is going on. If it looks like it is going to overflow, set it in its rack and stand back.

P.T.Ruda, J.Chem.Educ., 55, 652 (1978).