Discussion

A catalyst alters the rate of a chemical reaction without itself being consumed. An intermediate is formed during one step of a chemical reaction and consumed in a later step, so that, like the catalyst, it does not appear in the net equation for a reaction. The intermediate in this reaction is a cobalt(III)-tartrate complex, [Co(C₄H₄O₆)₃)]^3-.

The overall reaction taking place is

C₄H₂O₆^4-(aq) + 5 H₂O₂(aq) → 4 CO₂(g) + 4 OH^-(aq) + 4 H₂O(l)

Narration

Four vials contain sodium potassium tartrate solutions in hydrogen peroxide at different temperatures. One milliliter of pink cobalt(II) chloride catalyst solution is added to three of the vials.

The vial on the right is initially at 55°C. When the pink cobalt(II) chloride catalyst is added, the solution turns green and bubbles form, and some of the solution foams out of the vial. The color change is indicative of an intermediate chemical species of cobalt.

The second vial from the right was initially at 35°C. When the catalyst is added to this solution, the reaction proceeds more slowly than in the solution on the right. The role of the cobalt is to serve as a catalyst. Hydrogen peroxide oxidizes the cobalt(II) to cobalt(III). The cobalt(III) intermediate subsequently oxidizes the tartrate to carbon dioxide gas, and returns to the original cobalt(II).

The next vial is at room temperature, so the catalyzed reaction proceeds even more slowly. It takes over an hour for this reaction to reach completion.

The control vial on the left was initially at 55°C. The absence of bubbles in the control solution indicates that even at this elevated temperature, without a catalyst, no reaction occurs between the peroxide and the tartrate.