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Catalyst demonstration

Watch the video in which the addition of a pink, aqueous cobalt(II) salt affects the rate of reaction between potassium sodium tartrate and hydrogen peroxide solutions. What important feature about catalysts is visually illustrated in this reaction?

Video - catalysis by a cobalt(II) salt

This experiment illustrates an important point that a true catalyst can be recovered chemically unchanged at the end of the reaction. This is not to say that it does not change during the reaction. In this reaction we see the pink colour of the cobalt(II) catalyst change to green (this is a different oxidation state of the cobalt). However, at the end of the experiment the original pink colour of the cobalt(II) ion is recovered.

Another example of a transition element compound acting as a catalyst as a result of its variable oxidation number occurs with the following redox reaction:

This reaction is slow as it has a high activation energy. Try to think why these reactants might be reluctant to react with each other and check your answer here .

The reaction is catalyzed by Fe²⁺ ions. These are first oxidized to Fe³⁺ by the peroxodisulfate ion:

The Fe³⁺ ions then oxidize the iodide to iodine:

These last two reactions are much faster than the first one (they have a lower activation energy). If you know the answer to the last question you should be able to see why. Check your answer here . Taking the two reactions together we get the original reaction. Although the Fe²⁺ catalyst was oxidized, it is subsequently returned to its original state and can be recovered.