When the concept of activity is taught in the context of chemical equilibrium, it is often described as an “effective concentration.” What this approach often misses, however, is the relationship between activity and how molecules act and react. This connection can be made better in the context of kinetics, where a difference in activity affects how long a reaction takes to reach completion. To demonstrate this effect, the iodine clock reaction can be modified by the addition of an unreactive salt. This increase in the ionic activity of the solution leads to a slower reaction, which is readily seen by the longer time required for the clock reaction to complete. This simple demonstration allows for discussion of the actual role of “spectator ions” in chemical reactions and can help students better appreciate what is meant by activity, which should also enrich their understanding of chemical equilibrium.

Teaching activity in the context of kinetics

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