Under what circumstances might be important to know the activity tendencies of elements?

It is very important to know the activity tendencies of the elements. The activity tendencies tells us about whether the element is reactive or not.

In the redox-reaction where there is a need to know the oxidizing agent and reducing agent, we can know it easily from the activity tendencies. The elements lying above the reactivity series are better reducing agents.

In the substitution reactions, the activity tendencies helps us to know which element will replace the other. The element lying above in the series will replace the element lying below it.

where, N is an element that lies above in the reactivity series

M is an element that lies below in the reactivity series