What do you mean by activity and selectivity of catalysis?

Explanation

Activity : Activity of a catalyst refer to the chemisorption of reactants reasonably strong. It has ability to increase chemical reaction.
For example : Transition metals show catalytic activity. (Maximum activity group 7 - 9 metal).
Selectivity : Selectivity of a cataysts is its ability to speed up a reaction to yield a particular product.
For example : `(i) CO + H_2` Combine to form `CH_4` when `Ni ` is the catalyst and methanol when `Cr_2O_3` is catalyst.
`CO + 3H_2 overset("Ni")(rarr) underset("Methane")(CH_4)+H_2O`
`CO + 2H_2 underset(Cr_2O_3)overset(Cu//ZnO)(rarr)underset("Methyl alcohol")(CH_3OH)`.