What do you mean by activity and selectivity of catalysts ?

Explanation

Activity means the ability of catalysts to increase the chemical reaction. A common example of activity is the reaction `H_2` and `O_2` to form water in the presence of platinum. Without the presence of the catalyst, the mixture of `H_2 and O_2` can be safely stored. But in the presence of catalyst, the reaction occurs with explosive voilence. In some cases the catalyst can accelerate the reaction as high as `10^8` times.
The selectivity means the ability of the catalyst to direct reaction to give particular products. For example, n-heptane in the presence of platinum catalyst gives toluene.
`nC_7H_16 overset(Pt)to C_6H_5CH_3`
Similarly, propylene `(CH_3CH = CH_2)` reacts with `O_2` in the presence of bismuth molybdate catalyst to selectively give acrolein `(CH_2CHCHO)`.