Question

What do you mean by activity and selectivity of catalysts?

Answer 1

Activity is the ability of the catalyst to increase the rate of reaction. For example, in absence of a catalyst, the reaction between hydrogen and oxygen to form water does not occur at an appreciable rate. Infact, hydrogen and oxygen can be safely stored together without reaction. In the presence of platinum catalyst, the reaction occurs with explosive violence. In few reactions, the catalyst can accelerate the rate of reaction to as high as ${10}^8$ times.

The ability of the catalyst to direct the reaction to give particular products is its selectivity. For example, n-heptane in presence of platinum catalyst gives toluene.

$n{C}_7{H}_{16}\stackrel{Pt}{\to }{C}_6{H}_{5}C{H}_3$

Also, propylene $(C{H}_3-CH=C{H}_2)$ reacts with oxygen in the presence of bismuth molybdate catalyst to selectively give acrolein, $C{H}_2=CHCHO$.