The role of a catalyst is to change $ . . . . . . $.

Why is desorption important for a substance to act as a good catalyst?

Match the following:

List-$I$ (Reaction)	List-$II$ (Catalyst)
$A$. Hydrogenation of vegetable oils	$I$. $Ni$
$B$. Decomposition of potassium chlorate	$II$. $MnO_2$
$C$. Oxidation of $SO_2$ in lead chamber process	$III$. $Pt$
$D$. Oxidation of ammonia in Ostwald's process	$IV$. $NO_{(g)}$

The correct answer is

Which is used in the Haber process for the manufacture of $NH_3$?

What is the role of a catalyst in a chemical reaction?

When $KMnO_4$ solution is added to hot oxalic acid solution,the decolorization is slow in the beginning but becomes instantaneous after some time. This is because