Identify the catalytic reaction in which both reactants are in different phases.

Match the catalyst (Column-$I$) with products (Column-$II$)

Column-$I$ Catalyst	Column-$II$ Product
$(a)$ $V_2O_5$	$(i)$ Polyethylene
$(b)$ $TiCl_4/Al(Me)_3$	$(ii)$ Ethanal
$(c)$ $PdCl_2$	$(iii)$ $H_2SO_4$
$(d)$ Iron oxide	$(iv)$ $NH_3$

$A$ catalyst is used to

Catalysts in the following reactions are:
$(I) CH_3COOCH_{3(l)} + H_2O_{(l)} \rightarrow CH_3COOH_{(aq)} + CH_3OH_{(l)}$
$(II) 2SO_{2(g)} + O_{2(g)} \rightarrow 2SO_{3(g)}$ (Contact Process)
$(III) 2SO_{2(g)} + O_{2(g)} \rightarrow 2SO_{3(g)}$ (Lead Chamber Process)
$(IV) N_{2(g)} + 3H_{2(g)} \rightarrow 2NH_{3(g)}$ (Haber Process)

In aqueous solution,$Na_2SO_3$ is oxidized by air,but $Na_3AsO_3$ is not. However,when air is passed through a solution containing both $Na_2SO_3$ and $Na_3AsO_3$,both are oxidized. This is an example of:

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