Identify the redox reaction out of the following reacitons and identify the oxidising and reducing agents in them.
(a) `3HCl(aq)+HNO_(3)(aq)toCl_(2)(g)+NOCl(g)+2H_(2)O(l)`
(b) `HgCl_(2)(aq)+2KI(aq)toHgI_(2)(s)+2KCl(aq)`
( c) `Fe_(2)O_(3)(s)+3CO(g)overset(Delta" ")(to)2Fe(s)+3CO_(2)(g)`
(d) `PCl_(2)(l)+3H_(2)O(l)to3HCl(aq)+H_(2)PO_(3)(aq)`
(e) `4NH_(3)(aq)+3O_(2)(g)to2N_(2)(g)+6H_(2)O(g)`

Explanation

(i) Writing the O.N. on each atom,
`overset(+1-1)(3HCl)(aq)+overset(+1+5-2)(HNO_3)(aq) rarroverset(0)(Cl_2)(g)+overset(+3-2-1)(NOCl)(g)+overset(+1-2)(2H_2O)(l)`
Here, O.N. of Cl increases from -1 (in HCl) to 0 (in `Cl_2` ) Therefore , `Cl^(-)` is oxidised and hence HCl acts as a reducing agent.
The O.N. of N decreases from +5 (in `HNO_3` ) to +3 (in NOCl) and therefore , `HNO_3` acts an oxidising agent.
Thus, reaction (i) is redox reaction.
(ii) `overset(+2-1)(HgCl_2)(aq)+overset(+1-1)(2KI(aq)) rarroverset(+2-1)("Hg I"_2)(s)+overset(+1-1)("2K Cl")(aq)`
Here O.N. of none of the atoms undergo a change and therefore , this is not a redox reaction.
(iii) `overset(+3-2)(Fe_2O_3)(s)+overset(+2-2)(3CO)(g) overset(Delta)rarroverset(0)2(Fe)(s)+3overset(+4-2)(CO_2)(g)`
Here, O.N. of Fe decreases form +3 (in `Fe_2O_3`) to 0(in Fe) and therefore , `Fe_2O_3` acts as an oxidising agent.
O.N of C increases from +2 (in CO) to +4 (in `CO_2` ) and therefore , CO acts as a reducing agent.
Thus , this is a redox reaction.
(iv) `overset(+3-1)(PCl_3)(l)+overset(+1-2)(3H_2O)(l)rarroverset(+1-1)(3HCl)(aq)+overset(+1+3-2)(H_3PO_3)(aq)`
Here O.N of none of the atoms undergo a change and therefore , it is not a redox reaction.
(v) `overset(-3+1)(4NH_3)(g)+overset(0)(3O_2)(g)rarroverset(0)(2N_2)(g)+overset(+1-2)(6H_2O)(g)`
Here O.N. of N increases from -3 - (in `NH_3` ) to 0 on `(N_2)` and therefore , `NH_3` acts as a reducing agent . O. N of O decreases from 0 (in `O_2`) to -2 (in `H_2O`) and therefore , `O_2` acts as an oxidising agent.
Thus, reaction (v) is a redox reaction.