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

Writing the O.N. on each atom above its symbol, then
`3overset(+1)(H)overset(-1)(Cl)(aq)+overset(+1)(H)overset(+5)(N)overset(-2)(O_(3))(aq)tooverset(0)(Cl_(2))(g)+overset(+3)(N)overset(-2)(O)overset(-1)(Cl)(g)+2overset(+1)(H_(2))overset(-2)(O)(l)`
Here, the O.N. of Cl in increases from -1 in HCl to O in `Cl^(-)` is oxidised and hence HCl acts as the reducing agent.
The O.N. of N decreases from +5 in `HNO_(3)` to +3 in `NOCl`, therefore, `HNO_(3)` acts as the oxidising agent.
Thus, this reaction is a redox reaction.
(b) Writing the O.N. of each atom above its symbol, we have,
`overset(+2)(Hg)overset(-1)(Cl_(2))(aq)+2overset(+1)(K)overset(-1)(I)(aq)tooverset(+2)(Hg)overset(-1)(I_(2))(s)+2overset(+1)(K)overset(-1)(Cl^(-))(aq)`
Here, tehe O.N. of none of the atoms undergo a change. therefore, this reaction is not a redox reaction.
( c) `overset(+3)(Fe_(2))overset(-1)(O_(3))(s)+3overset(+2)(C)overset(-2)(O)(g)overset(Delta" ")(to)2overset(0)(Fe)(s)+3overset(+4)(C)overset(-2)(O_(2))(g)`
Here, O.N. of Fe decreases from +3 in `F_(2)O_(3)` to 0 in Fe, therefore, `Fe_(2)O_(3)` acts as an oxidising agent. Further O.N. of C increases from +2 in CO to +4 in `CO_(2)`, therefore, CO acts as a reducing agent
Thus, this reaction is an example of redox reaction.
(d) Writing the O.N. of each atom above its symbol, then
`overset(+3)(P)overset(-1)(Cl_(3))(l)+3overset(+1)(H_(2))overset(-2)(O)(l)to3overset(+1)(H)overset(-1)(Cl)(aq)+overset(+1)(H_(3))overset(+3)(P)overset(-2)(O_(3))(aq)`
Here, O.N. of none of the atoms undergo a change, therefore, this reaction is not a redox reaction.
(e) Writing the O.N. of each atom above its symbol, then
`4overset(-3)(N)overset(+1)(H_(3))+3overset(0)(O_(2))(g)to2overset(0)(N_(2))(g)+6overset(+1)(H_(2))overset(-2)(O)(l)`
Here, O.N. of N increases from -3 to 0 in `N_(2)`, therefore `NH_(3)` acts as a reducing agent.
Further, O.N. of O decreases from 0 to `O_(2)` to -2 in `H_(2)O`, therefore, `O_(2)` acts as a oxidising agent. Thus, this reaction is a redox reaction.