25. Identify the redox reaction out of the following reactions and identify the oxidising and reducing agents in them.

(a) $3\mathrm{HCl}\left(\mathrm{aq}\right)+{\mathrm{HNO}}_{3}\left(\mathrm{aq}\right)\to {\mathrm{Cl}}_{2}\left(\mathrm{g}\right)+\mathrm{NOCl}\left(\mathrm{g}\right)+2{\mathrm{H}}_{2}\mathrm{O}\left(\mathrm{l}\right)$

(b) ${\mathrm{HgCl}}_{2}\left(\mathrm{aq}\right)+2\mathrm{KI}\left(\mathrm{aq}\right)\to {\mathrm{HgI}}_{2}\left(\mathrm{s}\right)+2\mathrm{KCl}\left(\mathrm{aq}\right)$

(c) ${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}\left(\mathrm{s}\right)+3\mathrm{CO}\left(\mathrm{g}\right)\stackrel{∆}{\to }2\mathrm{Fe}\left(\mathrm{s}\right)+3{\mathrm{CO}}_{2}\left(\mathrm{g}\right)$

(d) ${\mathrm{PCl}}_{3}\left(\mathrm{l}\right)+3{\mathrm{H}}_{2}\mathrm{O}\left(\mathrm{l}\right)\to 3\mathrm{HCl}\left(\mathrm{aq}\right)+{\mathrm{H}}_{2}{\mathrm{PO}}_{3}\left(\mathrm{aq}\right)$

(e) $4{\mathrm{NH}}_{3}\left(\mathrm{aq}\right)+3{\mathrm{O}}_{2}\left(\mathrm{g}\right)\to 2{\mathrm{N}}_{2}\left(\mathrm{g}\right)+6{\mathrm{H}}_{2}\mathrm{O}\left(\mathrm{g}\right)$

(a) Writing the O.N. on each atom above its symbol, then
$3\stackrel{+1}{\mathrm{H}}\stackrel{-1}{\mathrm{Cl}}\left(\mathrm{aq}\right)+\stackrel{+1}{\mathrm{H}}\stackrel{+5}{\mathrm{N}}{\stackrel{-2}{\mathrm{O}}}_{3}\left(\mathrm{aq}\right)\to \stackrel{0}{\mathrm{C}}{\mathrm{l}}_{2}\left(\mathrm{g}\right)+\stackrel{+3}{\mathrm{N}}\stackrel{-2}{\mathrm{O}}\stackrel{-1}{\mathrm{C}}\mathrm{l}\left(\mathrm{g}\right)+2{\stackrel{+1}{\mathrm{H}}}_{2}\stackrel{-2}{\mathrm{O}}\left(\mathrm{l}\right)$
Here, the O.N. of Cl increases from -1 in HCl to 0 in Cl2, therefore, Cl- is oxidised and hence HCl acts as the reducing agent.
The O.N. of N decreases from +5 in HNO3 to +3 in NOCl, therefore, HNO3 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,
$\stackrel{+2}{\mathrm{Hg}}{\stackrel{-1}{\mathrm{Cl}}}_{2}\left(\mathrm{aq}\right)+2\stackrel{+1}{\mathrm{K}}\stackrel{-1}{\mathrm{I}}\left(\mathrm{aq}\right)\to \stackrel{+2}{\mathrm{Hg}}{\stackrel{-1}{\mathrm{I}}}_{2}\left(\mathrm{s}\right)+2\stackrel{+1}{\mathrm{K}}\stackrel{-1}{\mathrm{Cl}}\left(\mathrm{aq}\right)$
Here, the O.N. of none of the atoms undergo a change, therefore, this reaction is not a redox reaction.
(c) ${\stackrel{+3}{\mathrm{Fe}}}_{2}{\stackrel{-2}{\mathrm{O}}}_{3}\left(\mathrm{s}\right)+3\stackrel{+2}{\mathrm{C}}\stackrel{-2}{\mathrm{O}}\left(\mathrm{g}\right)\stackrel{∆}{\to }2\stackrel{0}{\mathrm{Fe}}\left(\mathrm{s}\right)+3\stackrel{+4}{\mathrm{C}}{\stackrel{-2}{\mathrm{O}}}_{2}\left(\mathrm{g}\right)$
Here, O.N. of Fe decreases from +3 in Fe2O3 to 0 in Fe, therefore, Fe2O3 acts as an oxidising agent. Further, O.N. of C increases from +2 in CO to +4 in CO2, 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
$\stackrel{+3}{\mathrm{P}}{\stackrel{-1}{\mathrm{Cl}}}_{3}\left(\mathrm{l}\right)+3{\stackrel{+1}{\mathrm{H}}}_{2}\stackrel{-2}{\mathrm{O}}\left(\mathrm{l}\right)\to 3\stackrel{+1}{\mathrm{H}}\stackrel{-1}{\mathrm{Cl}}\left(\mathrm{aq}\right)+{\stackrel{+1}{\mathrm{H}}}_{2}\stackrel{+3}{\mathrm{P}}{\stackrel{-2}{\mathrm{O}}}_{3}\left(\mathrm{aq}\right)$
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
$4\stackrel{-3}{\mathrm{N}}{\stackrel{+1}{\mathrm{H}}}_{3}\left(\mathrm{aq}\right)+3{\stackrel{0}{\mathrm{O}}}_{2}\left(\mathrm{g}\right)\to 2{\stackrel{0}{\mathrm{N}}}_{2}\left(\mathrm{g}\right)+6{\stackrel{+1}{\mathrm{H}}}_{2}\stackrel{-2}{\mathrm{O}}\left(\mathrm{g}\right)$
Here, O.N. of N increases from -3 to 0 in N2, therefore, NH3 acts as a reducing agent. Further, O.N. of O decreases from 0 in O2 to -2 in H2O, therefore, O2 acts as a oxidising agent. Thus, this reaction is a redox reaction.