Identify the redox reactions out of the following reactions and identify the oxidising and reducing agents in them.
(i) 3HCI(aq) + HNO3 (aq) → CI2(g) + NOCI (g) + 2H2O(l)
(ii) HgCI2 (aq) + 2KI (aq) → HgI2 (s) + 2KCI (aq)
(iii) Fe2O3 (s) + 3CO(g) → 2Fe (s) + 3CO2(g)
(iv) PCI3 (I) + 3H2O (I) → 3HCI (aq) + H3 Po3 (aq)
(v) 4NH3 + 3O2 (g) → 2N2 (g) + 6H2 O (g)
Option (i) 
Cl oxidizes from -1 to 0 and Nitrogen reduced from +5 to +3. Thus as Cl oxidizes it acts as reducing agent and nitric acid acts as an oxidizing agent.
Reducing agent: HCl
Oxidizing agent: HNO3
Option (ii) 
This reaction is an example of displacement reaction, and none of the components undergo oxidation or reduction, as the oxidation states oh Hg, Cl, K, & I are same in both reactants and products.
Option (iii) 
Oxidising agent: Fe2O3
Reducing agent: CO
Option (iv) 
Oxidising agent: O2
Reducing agent: NH3