JEE Main & Advanced Chemistry Redox Reactions / रेडॉक्स अभिक्रियाएँ Oxidation-Reduction And Redox Reactions

(1) Oxidation : Oxidation is a process which involves; addition of oxygen, removal of hydrogen, addition of non-metal, removal of metal, Increase in +ve valency, loss of electrons and increase in oxidation number.

(i)  Addition of oxygen : 2Mg + O_{2 \[\to \]}2MgO                    

(ii) Removal of hydrogen : H₂S+Cl_{2\[\to \]}2HCl + S     

(iii) Addition of Non-metal : Fe + S \[\to \] FeS              

(iv) Removal of metal : 2KI+H₂O_{2\[\to \]}2KOH+I₂

(v) Increase in +ve valency : \[F{{e}^{2+}}\,\to \,F{{e}^{3+}}+{{e}^{-}}\]                        

(vi) Loss of electrons (also known as de-electronation)

(a) \[{{H}^{0}}\to {{H}^{+}}+{{e}^{-}}\]            (Formation of proton)

(b) \[MnO_{4}^{2-}\to MnO_{4}^{-}+{{e}^{-}}\]      (De-electronation of \[MnO_{4}^{2-}\])

(c) \[2F{{e}^{0}}\to 2Fe_{{}}^{3+}+6{{e}^{-}}\]      (De-electronation of iron)

(vii) Increase in oxidation number

(a) \[M{{g}^{0}}\to M{{g}^{2+}}\]                         (From 0 to +2)

(b) \[{{\left[ F{{e}^{+2}}{{(CN)}_{6}} \right]}^{4-}}\to {{\left[ F{{e}^{+3}}{{(CN)}_{6}} \right]}^{3-}}\]   (From +2 to +3)

(c) \[2C{{l}^{-}}\to Cl_{2}^{0}\]                             (From –1 to 0)

(2) Reduction : Reduction is just reverse of oxidation. Reduction is a process which involves; removal of oxygen, addition of hydrogen, removal of non-metal, addition of metal, decrease in +ve valency, gain of electrons and decrease in oxidation number.

(i) Removal of oxygen : \[CuO+C\to Cu+CO\]           

(ii) Addition of hydrogen : \[C{{l}_{2}}+{{H}_{2}}\to 2HCl\]

(iii) Removal of non-metal 

\[2HgC{{l}_{2}}+SnC{{l}_{2}}\to H{{g}_{2}}C{{l}_{2}}+SnC{{l}_{4}}\]

(iv) Addition of metal : \[HgC{{l}_{2}}+Hg\to H{{g}_{2}}C{{l}_{2}}\]

(v) Decrease in +ve valency 

(a) \[F{{e}^{3+}}\,\to \,F{{e}^{2+}}\]     (+ve valency decreases)

(b) \[{{[Fe\,{{(CN)}_{6}}]}^{3-}}\to {{[Fe{{(CN)}_{6}}]}^{4-}}\] (–ve valency increases)

(vi) Gain of electrons (also known as electronation)

(a) \[Z{{n}^{2+}}(aq)+2{{e}^{-}}\to Zn(S)\] (Electronation of \[Z{{n}^{2+}}\])

(b) \[P{{b}^{2+}}+2{{e}^{-}}\to P{{b}^{0}}\] (Electronation of \[P{{b}^{2+}}\])

(c) \[{{[Fe{{(CN)}_{6}}]}^{3-}}+{{e}^{-}}\to {{[Fe{{(CN)}_{6}}]}^{4-}}\]                    

(Electronation of \[{{[Fe{{(CN)}_{6}}]}^{3-}}\])

(vii) Decrease in oxidation number

(a) \[M{{g}^{2+}}\to M{{g}^{0}}\]                         (From +2 to 0)

(b) \[{{\left[ Fe{{(CN)}_{6}} \right]}^{3-}}\to {{\left[ Fe{{(CN)}_{6}} \right]}^{4-}}\]               (From +3 to +2)

(c) \[Cl_{2}^{0}\to 2C{{l}^{-}}\]                             (From 0 to –1)

(3) Redox-reactions

(i) An overall reaction in which oxidation and reduction takes place simultaneously is called redox or oxidation-reduction reaction. These reactions involve transfer of electrons from one atom to another. Thus every redox reaction is made up of two half reactions; One half reaction represents the oxidation and the other half reaction represents the reduction.

(ii) Types of redox reaction

(a) Direct redox reaction : The reactions in which oxidation and reduction takes place in the same vessel are called direct redox reactions.

(b) Indirect redox reaction : The reactions in which oxidation and reduction takes place in different vessels are called indirect redox reactions. Indirect redox reactions are the basis of electro-chemical cells.

(c) Intermolecular redox reactions : In which one substance is oxidised while the other is reduced.

For example, \[2\,Al+F{{e}_{2}}{{O}_{3}}\to A{{l}_{2}}{{O}_{3}}+2Fe\]

Here, Al is oxidised to \[A{{l}_{2}}{{O}_{3}}\] while \[F{{e}_{2}}{{O}_{3}}\]is reduced to Fe.

(d) Intramolecular redox reactions : In which one element of a compound is oxidised while the other is reduced.

For example, \[2\,KCl{{O}_{3}}\xrightarrow{\Delta }2\,KCl+3\,{{O}_{2}}\]

Here, \[C{{l}^{+5}}\] in \[KCl{{O}_{3}}\] is reduced to \[C{{l}^{-1}}\] in KCl while \[{{O}^{2-}}\] in \[KCl{{O}_{3}}\] is oxidised to \[O_{2}^{0}\].