JEE Main & Advanced Chemistry Redox Reactions / रेडॉक्स अभिक्रियाएँ .Auto Oxidation and Disproportionation

Auto Oxidation and Disproportionation

Autoxidation.

            (1) Turpentine and numerous other olefinic compounds, phosphorus and certain metals like Zn and Pb can absorb oxygen from the air in presence of water. The water is oxidised to hydrogen peroxide. This phenomenon of formation of \[{{H}_{2}}{{O}_{2}}\] by the oxidation of \[{{H}_{2}}O\] is known as autoxidation. The substance such as turpentine or phosphorus or lead which can activate the oxygen is called activator. The activator is supposed to first combine with oxygen to form an addition compound, which acts as an autoxidator and reacts with water or some other acceptor so as to oxidise the latter. For example;

                        \[\underset{(\text{activator})}{\mathop{Pb}}\,+{{O}_{2}}\to \,\underset{\text{(autoxidator)}}{\mathop{Pb{{O}_{2}}}}\,\]                   \[Pb{{O}_{2}}+\underset{(\text{acceptor})}{\mathop{{{H}_{2}}O}}\,\to PbO+{{H}_{2}}{{O}_{2}}\]

(2) The turpentine or other unsaturated compounds which act as activators are supposed to take up oxygen molecule at the double bond position to form unstable peroxide called moloxide, which then gives up the oxygen to water molecule or any other acceptor.

                        \[\begin{matrix}

   RCH=CHR+{{O}_{2}} & \to  & RHC & CHR  \\

   {} & {} & \,\,\,\,\,\,\,O & O\,\,\,\,\,\,\,  \\

\end{matrix}\]

                        \[\begin{array}{*{35}{l}}

   RHC & CHR+2{{H}_{2}}O & \to  & RCH=CHR+2{{H}_{2}}{{O}_{2}}  \\

   \,\,\,\,\,\,O & O & {} & {}  \\

\end{array}\]

                                                \[2KI+{{H}_{2}}{{O}_{2}}\to 2KOH+{{I}_{2}}\]

            The evolution of iodine from KI solution in presence of turpentine can be confirmed with starch solution which turns blue.

(3) The concept of autoxidation help to explain the phenomenon of induced oxidation. \[N{{a}_{2}}S{{O}_{3}}\] solution is oxidised by air but \[N{{a}_{3}}As{{O}_{3}}\] solution is not oxidised by air. If a mixture of both is taken, it is observed both are oxidised. This is induced oxidation.

                                                \[N{{a}_{2}}S{{O}_{3}}+{{O}_{2}}\to N{{a}_{2}}S{{O}_{5}}\]

                                                                              Moloxide

                                        \[N{{a}_{2}}S{{O}_{5}}+N{{a}_{3}}As{{O}_{3}}\to N{{a}_{3}}As{{O}_{4}}+N{{a}_{2}}S{{O}_{4}}\]    

                                                            \[N{{a}_{2}}S{{O}_{3}}+N{{a}_{3}}As{{O}_{3}}+{{O}_{2}}\to N{{a}_{2}}S{{O}_{4}}+N{{a}_{3}}As{{O}_{4}}\]

Disproportionation.

            One and the same substance may act simultaneously as an oxidising agent and as a reducing agent with the result that a part of it gets oxidised to a higher state and rest of it is reduced to lower state of oxidation. Such a reaction, in which a substance undergoes simultaneous oxidation and reduction is called disproportionation and the substance is said to disproportionate.

            Following are the some examples of disproportionation,

(1) \[{{H}_{2}}{{O}_{2}}+{{H}_{2}}{{\overset{-1}{\mathop{O}}\,}_{2}}={{H}_{2}}O+{{\overset{0}{\mathop{O}}\,}_{2}}\]  

(2) \[4K\overset{+5}{\mathop{Cl}}\,{{O}_{3}}\to 3K\overset{+7}{\mathop{Cl}}\,{{O}_{4}}+\overset{-1}{\mathop{KCl}}\,\]

(3) \[\overset{0}{\mathop{4P}}\,+3NaOH+3{{H}_{2}}O\to 3Na{{H}_{2}}{{\overset{+1\,\,\,}{\mathop{PO}}\,}_{2}}+\overset{-3\,\,\,\,\,\,\,\,\,}{\mathop{P{{H}_{3}}}}\,\]

(4) \[\overset{0}{\mathop{3C{{l}_{2}}}}\,+\underset{(conc.)}{\mathop{6NaOH}}\,\xrightarrow{hot}5Na\overset{-1}{\mathop{Cl}}\,+Na\overset{+5\,\,\,\,\,}{\mathop{Cl{{O}_{3}}}}\,+3{{H}_{2}}O\]

Important applications of redox-reactions.

            Many applications are based on redox reactions which are occuring in environment. Some important examples are listed below;

            (1) Many metal oxides are reduced to metals by using suitable reducing agents. For example \[A{{l}_{2}}{{O}_{3}}\] is reduced to aluminium by cathodic reduction in electrolytic cell. \[F{{e}_{2}}{{O}_{3}}\] is reduced to iron in a blast furnace using coke.

(2)  Photosynthesis is used to convert carbon dioxide and water by chlorophyll of green plants in the presence of sunlight to carbohydrates.

                                                \[6C{{O}_{2(g)}}+6{{H}_{2}}{{O}_{(l)}}\,\,\xrightarrow{Chlorophyll}\,{{C}_{6}}{{H}_{12}}{{O}_{6(aq.)}}+6{{O}_{2}}_{(g)}\]

            In this case, \[C{{O}_{2}}\] is reduced to carbohydrates and water is oxidised to oxygen. The light provides the energy required for the reaction.

(3) Oxidation of fuels is an important source of energy which satisfies our daily need of life.

                                                            Fuels \[+{{O}_{2}}\to \,C{{O}_{2}}+{{H}_{2}}O+\]Energy

            In living cells, glucose \[\left( {{C}_{6}}{{H}_{12}}{{O}_{6}} \right)\] is oxidised to \[C{{O}_{2}}\] and \[{{H}_{2}}O\] in the presence of oxygen and energy is released,  \[{{C}_{6}}{{H}_{12}}{{O}_{6}}_{(aq.)}+6{{O}_{2}}(g)\to \,6C{{O}_{2}}(g)+6{{H}_{2}}{{O}_{(l)}}+\]Energy

(4) The electrochemical cells involving reaction between hydrogen and oxygen using hydrogen and oxygen electrodes in fuel cells meet our demand of electrical energy in space capsule.

            (5) Respiration in animals and humans is also an important application of redox reactions.