MCQ
Identify disproportionation reaction:
  • A
    $\text{CH}_4+2\text{O}_2\xrightarrow{ \ \ \ \ \ \ \ \ }\text{CO}_2+2\text{H}_2\text{O}$
  • B
    $\text{CH}_4+4\text{Cl}_2\xrightarrow{ \ \ \ \ \ \ \ \ }\text{CCl}_4+4\text{HCl}$
  • C
    $2\text{F}_2+2\text{OH}^-\xrightarrow{ \ \ \ \ \ \ \ \ }2\text{F}^-+\text{OF}_2+\text{H}_2\text{O}$
  • D
    $2\text{NO}_2+2\text{OH}^-\xrightarrow{ \ \ \ \ \ \ \ \ }\text{NO}^-_2+\text{NO}^-_3+\text{H}_2\text{O}$

Answer

  1. $2\text{NO}_2+2\text{OH}^-\xrightarrow{ \ \ \ \ \ \ \ \ }\text{NO}^-_2+\text{NO}^-_3+\text{H}_2\text{O}$

Explanation:

Reactions in which the same substance is oxidized as well as reduced are called disproportionation reactions. Writing the O.N. of each element above its symbol in the given reactions,

  1. $\stackrel{-4+1}{\text{CH}}_4+\stackrel{0}{\text{2O}}_2\xrightarrow{ \ \ \ \ \ \ \ \ }\stackrel{+4-2}{\text{CO}}_2+\stackrel{+1-2}{\text{2H}}_2\text{O}$

  2. $\stackrel{-4+1}{\text{CH}}_4+\stackrel{0}{\text{4Cl}}_2\xrightarrow{ \ \ \ \ \ \ \ \ }\stackrel{+4-1}{\text{CCl}}_4+\stackrel{+1-1}{\text{4HCl}}\text{}$

  3. $2\stackrel{0}{\text{F}}_2+\stackrel{-2+1}{\text{2OH}^-}\xrightarrow{ \ \ \ \ \ \ \ \ }\stackrel{-1}{\text{2F}^-+}\stackrel{+2-1}{\text{OF}_2}+\stackrel{+1-2}{\text{H}_2\text{O}}$

  4. $\stackrel{+4-2}{\text{2NO}_2}+\stackrel{-2+1}{\text{2OH}^-}\xrightarrow{ \ \ \ \ \ \ \ \ }\stackrel{+3-2}{\text{NO}^-_2}+\stackrel{+5-2}{\text{NO}^-_3}+\stackrel{+1-2}{\text{H}_2\text{O}}$

Thus, in reaction (d), N is both oxidized as well as reduced since the O.N. of N increases from +4 in NO2 to +5 in NO3 and decreases from +4 in NO2 to +3 in NO2.

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