Question 21

The Mn3+ ion is unstable in solution and undergoes disproportionation to give Mn2+, MnO2, and H+ ion. Write a balanced ionic equation for the reaction.

Answer

The given reaction can be represented as:

Mn3+(aq)  →  Mn2+(aq)  + MnO2(s)  +  H+(aq)

The oxidation half equation is:

M+3 n3+(aq)  →  M+4 nO2(s)

The oxidation number is balanced by adding one electron as:

Mn3+(aq)     →  M nO2(s) + e-

The charge is balanced by adding 4H+ ions as:

Mn3+(aq)     →  M nO2(s)  +  4H+(aq)   +    e-

The O atoms and H+ ions are balanced by adding 2H2O molecules as:

Mn3+(aq)   +  2H2O(l)   →    MnO2(s)  + 4H+(aq)   +    e..... (i)

The reduction half equation is:

Mn3+(aq)     →   Mn2+(aq)  

The oxidation number is balanced by adding one electron as:

Mn3+(aq)   +    e-    →   Mn2+(aq)   ....... (ii)

The balanced chemical equation can be obtained by adding equation (i) and (ii) as:

2Mn3+(aq)   +  2H2O(l)   →   MnO2(s)  + 2Mn2+(aq) + 4H+(aq)

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