Question

balance reaction show half reaction

21. Balance the following equation in acidic solution. wri MnO4 (aq) Cr(aq) Mn2+ (aq) +Cr2O72 (aq)

Answer 1

Balancing equation on the presence of acidic medium:
* separate the reaction into Oxidation Half Reaction (OHR) and Reduction Half Reaction (RHR)
* Balance other than oxygen and hydrogen atoms.
* To Balance the oxygen atoms add H2O.
* To Balance the hydrogen atoms add H+
* To Balance the charge add electrons
* Multiply the both equations with suitable coefficient to get the same number of electrons in both reactions
* Combine both oxidation and reduction half reaction

MnO4^- + Cr^3+ ------> Mn^2+ + Cr2O7^-2

+7 -2 +3 +2 +6 -2

OHR : Cr^3+ ------> Cr2O7^-2

RHR: MnO4^- ------> Mn^2+

Balancing other than O and H

OHR : 2Cr^3+ ------> Cr2O7^-2

RHR: MnO4^- ------> Mn^2+

Balancing O by addition of H2O

OHR : 2Cr^3+ + 7H2O ------> Cr2O7^-2

RHR: MnO4^- ------> Mn^2+ + 4H2O

Balancing H by addition of H+

OHR : 2Cr^3+ + 7H2O ------> Cr2O7^-2 + 14H+

RHR: MnO4^- + 8H+ ------> Mn^2+ + 4H2O

Charge balance by addition of electrons

OHR : 2Cr^3+ + 7H2O ------> Cr2O7^-2 + 14H+ + 6e-

RHR: MnO4^- + 8H+ + 5e- -----> Mn^2+ + 4H2O

Multiply OHR with 5 and RHR with 6

OHR : 10Cr^3+ + 35H2O ------> 5Cr2O7^-2 + 70H+ + 30e-

RHR: 6MnO4^- + 48H+ + 30e- -----> 6Mn^2+ + 24H2O

Combine above two equations

6MnO4^- + 10Cr^3+ 11H2O -----> 6Mn^2+ + 5Cr2O7^-2 + 22H+