Question

How would you balance MnO4- + 1e- = MnO42- and SO32- = SO42- + 2e- in basic conditions? Please show all work. The end answer should be a basic redox equation with all reactants and all products that haven't been crossed out.

AND How would you balance MnO42- + 3e- = MnO(OH) and SO32- = SO42- + 2e- in basic conditions? Please show all work. The end answer should be a basic redox equation with all reactants and all products that haven't been crossed out.

Answer 1

Question 1.

The half reactions:

MnO4- + 1e- = MnO4-2

SO3-2 = SO4-2 + 2e-

FIRST: ignore electron charges, balance atoms such as O, by adding H2O

MnO4- + = MnO4-2

H2O + SO3-2 = SO4-2

balance H by adding H+

MnO4- + = MnO4-2

H2O + SO3-2 = SO4-2 + 2H+

balance charges

MnO4- + 1e- = MnO4-2

H2O + SO3-2 = SO4-2 + 2H+ + 2e-

balance e-

2MnO4- + 2e- = 2MnO4-2

H2O + SO3-2 = SO4-2 + 2H+ + 2e-

add all

H2O + SO3-2 + 2MnO4- + 2e- = 2MnO4-2 +  SO4-2 + 2H+ + 2e-

cancel common terms

H2O + SO3-2 + 2MnO4- = 2MnO4-2 +  SO4-2 + 2H+ (this is balanced already, in acidic conditions, since H+ is present)

in order to balance in basic conditions:

add OH- to cancel acidic protons

2OH- + H2O + SO3-2 + 2MnO4- = 2MnO4-2 +  SO4-2 + 2H+ + 2OH-

convert to water

2OH- + H2O + SO3-2 + 2MnO4- = 2MnO4-2 +  SO4-2 + 2H2O

cancel common terms

2OH- + SO3-2 + 2MnO4- = 2MnO4-2 +  SO4-2 + H2O( this is now balanced in basic conditions).

Question 2.

similar:

MnO4-2 + 3e- = MnO(OH)

SO3-2 = SO4-2 + 2e-

ignore OH- and electron

MnO4-2 = MnO+

SO3-2 = SO4-2

balance O

MnO4-2 = MnO+ + 3H2O

H2O + SO3-2 = SO4-2

balance H

6H+ + MnO4-2 = MnO+ + 3H2O

H2O + SO3-2 = SO4-2 + 2H+

balnace charge

6H+ + MnO4-2 +3e- = MnO+ + 3H2O

H2O + SO3-2 = SO4-2 + 2H+ + 2e-

balance e-

12H+ + 2MnO4-2 + 6e- = 2MnO+ + 6H2O

3H2O + 3SO3-2 = 3SO4-2 + 6H+ + 6e-

add all

3H2O + 3SO3-2 + 12H+ + 2MnO4-2 + 6e- = 2MnO+ + 6H2O + 3SO4-2 + 6H+ + 6e-

cancel common terms

3SO3-2 + 6H+ + 2MnO4-2= 2MnO+ + 3H2O + 3SO4-2

this is now acidic balanced

add OH- for basic

6OH- + 3SO3-2 + 6H+ + 2MnO4-2= 2MnO+ + 3H2O + 3SO4-2 + 6OH-

add H2O

3SO3-2 + 6H2O + + 2MnO4-2= 2MnO+ + 3H2O + 3SO4-2 + 6OH-

cancel common terms

3SO3-2 + 3H2O + + 2MnO4-2= 2MnO+ + 3SO4-2 + 6OH-