How to balance this redox reaction?

[Graffiti]

Can someone walk me through balancing this redox reaction?

(H2O2) + (VO^2+) ---> (VO2^+) + (OH-)

I keep encountering an issue with balancing the charges on both sides.

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[Graffiti]

This is from TBR page 263.

 

[Mcat35]

This is from TBR page 263.

although it is very unlikely for you to get something like this on the MCAT its good practice.

Break them down to half reactions first.

H2O2 ----- > OH-
Vo^2+ ------> VO2 ^+

balance all non Oxygen, Hydrogens first
balance Oxygens with adding H2O
Balance Hydrogens with H+
In the end balance the e-.
Multiply one of the half equations by 2 so you will get 2e- on both side when you combine them.

The way I was doing it is just to balance everything else the same as in acidic conditions, but in the end just add OH- to make all the H+ to H2O.. You should get the same results that way. I like that way because you don't have to memorize 2x different ways for both acidic and basic.

 

[ilovemcat]

Begin by balancing the Half-Reactions:

1: Write out unbalanced half-reaction: H2O2 ---> 2OH-
2: Identify the number of electrons in redox reaction: 2 electrons (Reduced)

Both oxygens go from a -1 to -2 oxidation state. Both gain 1 electron each.​

3: Write out unbalanced half-reaction: VO^2+ --> VO2^+
4: Identify the number of electrons in redox reaction: 1 electron (Oxidized)

Vanadium goes from a +4 to +5 oxidation state: It loses 1 electron.​

5: Balance the electrons in both half-reactions by cross multiplying:
(H2O2 + 2 electrons ---> 2OH-) x 1
(VO^2+ --> VO2^+ 1 electron) x 2

After multiplying (for simplicity I'm excluding electrons):
H2O2 ---> 2OH-
2VO^2---> 2VO2^+

6: Now balance the charges in each half-reaction by adding OH- or H+. Because this is a basic reaction, add OH-'s:

Right side has a net -2 charge; add 2OH-'s to left side.
2OH - + H2O2 ---> 2OH-

Right side has a net +2 charge; add 2OH-'s to right side.
2VO^2+ ---> 2VO2^+ 2OH-

7: Balance the Hydrogens in both half-reactions by adding water:

2OH- + H2O2 ---> 2OH- (already balanced)
2OH- + 2VO^2+ ---> 2VO2^+ +2H2O

8: And finally, add together the two half reactions: (making sure to cancel out molecules that appear twice)

2OH- + H2O2 ---> 2OH-
2OH- + 2VO^2+ ---> 2VO2^+ + 2H2O
--------------------------------------
2VO^2+ + H2O2 -+ 2OH- --> 2VO2^+ + 2H2O

Notice the oxygens don't balance until you add both half reactions together. This is why we don't bother balancing oxygen atoms because they'll balance in the end (provided you balanced everything else correctly). I definetly wouldn't go through this process for the MCAT. Instead I'd just check to see if the charges are the same and the #atoms equal each other for each choice.

 

[Graffiti]

Thanks!! That makes sense now.