Question

How many electrons must be added to the half reaction given below to balance the charge after the mass has been balanced?


a.
4 on the left side b.
4 on the right side c.
3 on the left side d.
3 on the right side e.
1 on the right side

Answer 1

I don't see the half‑reaction in your question — could you paste it (and say whether the medium is acidic or basic)? Then I can tell you exactly how many electrons and on which side.

Meanwhile, here are the quick steps you can use to determine how many electrons to add:
1. Balance all atoms except O and H.
2. Balance O by adding H2O.
3. Balance H by adding H+ (in acidic solution) or H2O and OH− (in basic solution).
4. Count the total charge on each side and add electrons (e−) to the more positive side until the charges are equal.
5. If needed, multiply the half‑reaction to cancel electrons when combining with another half‑reaction.

Example (acidic medium): MnO4− → Mn2+
- Balance Mn: already balanced.
- Balance O: add 4 H2O to right.
- Balance H: add 8 H+ to left.
- Now charges: left = −1 + 8(+1) = +7; right = +2. Add 5 e− to the left to reduce +7 to +2:
MnO4− + 8 H+ + 5 e− → Mn2+ + 4 H2O.

Send the half‑reaction and I’ll balance it for you and pick the correct choice.