Balancing the count
Show on a big screenH₂ + O₂ → H₂O. Count the atoms on both sides.
- H₂ + O₂ → H₂O. Left: 2 H, 2 O. Right: 2 H, 1 O.
- Oxygen is short. Put a 2 in front of the water.
- Hydrogen is now short on the left: 2 against 4. Put a 2 in front of the hydrogen too.
- 2H₂ + O₂ → 2H₂O. H 4 = 4. O 2 = 2.
Tap a formula to change its number. Make both sides match.
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The rule
Atoms only rearrange in a reaction. Each atom's count must match on both sides. A coefficient multiplies one whole formula.
| Atom | Left, before | Right, before | Left, after | Right, after |
|---|---|---|---|---|
| Hydrogen | 2 | 2 | 4 | 4 |
| Oxygen | 2 | 1 | 2 | 2 |
A 2 in front of water makes oxygen 2 on each side. It also makes hydrogen 4 on the right. A 2 in front of hydrogen makes hydrogen 4 on the left. The balanced sentence is 2H₂ + O₂ → 2H₂O.
Aluminium burning in air needs three numbers. Al + O₂ → Al₂O₃ becomes 4Al + 3O₂ → 2Al₂O₃.
Oxygen first: O₂ gives 2 and Al₂O₃ gives 3. Both reach 6, so write 3O₂ and 2Al₂O₃. Then 2Al₂O₃ holds 4 Al, so write 4Al.
Use the smallest whole numbers. 4H₂ + 2O₂ → 4H₂O is balanced. 2H₂ + O₂ → 2H₂O is smaller.
Some sentences need no fix. Ca + Cl₂ → CaCl₂ has 1 calcium and 2 chlorine atoms each side.
Watch out!
Do not fix a count by changing a formula's small numbers. H₂O₂ is not water. Change the number in front.
Try the lab to finish this lesson.