Can you change subscripts to balance a chemical equation?
Updated
The short answer
You can't, because a subscript is part of what the substance is: H₂O is water and H₂O₂ is hydrogen peroxide. Balance the equation by changing only the coefficients in front of each formula, as in 2H₂ + O₂ → 2H₂O.
What it looks like
Balance H₂ + O₂ → H₂O.
| Wrong | Right |
|---|---|
| H₂ + O₂ → H₂O₂ | 2H₂ + O₂ → 2H₂O |
Count the atoms in the wrong line and they match, with 2 H and 2 O on each side. That's why it feels finished. But the product is now hydrogen peroxide, so the equation describes a different reaction from the one in the problem. In the right line, the 2 in front of H₂O means two water molecules, and the formula itself is untouched. Each side has 4 H and 2 O.
The same slip can show up on the other side of the arrow, as H₂ + O → H₂O. Oxygen gas is made of O₂ molecules, so writing a lone O changes a formula too.
In alai, an iPad app that reads your handwriting as you work, Ally, the tutor, would point at the H₂O₂ in the wrong line and ask about it.
Why it happens
Changing a subscript is the fastest way to make the counts match, and counting atoms, the check you were taught, still passes.
Coefficients and subscripts are both small numbers next to a formula, so they look like the same kind of thing. They mean different things: the coefficient counts molecules, and the subscript says how many atoms of an element are inside one molecule. As OpenStax Chemistry 2e explains, the subscripts are part of what identifies a substance, so changing one changes what the equation says.
Seven elements exist as two-atom molecules on their own: hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine, written H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂. A lone O or Cl is tempting because it makes the counting easier, and it's the same slip.
How to catch it yourself
When you think you're done, cover the coefficients with your finger and ask:
Is every formula left exactly the one the problem gave you?
Try these
Balance each equation.
- Na + Cl₂ → NaCl
- CH₄ + O₂ → CO₂ + H₂O
- Al + O₂ → Al₂O₃
Answers
- 2Na + Cl₂ → 2NaCl, with 2 Na and 2 Cl on each side. Writing NaCl₂ would balance the chlorine by changing the product.
- CH₄ + 2O₂ → CO₂ + 2H₂O, with 1 C, 4 H and 4 O on each side.
- 4Al + 3O₂ → 2Al₂O₃, with 4 Al and 6 O on each side.
How alai helps
alai is a notes app for iPad and the web. In Homework help, its tutor, Ally, follows each line you write, and changing a subscript is one of the slips Ally is set up to recognize: Ally points at the changed formula and asks a question, and you do the balancing. Ally also remembers slips from past sessions, so if this one comes back, Ally can point back to last time.