Chemical Reactions & Balancing
Burning, rusting, baking — every reaction is the same move: old bonds break, new bonds form, and every single atom survives.
Builds on: 1.2 Covalent Bonds: Sharing Pairs
Rearrangement, not magic
When methane burns, it looks like the gas vanishes into heat. It doesn’t. A chemical reaction only does one thing: it breaks some bonds and forms others. The atoms themselves — every carbon, every hydrogen, every oxygen — come out the other side, merely reshuffled into new molecules. Antoine Lavoisier nailed this in 1789 by weighing everything obsessively: in a closed system, mass before equals mass after. This is the law of conservation of mass, and it is bookkeeping you can bet your life on (your cells do, constantly).
The chemical sentence
We write reactions as equations: reactants → products. The arrow reads “react to form”. But a raw equation like H₂ + O₂ → H₂O is a lie — count the oxygens: two on the left, one on the right. An atom vanished, which Lavoisier forbids. The fix is balancing: put multipliers (coefficients) in front of whole molecules until every element counts equal on both sides:
The iron rule: you may only change coefficients, never the little subscripts. Writing H₂O₂ to “fix” the oxygen count doesn’t balance water — it invents hydrogen peroxide, a completely different substance (please don’t drink it).
A strategy that always works
- Balance elements that appear in only one place on each side first.
- Save lone elements (like O₂ or Fe) for last — their coefficient is a free knob.
- If you end up with a fraction, multiply everything to clear it.
- Finish by checking every element — and that the coefficients can’t be divided down.
2 H₂ + O₂ → 2 H₂O doesn’t just mean “2 molecules + 1 molecule”. It means any 2 : 1 : 2 ratio — 2 dozen, 2 billion, or 2 moles. Coefficients are the recipe’s proportions, and the next two lessons turn that recipe into grams you can weigh.
⚗️ Lab — The Atom Accountant
Four unbalanced equations, one balance beam per element. Make every beam level.
- Start with H₂ + O₂ → H₂O — two sliders solve it.
- Rust (Fe + O₂ → Fe₂O₃) needs the fraction-clearing trick: it balances at 4 : 3 : 2.
- Balance one with doubled coefficients and read the fine print.