Chapter 7 · Chemical Reactions
7.4How to Write Balanced Chemical Equations
14 min · two checks
Predict
What coefficient goes in front of O₂ when hydrogen burns to form water?
The idea
- Balance a skeleton equation using coefficients only.
- Balance polyatomic ions as groups when they stay intact.
Write correct formulas first. Then inventory atoms on each side. Balance elements that appear in only one formula on each side before elements that appear in several. Leave H and O for late in combustion equations; balance O last with a fractional coefficient if you must, then multiply through to clear the fraction. If a polyatomic ion such as SO₄²⁻ appears unchanged on both sides, balance it as a unit.
A standard order that rarely fails: metals, then nonmetals other than H and O, then hydrogen, then oxygen. Check every element when you think you are done, and check charge if ions are written. Odd-even trouble — one O₂ versus an odd number of oxygen atoms in products — is fixed by doubling the carbon-containing product and retrying, or by using 1/2 O₂ and doubling at the end.
Keep these
- Formulas first, then coefficients. Never change subscripts.
- Balance atoms one element at a time and recount at the end.
- Fractions are a temporary tool; multiply them away in the final equation.
Worked path
Balance C₃H₈ + O₂ → CO₂ + H₂O.
Observe
Three C and eight H on the left. Oxygen is in two products and one reactant.