Why balance equations?
The law of conservation of mass says atoms are neither created nor destroyed in a reaction. A balanced equation shows the same number of each kind of atom on both sides.
How to balance
- Write correct formulas for reactants and products.
- Count atoms of each element on each side.
- Add coefficients to balance one element at a time. Never change subscripts.
- Balance polyatomic ions (like SO₄²⁻) as a unit if they appear unchanged on both sides.
- Leave H and O (or any element appearing alone) until last.
- Reduce coefficients to the lowest whole-number ratio.
- Check your work.
Worked example: propane combustion
C₃H₈ + O₂ → CO₂ + H₂O
- Carbon: 3 on the left, so put 3 in front of CO₂.
- Hydrogen: 8 on the left, so put 4 in front of H₂O.
- Oxygen on the right: 3(2) + 4(1) = 10, so put 5 in front of O₂.
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Check: C 3 = 3; H 8 = 8; O 10 = 10. ✓
Worked example: a fraction trick
C₂H₆ + O₂ → CO₂ + H₂O
- C: 2CO₂. H: 3H₂O. O on right = 4 + 3 = 7, so O₂ needs 7/2.
- Multiply everything by 2: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O
The five main reaction types
| Type | General pattern | Example |
|---|---|---|
| Synthesis reaction | A + B → AB | 2Mg + O₂ → 2MgO |
| Decomposition reaction | AB → A + B | 2H₂O₂ → 2H₂O + O₂ |
| Single displacement | A + BC → AC + B | Zn + 2HCl → ZnCl₂ + H₂ |
| Double displacement | AB + CD → AD + CB | AgNO₃ + NaCl → AgCl + NaNO₃ |
| Combustion | Fuel + O₂ → CO₂ + H₂O | CH₄ + 2O₂ → CO₂ + 2H₂O |
Synthesis
Two or more reactants combine into one product. Example: iron and sulfur heated together make iron(II) sulfide.
Decomposition
One compound breaks apart, often when heated or when electricity is passed through it. Example: electrolysis of water, 2H₂O → 2H₂ + O₂.
Single displacement and the activity series
A more reactive element replaces a less reactive one. The activity series ranks metals by reactivity:
K > Na > Ca > Mg > Al > Zn > Fe > Pb > (H) > Cu > Ag > Au
- Zinc is above hydrogen, so it displaces hydrogen from acid.
- Copper is below hydrogen, so it doesn’t react with dilute acid.
- Copper can displace silver: Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag (silver crystals grow on a copper wire!).
Double displacement
Two ionic compounds swap partners. These reactions usually happen if one product is:
- a precipitate (an insoluble solid), like AgCl
- a gas, like CO₂
- water (in neutralization: HCl + NaOH → NaCl + H₂O)
Combustion
- Complete combustion (plenty of oxygen): produces CO₂ and H₂O, with a blue flame.
- Incomplete combustion (limited oxygen): also produces carbon monoxide (CO) and soot (C), with a yellow, smoky flame. CO is a colourless, odourless, poisonous gas, which is why homes need CO detectors.
Identifying reaction types
| Clue | Likely type |
|---|---|
| One product only | Synthesis |
| One reactant only | Decomposition |
| An element + a compound on both sides | Single displacement |
| Two compounds → two compounds | Double displacement |
| O₂ reactant, CO₂ + H₂O products | Combustion |
Common mistakes
- Changing subscripts. H₂O₂ is hydrogen peroxide, not water.
- Forgetting diatomic elements. H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂ exist as pairs.
- Predicting displacement without checking the activity series.
Try it yourself
Balance each equation and name the reaction type.
- Al + O₂ → Al₂O₃
- KClO₃ → KCl + O₂
- Fe + CuSO₄ → FeSO₄ + Cu
- Pb(NO₃)₂ + KI → PbI₂ + KNO₃
- C₆H₁₂O₆ + O₂ → CO₂ + H₂O
(Answers: 4Al + 3O₂ → 2Al₂O₃, synthesis; 2KClO₃ → 2KCl + 3O₂, decomposition; already balanced, single displacement; Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃, double displacement; C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, combustion/respiration)
Quick recap
- Balanced equations obey conservation of mass; adjust only coefficients.
- Synthesis combines; decomposition breaks apart.
- Single displacement depends on the activity series.
- Double displacement forms a precipitate, gas or water.
- Combustion of fuels produces CO₂ and H₂O (and CO if oxygen is limited).