Predicting & Balancing Reactions
Balancing, the five reaction families, product prediction, the activity series, and when there is no reaction.
Balancing basics
CORE RULEAtoms are conserved: same count of each element on both sides. Balance with coefficients, never by changing subscripts (that changes the substance).
Balancing by inspection
METHODStart with the most complex species; balance elements in the fewest formulas first; do O and H last; clear fractions by multiplying through; reduce to smallest whole numbers.
The five reaction families
REFERENCE| Family | Pattern | Example |
|---|---|---|
| Synthesis | A + B → AB | 2Mg + O₂ → 2MgO |
| Decomposition | AB → A + B | 2HgO → 2Hg + O₂ |
| Single replacement | A + BC → AC + B | Zn + 2HCl → ZnCl₂ + H₂ |
| Double replacement | AB + CD → AD + CB | AgNO₃ + NaCl → AgCl + NaNO₃ |
| Combustion | fuel + O₂ → oxides | CH₄ + 2O₂ → CO₂ + 2H₂O |
Combustion routine (C, H, O fuels)
METHODProducts CO₂ + H₂O. Balance C, then H (sets H₂O), then O (adjust O₂, often fractional), then multiply through to clear fractions and reduce.
Activity series
REFERENCEMetals (most → least active): Li > K > Ca > Na > Mg > Al > Mn > Zn > Fe > Ni > Sn > Pb > H > Cu > Ag > Au. A metal displaces one below it; metals above H displace H₂ from acids. Halogens: F > Cl > Br > I.
Single-replacement products
CORE RULEA + BX → AX + B (A above B): Zn + CuSO₄ → ZnSO₄ + Cu. Metal + acid → salt + H₂ (metal takes its low charge, Fe → Fe²⁺). Halogen + halide: Cl₂ + 2NaBr → 2NaCl + Br₂.
Metal + nonmetal → salt
CORE RULECation from group (1→+1, 2→+2, 13→+3), anion from group (15→−3, 16→−2, 17→−1); balance charges. 2Na + Cl₂ → 2NaCl; 3Mg + N₂ → Mg₃N₂.
Active metal + water
CORE RULEGroup 1: 2M + 2H₂O → 2MOH + H₂ (all, cold water). Group 2 varies: Be none; Mg very slow in cold water, readily with steam (→ MgO + H₂); Ca, Sr, Ba: M + 2H₂O → M(OH)₂ + H₂. Al passivated by its oxide layer.
Oxides
CORE RULESoluble (active-metal) oxide + H₂O → hydroxide: Na₂O + H₂O → 2NaOH; CaO + H₂O → Ca(OH)₂ (most metal oxides are insoluble and don't react). Most nonmetal oxides + H₂O → oxoacid, oxidation state preserved: SO₂ + H₂O → H₂SO₃; SO₃ + H₂O → H₂SO₄; CO₂ + H₂O → H₂CO₃.
Metal oxide + acid → salt + H₂O. Nonmetal oxide + base → salt (+ H₂O): CO₂ + 2NaOH → Na₂CO₃ + H₂O.
Nonmetal + O₂ → oxide (default combustion): C → CO₂ (CO if O₂ limited), S → SO₂, P₄ → P₄O₁₀.
Thermal decomposition
CORE RULEΔ signals: carbonate → oxide + CO₂ (CaCO₃ → CaO + CO₂); bicarbonate → carbonate + CO₂ + H₂O; heavy hydroxide → oxide + H₂O; chlorate → chloride + O₂ (2KClO₃ → 2KCl + 3O₂).
When to write 'no reaction'
WATCH OUTNo reaction when a single-replacement element is below its target, a halide meets a weaker halogen, or no family pattern fits. The test is whether a pattern applies, not whether a change is visible.