Contents
1. Writing Chemical Formulae
A chemical formula shows the type and number of each atom in a compound. For ionic compounds, combine the ions so that the overall charge is zero.
Calcium chloride: Ca²⁺ and Cl⁻. Criss-cross: 2 chloride ions needed to balance 2+ charge. Formula = CaCl₂.
Aluminium oxide: Al³⁺ and O²⁻. Criss-cross: 2 Al³⁺ and 3 O²⁻ (total charge: 6+ and 6−). Formula = Al₂O₃.
Naming compounds
| Compound type | Naming rule | Example |
|---|---|---|
| Metal + non-metal | Metal name + non-metal ending in -ide | Sodium chloride, Iron(II) sulfide |
| Metal + oxygen | Metal name + oxide | Copper(II) oxide, Iron(III) oxide |
| Acid + metal oxide/carbonate | Metal name + acid anion (-ate or -ide) | Copper sulfate, Calcium carbonate |
2. Balancing Equations
Atoms are neither created nor destroyed in a reaction. The number of each type of atom must be the same on both sides. Only change coefficients (the numbers in front of formulae) — never change subscripts within a formula.
Balance: Fe + O₂ → Fe₂O₃
Count: Left: 1 Fe, 2 O. Right: 2 Fe, 3 O. Neither balanced.
Balance Fe: 4Fe + O₂ → 2Fe₂O₃ (now 4 Fe each side, but O: left 2, right 6)
Balance O: 4Fe + 3O₂ → 2Fe₂O₃
Check: Left 4 Fe, 6 O. Right 4 Fe, 6 O. ✓ Balanced.
Key equations to know
| Reaction | Word equation | Notes |
|---|---|---|
| Acid + metal | acid + metal → salt + hydrogen | H₂ gas produced |
| Acid + carbonate | acid + carbonate → salt + water + CO₂ | CO₂ causes effervescence |
| Acid + base/alkali | acid + alkali → salt + water | Neutralisation |
| Acid + metal oxide | acid + metal oxide → salt + water | No gas produced |
| Combustion (complete) | fuel + oxygen → CO₂ + H₂O | Full oxidation |
| Combustion (incomplete) | fuel + oxygen → CO + H₂O (or C + H₂O) | Insufficient O₂ |
3. State Symbols
| Symbol | Meaning |
|---|---|
| (s) | Solid |
| (l) | Liquid (pure liquid, e.g. water) |
| (g) | Gas |
| (aq) | Aqueous — dissolved in water |
Use (aq) when a substance is dissolved in water, e.g. HCl(aq), NaOH(aq), NaCl(aq). Pure water is H₂O(l). Do not write H₂O(aq) — water is not dissolved in itself.
4. Ionic Equations
An ionic equation shows only the ions that actually change (react) — the spectator ions (which remain unchanged in solution) are cancelled out.
Full equation: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Full ionic: H⁺(aq) + Cl⁻(aq) + Na⁺(aq) + OH⁻(aq) → Na⁺(aq) + Cl⁻(aq) + H₂O(l)
Cancel spectators (Na⁺ and Cl⁻ appear on both sides):
H⁺(aq) + OH⁻(aq) → H₂O(l) — this is the ionic equation for any acid-alkali neutralisation.
Common ionic equations
| Reaction | Ionic equation |
|---|---|
| Any acid + any alkali (neutralisation) | H⁺(aq) + OH⁻(aq) → H₂O(l) |
| Precipitation of silver chloride | Ag⁺(aq) + Cl⁻(aq) → AgCl(s) |
| Precipitation of barium sulfate | Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) |
| Carbonate + acid | CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l) |
- Balance by changing coefficients only - never change subscripts inside formulae.
- Balance order: metal/non-metal elements first, then H, then O last.
- State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution. Required in full ionic equations.
- Ionic equations: remove spectator ions (appear unchanged on both sides). Net ionic equation shows actual reaction.
- Common ions: SO42- (2-), NO3- (1-), CO32- (2-), OH- (1-), NH4+ (1+), Cu2+, Fe2+, Fe3+, Zn2+.
- Mr = sum of all Ar. e.g. CaCO3 = 40+12+(3x16) = 100. Always add up every atom in the formula.
5. Common Exam Traps
You cannot write H₃O to make water balance — H₂O is water and H₃O is a different compound. Only change the coefficient in front of the formula.
When NaCl dissolves in water it is NaCl(aq) — not NaCl(s). Solid ionic compounds that are insoluble in water use (s) as their state symbol in ionic equations (e.g. AgCl(s), BaSO₄(s)).
In the full equation, HCl(aq) and NaOH(aq) are written as molecules. Only in the ionic equation do you split them into ions: H⁺ + Cl⁻ and Na⁺ + OH⁻.
Key Terms — Flashcard Review
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🎯 Practice Quiz — Test Yourself
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Original study notes for Singapore students. Not affiliated with MOE, SEAB or Cambridge.