From balancing atoms to writing ionic equations and identifying all 7 reaction types — master the language of chemistry.
Section 1 — What Is a Chemical Equation?
A chemical equation is a shorthand representation of a chemical reaction using symbols and formulae. It's the universal language of chemistry — a chemist anywhere in the world can read the same equation.
Conventions You Must Know
- 1Reactants on the LEFT, products on the RIGHT, separated by →
- 2State symbols after each formula: (s) solid · (l) liquid · (g) gas · (aq) aqueous solution
- 3Conditions (temperature, pressure, catalyst) go ABOVE the arrow
- 4Use double arrow ⇌ for reversible reactions
- 5NEVER change a formula to balance — only change the coefficients (numbers in front)
Section 2 — Balancing Chemical Equations
Step-by-Step Balancing Method
- 1Write the correct formulae of all reactants and products with state symbols.
- 2Count atoms on each side.
- 3Balance elements that appear in only one compound first. Leave free-state elements (H₂, O₂) for last.
- 4If H or O are in compounds, balance H second-to-last and O last.
- 5Adjust coefficients (numbers in front) — NEVER change subscripts!
- 6Check coefficients are in the lowest possible ratio.
Worked Example 1 — Mg burns in O₂
Worked Example 2 — Propane (C₃H₈) burns
Section 3 — State Symbols & Solubility Rules
Whether an ionic compound is written as (aq) or (s) depends on its solubility in water. You need to know these rules to correctly label equations.
| Compound / Ion | Solubility | Important Exceptions |
|---|---|---|
| All Group I (Li, Na, K) compounds | ✅ Soluble | None |
| Ammonium (NH₄⁺) compounds | ✅ Soluble | None |
| Nitrates (NO₃⁻) | ✅ Soluble | None |
| Chlorides, Bromides, Iodides | ✅ Soluble | AgCl, PbCl₂ — insoluble |
| Sulfates (SO₄²⁻) | ✅ Soluble | BaSO₄, PbSO₄ — insoluble; CaSO₄ — slightly soluble |
| Carbonates & Phosphates | ❌ Insoluble | Group I + ammonium carbonates/phosphates are soluble |
| Most Hydroxides (OH⁻) | ❌ Insoluble | NaOH, KOH soluble; Ba(OH)₂ moderately; Ca(OH)₂ slightly |
Section 4 — Ionic Equations
Ionic equations show ONLY the ions that actually change during a reaction. Ions that don't change are called spectator ions — they get cancelled out.
4 Steps to Write an Ionic Equation
- 1Write the balanced molecular equation.
- 2Rewrite it, splitting ALL (aq) ionic compounds into their individual ions.
- 3Cancel (cross out) spectator ions — those that appear identically on BOTH sides.
- 4Write the final net ionic equation with only the ions that changed.
Example 1 — Lead Nitrate + Sodium Chloride
Example 2 — KOH + H₂SO₄ (Neutralisation)
Section 5 — The 7 Types of Chemical Reactions
Two or more substances combine to form a SINGLE product.
Pattern: A + B → AB
2Mg(s) + O₂(g) → 2MgO(s)
One reactant breaks down into two or more products. Can be thermal or electrolytic.
Pattern: AB → A + B
CaCO₃(s) →(heat) CaO(s) + CO₂(g)
A more reactive element displaces a less reactive one from a compound.
Pattern: A + BX → AX + B
Mg(s) + CuSO₄(aq) → MgSO₄(aq) + Cu(s)
Two ionic solutions exchange ions; one product is INSOLUBLE (precipitate).
Pattern: AX + BY → AY↓ + BX
AgNO₃(aq) + KBr(aq) → AgBr(s) + KNO₃(aq)
Acid + base/alkali → salt + water.
Pattern: Acid + Base → Salt + H₂O
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
One reactant is OXIDISED and another is REDUCED (electron transfer).
Memory: OIL RIG (Oxidation Is Loss, Reduction Is Gain)
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)
Reaction can proceed in BOTH directions. Products can reform reactants.
Pattern: A + B ⇌ C + D
NH₄Cl(s) ⇌ NH₃(g) + HCl(g)
Section 6 — Resources & Simulations
Section 7 — CSEC Practice Questions
(a) Al(s) + O₂(g) → Al₂O₃(s)
(b) Fe(s) + Cl₂(g) → FeCl₃(s)
(c) N₂(g) + H₂(g) ⇌ NH₃(g)
a Al needs 2 on right (Al₂O₃), so 4 Al on left. O₂: right has 3 O, so put 3/2 O₂ → multiply all by 2: 4Al(s) + 3O₂(g) → 2Al₂O₃(s)
Check: Al = 4 both sides ✅ O = 6 both sides ✅
b FeCl₃ needs 3 Cl. Cl₂ comes in pairs, so need 3 Cl₂ to give 6 Cl → 2 FeCl₃: 2Fe(s) + 3Cl₂(g) → 2FeCl₃(s)
c NH₃ has 1 N and 3 H. Need 2 NH₃ to use 1 N₂. Then H side needs 3 H₂: N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
a AgI — Iodides are usually soluble, BUT AgI is an exception → INSOLUBLE
b Fe(NO₃)₃ — all nitrates are soluble → SOLUBLE
c Na₂CO₃ — Group I compounds are all soluble (Na is Group I) → SOLUBLE
d BaSO₄ — Sulfates are usually soluble, BUT BaSO₄ is an exception → INSOLUBLE
e (NH₄)₃PO₄ — Phosphates are usually insoluble, BUT ammonium compounds are all soluble → SOLUBLE
1 Write molecular equation. BaSO₄ is insoluble (exception), NaNO₃ is soluble:
Ba(NO₃)₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaNO₃(aq)
2 Split all (aq) compounds into ions:
Ba²⁺(aq) + 2NO₃⁻(aq) + 2Na⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) + 2Na⁺(aq) + 2NO₃⁻(aq)
3 Cancel spectator ions: Na⁺ and NO₃⁻ appear on both sides — cancel them.
A white precipitate of barium sulfate forms.
(a) Mg(NO₃)₂(s) → MgO(s) + NO₂(g) + O₂(g)
(b) Zn(s) + HCl(aq) → ZnCl₂(aq) + H₂(g)
(c) AgNO₃(aq) + MgBr₂(aq) → AgBr(s) + Mg(NO₃)₂(aq)
a One reactant breaks into multiple products → Decomposition. (Thermal decomposition of magnesium nitrate.)
b Zn (more reactive) displaces H from HCl → Single Displacement. Zinc displaces hydrogen because Zn is higher in the reactivity series than H.
c Two ionic solutions swap partners; AgBr(s) is an insoluble precipitate → Ionic Precipitation (Double Displacement).
a Zinc displaces copper from copper(II) sulfate. Zinc sulfate is soluble (colorless), copper metal is a pink-brown solid:
Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
b A more reactive element (Zn) displaces a less reactive element (Cu) from a compound → Single Displacement reaction.
c Split (aq): Zn(s) + Zn²⁺(aq)? No — Zn is a solid reactant, doesn't split. Split CuSO₄ and ZnSO₄:
Zn(s) + Cu²⁺(aq) + SO₄²⁻(aq) → Zn²⁺(aq) + SO₄²⁻(aq) + Cu(s)
Cancel SO₄²⁻ (spectator ion):