Writing Equations
Section: 3. Stoichiometry | Syllabus: Cambridge IGCSE Chemistry (0620)
Word and Symbol Equations
A chemical equation represents a reaction, showing the reactants (starting materials, on the left) reacting to form the products (on the right), separated by an arrow (→, meaning "produces" or "forms").
- Word equation: uses the names of substances, e.g. magnesium + oxygen → magnesium oxide
- Symbol equation: uses chemical formulae, e.g. Mg + O₂ → MgO
Converting Word to Symbol Equations
- Write the word equation
- Replace each name with its correct chemical formula
- Balance the equation (see below)
- Add state symbols if required
Balancing Chemical Equations
A balanced equation has the same number of atoms of each element on both sides, following the law of conservation of mass - atoms cannot be created or destroyed in a chemical reaction, only rearranged.
- Only change the large numbers in front of formulae (coefficients)
- Never change a subscript within a formula - that would turn it into a different substance (H₂O₂ is hydrogen peroxide, not water!)
- Balance one element at a time; leave hydrogen and oxygen until last, since they often appear in several formulae
- Remember the seven diatomic elements (H₂, N₂, F₂, O₂, I₂, Cl₂, Br₂) exist as pairs, not single atoms
A balanced equation has exactly the same number of each type of atom on both sides
Worked Example: Balancing Step by Step
- Question: balance the equation for the combustion of methane: CH₄ + O₂ → CO₂ + H₂O [2]
- Answer: balance H first by putting 2 in front of H₂O: CH₄ + O₂ → CO₂ + 2H₂O; now O is unbalanced (2 left, 4 right), so put 2 in front of O₂: CH₄ + 2O₂ → CO₂ + 2H₂O; check - C: 1=1, H: 4=4, O: 4=4 ✓ [2]
Common Mistakes
- ❌ Changing H₂O to H₂O₂ to try to balance oxygen
- ✓ only add coefficients in front of formulae (e.g. 2H₂O); never edit a subscript, since that changes the substance itself
- ❌ Writing "Mg + O → MgO" (unbalanced, and O should be O₂)
- ✓ oxygen is diatomic, and every element must balance: 2Mg + O₂ → 2MgO
Equations with State Symbols
State symbols show the physical state of each reactant and product: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water).
- 2Mg(s) + O₂(g) → 2MgO(s)
- CaCO₃(s) → CaO(s) + CO₂(g)
- HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Worked Example: Deducing a Symbol Equation with State Symbols
- Question: aqueous copper(II) chloride reacts with aqueous sodium hydroxide to form a precipitate of copper(II) hydroxide and a solution of sodium chloride. Write the balanced symbol equation, including state symbols. [3]
- Answer: copper(II) chloride is CuCl₂ (Cu²⁺ needs 2 Cl⁻); Cu²⁺ needs 2 OH⁻ to form Cu(OH)₂, so 2 NaOH are needed, giving 2 NaCl; CuCl₂(aq) + 2NaOH(aq) → Cu(OH)₂(s) + 2NaCl(aq) [3]
Ionic Equations
An ionic equation shows only the ions that actually react, leaving out spectator ions - ions that appear unchanged on both sides and take no real part in the reaction.
Worked Example: Deriving an Ionic Equation
- Question: derive the ionic equation for the neutralisation of hydrochloric acid by sodium hydroxide solution: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) [2]
- Answer: split the aqueous ionic compounds into ions: H⁺(aq) + Cl⁻(aq) + Na⁺(aq) + OH⁻(aq) → Na⁺(aq) + Cl⁻(aq) + H₂O(l); Na⁺ and Cl⁻ are unchanged on both sides (spectators) - cancel them: H⁺(aq) + OH⁻(aq) → H₂O(l) [2]
Common Types of Reaction
| Type | General pattern | Example |
|---|---|---|
| Combustion | fuel + O₂ → oxide(s) | CH₄ + 2O₂ → CO₂ + 2H₂O |
| Thermal decomposition | compound → 2 or more products (heat) | CaCO₃ → CaO + CO₂ |
| Neutralisation | acid + base → salt + water | HCl + NaOH → NaCl + H₂O |
| Displacement | more reactive element displaces less reactive one | Zn + CuSO₄ → ZnSO₄ + Cu |
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