Writing Equations
Section: 3. Stoichiometry | Syllabus: Cambridge AS Level Physics 9702
What is a Chemical Equation? Chemical Equation A representation of a chemical reaction showing the reactants (starting materials) and products (substances formed) using chemical formulae and symbols. Chemical equations are like sentences in chemistry - they tell the story of what happens in a reaction.
Reactants go on the left (what you start with) Products go on the right (what you make) An arrow (→) separates them (means "reacts to form" or "produces") Plus signs (+) separate multiple reactants or products General form: Reactants → Products Example: hydrogen + oxygen → water or in symbols: H₂ + O₂ → H₂O (unbalanced) Word Equations Word Equation A chemical equation written using the names of substances instead of chemical formulae.
Word equations are the simplest way to describe a reaction. Examples of Word Equations methane + oxygen → carbon dioxide + water magnesium + hydrochloric acid → magnesium chloride + hydrogen sodium hydroxide + sulfuric acid → sodium sulfate + water Key Rule Reactants always on the left, arrow in the middle, products always on the right!
Symbol Equations Symbol Equation A chemical equation using chemical formulae and symbols instead of words. Symbol equations are more precise than word equations. Converting Word to Symbol Equations Write the word equation Replace each name with its chemical formula Add state symbols if required Balance the equation (see next section) Example: Converting word to symbol equation Word: magnesium + oxygen → magnesium oxide Replace with formulae: Mg + O₂ → MgO (unbalanced) Add state symbols: Mg(s) + O₂(g) → MgO(s) (unbalanced) Balance (next section): 2Mg(s) + O₂(g) → 2MgO(s) ✓ Balancing Chemical Equations Balanced Equation An equation where the number of atoms of each element is the same on both sides.
This follows the Law of Conservation of Mass. Why balance equations? Atoms cannot be created or destroyed in chemical reactions The same atoms that go in must come out (just rearranged) Total mass stays the same Rules for Balancing Only change the large numbers in front of formulae (coefficients) Never change subscripts in the formulae (that would change the substance!) Balance one element at a time Leave hydrogen and oxygen until last (they appear in many compounds) Check all elements are balanced at the end Important H₂O is water.
If you change it to H₂O₂, that's hydrogen peroxide (completely different substance)! Never change subscripts - only add numbers in front. Balancing Equations - Step by Step Example 1: Hydrogen + Oxygen → Water Step 1: Write unbalanced equation H₂ + O₂ → H₂O Step 2: Count atoms on each side Left: H = 2, O = 2 Right: H = 2, O = 1 ❌ (oxygen not balanced) Step 3: Balance oxygen by putting 2 in front of H₂O H₂ + O₂ → 2H₂O Step 4: Recount Left: H = 2, O = 2 Right: H = 4, O = 2 ❌ (hydrogen not balanced now) Step 5: Balance hydrogen by putting 2 in front of H₂ 2H₂ + O₂ → 2H₂O ✓ Step 6: Final check Left: H = 4, O = 2 Right: H = 4, O = 2 ✓ Balanced!
More Balancing Examples Example 2: Magnesium + Oxygen → Magnesium Oxide Unbalanced: Mg + O₂ → MgO Count: Left (Mg = 1, O = 2), Right (Mg = 1, O = 1) ❌ Balance oxygen: Mg + O₂ → 2MgO Count: Left (Mg = 1, O = 2), Right (Mg = 2, O = 2) ❌ Balance magnesium: 2Mg + O₂ → 2MgO Count: Left (Mg = 2, O = 2), Right (Mg = 2, O = 2) ✓ Balanced equation: 2Mg + O₂ → 2MgO Example 3: Methane Combustion Word equation: methane + oxygen → carbon dioxide + water Symbol equation (unbalanced): CH₄ + O₂ → CO₂ + H₂O Count atoms: Left: C = 1, H = 4, O = 2 Right: C = 1, H = 2, O = 3 ❌ Balance hydrogen first (put 2 in front of H₂O): CH₄ + O₂ → CO₂ + 2H₂O Count: Left (C = 1, H = 4, O = 2), Right (C = 1, H = 4, O = 4) ❌ Balance oxygen (put 2 in front of O₂): CH₄ + 2O₂ → CO₂ + 2H₂O Count: Left (C = 1, H = 4, O = 4), Right (C = 1, H = 4, O = 4) ✓ Balanced: CH₄ + 2O₂ → CO₂ + 2H₂O Equations with State Symbols State symbols show the physical state of each substance in the equation.
Symbol Meaning Example (s) Solid Mg(s), NaCl(s) (l) Liquid H₂O(l), Br₂(l) (g) Gas O₂(g), CO₂(g) (aq) Aqueous (dissolved in water) HCl(aq), NaCl(aq) Examples with State Symbols 2Mg(s) + O₂(g) → 2MgO(s) 2H₂(g) + O₂(g) → 2H₂O(l) CaCO₃(s) → CaO(s) + CO₂(g) HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) Special Symbols in Equations Symbol Meaning Example → Produces / forms / yields H₂ + Cl₂ → 2HCl ⇌ Reversible reaction (goes both ways) N₂ + 3H₂ ⇌ 2NH₃ + Plus / and / reacts with Used between reactants or products Heat / Δ Heat is required Often written above arrow Catalyst name Catalyst needed Written above arrow Example with conditions: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (Conditions: 450°C, 200 atm pressure, iron catalyst) Common Types of Reactions 1.
Combustion (Burning) Substance + oxygen → oxides (+ water if hydrogen present) C + O₂ → CO₂ 2Mg + O₂ → 2MgO CH₄ + 2O₂ → CO₂ + 2H₂O 2. Thermal Decomposition One substance breaks down when heated → two or more products CaCO₃ → CaO + CO₂ 2Cu(NO₃)₂ → 2CuO + 4NO₂ + O₂ 3.
Neutralisation Acid + base → salt + water HCl + NaOH →…
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