Lesson overview
Formulae and equations: How do you balance a reaction?
Methane burns in oxygen to make carbon dioxide and water: how many oxygen molecules does each methane molecule need? Predict, then take ball-and-stick models apart on a tray and rebuild the products from their atoms, fetching whole molecules from the jars until no atom is left over, for methane, hydrogen and, in the Supplement, ammonia.
Suggested sequence
- Lesson 1 · Reading a formula. Symbols, subscripts and brackets; molecular formulae from models; the formulae of the substances named in the course; Supplement: empirical formulae and ionic formulae from charges. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- Lesson 2 · Balancing equations. Word and symbol equations, balancing with the numbers in front, state symbols; Supplement: ionic equations and equations from given information. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 3 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 3 questions, 15 min, lesson closed.
Learning goals
Part 1: Reading a formula
- State the formulae of the elements and compounds named in the subject content
- Define the molecular formula of a compound as the number and type of different atoms in one molecule
- Deduce the formula of a simple compound from the relative numbers of atoms present in a model or a diagrammatic representation
- Define the empirical formula of a compound as the simplest whole number ratio of the different atoms or ions in a compound
- Deduce the formula of an ionic compound from the relative numbers of the ions present in a model or a diagrammatic representation or from the charges on the ions
Part 2: Balancing equations
- Construct word equations and symbol equations to show how reactants form products, including state symbols
- Construct symbol equations with state symbols, including ionic equations
- Deduce the symbol equation with state symbols for a chemical reaction, given relevant information
Misconceptions to listen for
- “The simplest whole number ratio of the atoms.” — That is the empirical formula (Supplement). The molecular formula counts the atoms in one molecule.
- “The formula shows how the atoms are arranged in space.” — A formula counts atoms; a model or a diagram shows their arrangement.
- “Write CH3, three hydrogen atoms on one carbon atom.” — CH3 is half the molecule. The formula counts every atom in the whole molecule: C2H6.
- “Count the sticks as atoms.” — Sticks are bonds. Count only the balls.
- “Carbon dioxide is CO.” — CO is carbon monoxide; "di" in dioxide means two oxygen atoms, CO2.
- “Ammonia is NH4.” — NH4+ is the ammonium ion. The ammonia molecule is NH3.
- “The number after a bracket multiplies only the last symbol.” — The number after a bracket multiplies everything inside the bracket.
- “Ca(NO3)2 has 3 oxygen atoms.” — There are 3 in each nitrate group, and the bracket doubles it: 6.
- “Count every atom in the diagram and write that as the formula.” — A formula describes one molecule. Count the atoms in one molecule only.
- “Hydrogen peroxide is HO.” — HO is the simplest ratio (the empirical formula). The molecule itself has two of each atom.
Scope
Formulae and equations (3.1 Core 1–4, Supplement 5–8): the formulae of the elements and compounds named in the subject content, the molecular formula, a formula from a model or diagram, word and symbol equations with state symbols and, in the Supplement, the empirical formula, ionic formulae from a model or from the charges, ionic equations and symbol equations from given information. Ions and their charges are in Ions and ionic bonding, masses and moles in later modules of topic 3, and the reactions themselves in later modules.