IGCSE Combined Science - C3 Stoichiometry Notes

C3 Stoichiometry

C3.1 Formulas

Core

  1. State the formulas of the elements and compounds named in the subject content.

  2. Define the molecular formula of a compound as the number and type of atoms in one molecule.

  3. Deduce the formula of a simple molecular compound from the relative numbers of atoms present in a model or a diagrammatic representation.

  4. Construct word equations to show how reactants form products.

Supplement

  1. Balance and interpret simple symbol equations, including state symbols.

  2. 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.

  3. Construct symbol equations with state symbols, including ionic equations.

Preparing for Assessments

  • Use the syllabus statement as a checklist.

  • Identify statements you're not confident with.

  • Review relevant sections in the chapter.

  • Utilize YouTube tutorials (Fuse School, FreeScienceLessons, Cognito).

Coursebook Usage Reminder

  • Material without a sidebar or with a dashed blue or black sidebar is part of the Combined Science course.

Balancing Equations

Example 1:

  • Methane + Oxygen → Carbon Dioxide + Water

  • CH4 + O2 → CO2 + H2O

  • Initial state: C:1, H:4, O:2 (reactants) vs. C:1, H:2, O:3 (products) - Not balanced

  • Balanced equation:CH4 + 2 O2 → CO2 + 2 H2O

Example 2:

  • Fe<em>2O</em>3+C→Fe+COFe<em>2O</em>3 + C → Fe + CO

  • Initial state: Fe:2, O:3, C:1 (reactants) vs. Fe:1, O:1, C:1 (products) - Not balanced

  • Balanced equation: Fe<em>2O</em>3+3C→2Fe+3COFe<em>2O</em>3 + 3 C → 2 Fe + 3 CO

Example 3:

  • Mg+HNO<em>3→Mg(NO</em>3)<em>2+H</em>2Mg + HNO<em>3 → Mg(NO</em>3)<em>2 + H</em>2

  • Initial state: Mg:1, H:1, N:1, O:3 (reactants) vs. Mg:1, H:2, N:2, O:6 (products) - Not balanced

  • Balanced equation: Mg+2HNO<em>3→Mg(NO</em>3)<em>2+H</em>2Mg + 2 HNO<em>3 → Mg(NO</em>3)<em>2 + H</em>2

State Symbols

  • (s) = solid

  • (l) = liquid

  • (g) = gas

  • (aq) = aqueous (dissolved in water)

Examples with State Symbols

  • Mg(s)+2HNO<em>3(aq)→Mg(NO</em>3)<em>2(aq)+H</em>2(g)Mg(s) + 2 HNO<em>3(aq) → Mg(NO</em>3)<em>2(aq) + H</em>2(g)

  • 2Na(s)+2H<em>2O(l)→2NaOH(aq)+H</em>2(g)2 Na(s) + 2 H<em>2O(l) → 2 NaOH(aq) + H</em>2(g)

Exercises

  • Coursebook Q. 3.01 (p. 309), Q. 3.02 - 3.03 (p. 310)

  • Chemsheets: Word equations 1 and 2

  • Chemsheets: Balancing equations 1 and 2

Word Equations

Example

  • Na<em>2CO</em>3+2HCl→2NaCl+CO<em>2+H</em>2ONa<em>2CO</em>3 + 2 HCl → 2 NaCl + CO<em>2 + H</em>2O

  • Word equation: sodium carbonate + hydrochloric acid → sodium chloride + water + carbon dioxide

Exercise: Word Equations 2

  1. 2Na+S→Na2S2 Na + S → Na_2S

Balancing Equations

Key Points

  • Balanced equations have the same number of atoms of each type on both sides.

  • Balance equations by adding numbers in front of formulas, not by changing the formulas themselves.

  • State symbols: (s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous (dissolved in water).

Exercise: Balancing Equations 2

  • Explain the meaning of the balanced equation, including information from the state symbols:

  • P<em>4(s)+5O</em>2(g)→P<em>4O</em>10(s)P<em>4(s) + 5 O</em>2(g) → P<em>4O</em>{10}(s)