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5.1.1Additional lessons

Balancing Equations — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 89 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5Rules for Balancing

A balanced equation is the foundation of every quantitative calculation in chemistry.

Conservation of mass
  • Conservation of mass The number of atoms of each element must be identical on both sides of an equation because atoms are neither created nor destroyed.
Coefficients
  • Coefficients Only the numbers placed in front of formulae may be changed;
    • subscripts inside formulae must never be altered.
State symbols
  • State symbols Use (s) for solid, (l) for liquid, (g) for gas and (aq) for substances dissolved in water.
Charge balance
  • Charge balance In ionic equations the total charge must also be equal on both sides, as well as the number of atoms.

Part 2 of 5Balancing by Inspection

1
Write the correct formulae of all reactants and products before attempting any balancing.
2
Balance elements that appear in only one reactant and one product first, usually starting with carbon or metals.
3
Balance hydrogen next, then leave oxygen until last because it often appears in several species.
4
Use fractions temporarily if needed, then multiply the whole equation to give whole numbers.
5
Check every element again at the end and add state symbols to each species.

Part 3 of 5Ionic Equations

Ionic Equations
Full ionic form
  • Full ionic form Write soluble ionic compounds as separated ions and keep solids, liquids, gases and molecules as formulae.
Spectator ions
  • Spectator ions Ions that appear unchanged on both sides, such as Na⁺ and NO₃⁻, are cancelled out.
Net ionic equation
  • Net ionic equation Ag⁺(aq) + Cl⁻(aq) → AgCl(s) shows only the particles that actually react.
Redox and acids
  • Redox and acids Net ionic equations are widely used for precipitation, neutralisation and redox reactions.

Part 4 of 5Worked Example

Question Balance the combustion of propane and state the mole ratio of propane to oxygen: C₃H₈(g) + O₂(g) → CO₂(g) + H₂O(l).

  1. Carbon3 atoms on the left, so 3CO₂.
  2. Hydrogen8 atoms on the left, so 4H₂O.
  3. Oxygen on the right = 3 × 2 + 4 = 10 atoms, so 5O₂ on the left.

Answer

  • C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l);
    • propane : oxygen = 1 : 5

Part 5 of 5Exam Checklist & Tips

5.1.1 – Key Points for the Exam

  • Coefficient The number in front of a formula in a balanced equation.
  • Spectator ion An ion present in a reaction mixture that does not take part in the reaction.
  • Net ionic equation An equation showing only the ions and species that change.
  • State symbol A letter in brackets showing the physical state of a species.

Common Mistakes

  • Changing subscripts in formulae, such as writing H₂O₂ to balance oxygen in a combustion equation.
  • Leaving out state symbols when the question asks for a full equation.
  • Failing to balance charge in an ionic equation even when atoms balance.
  • Including spectator ions in the net ionic equation.

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