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4.1.3Chemical Reactions

Balancing Chemical Equations — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 5 parts Lesson 13 of 18 Unit quiz

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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 5Why Equations Must Balance

A balanced equation shows the same number of each type of atom in the reactants and products.

Conservation of atoms
  • Conservation of atoms Atoms are neither created nor destroyed in a reaction, so each element has the same number of atoms on both sides.
Coefficients
  • Coefficients Whole-number coefficients in front of formulas show the relative number of particles or moles taking part.
Subscripts
  • Subscripts Subscripts inside a formula are fixed by the identity of the substance and must never be changed to balance an equation.
State symbols
  • State symbols The symbols (s), (l), (g) and (aq) show the physical state of each substance.

Part 2 of 5A Method That Works

1
Write the correct formulas for all reactants and products.
2
Count the atoms of each element on both sides of the arrow.
3
Balance elements that appear in only one substance on each side first, leaving hydrogen and oxygen until late.
4
Adjust coefficients only, and recount all atoms after every change.
5
Finish by checking that the coefficients are the smallest whole numbers and that the charges balance for ionic equations.

Part 3 of 5Common Patterns

Common Patterns
Combustion of hydrocarbons
  • Combustion of hydrocarbons Balance carbon as CO₂, then hydrogen as H₂O, and then oxygen last, doubling everything if an odd number of O₂ is needed.
Polyatomic ions
  • Polyatomic ions If an ion such as PO₄³⁻ or SO₄²⁻ appears unchanged on both sides, balance it as a single unit.
Synthesis and decomposition
  • Synthesis and decomposition Elements in diatomic form, such as O₂, N₂ and H₂, need an even number of atoms in total on their side of the arrow.
Net ionic equations
  • Net ionic equations Charge must balance as well as atoms, so the total charge on the left equals the total on the right.
UnbalancedBalanced
Fe + O₂ → Fe₂O₃4Fe(s) + 3O₂(g) → 2Fe₂O₃(s)
Ca(OH)₂ + H₃PO₄ → Ca₃(PO₄)₂ + H₂O3Ca(OH)₂(aq) + 2H₃PO₄(aq) → Ca₃(PO₄)₂(s) + 6H₂O(l)

Part 4 of 5Worked Example

Question Balance the combustion of butane: C₄H₁₀(g) + O₂(g) → CO₂(g) + H₂O(l).

  1. Balance carbon4 C on the left, so use 4CO₂.
  2. Balance hydrogen10 H on the left, so use 5H₂O.
  3. Count oxygen on the right4 × 2 + 5 = 13 O atoms, which needs 6.5 O₂.
  4. Remove the fraction by doubling everything2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O.
  5. CheckC 8 = 8, H 20 = 20, O 26 = 16 + 10 = 26.

Answer 2C₄H₁₀(g) + 13O₂(g) → 8CO₂(g) + 10H₂O(l)

Part 5 of 5Exam Checklist & Tips

4.1.3 – Key Points for the Exam

  • Coefficient A number placed in front of a formula to show how many particles or moles of that substance take part.
  • Subscript A number in a formula showing how many atoms of an element are in one unit.
  • Reactant A substance present at the start of a reaction that is changed.
  • Product A substance formed in a reaction.

Common Mistakes

  • Changing subscripts to balance an equation, which changes the substance into a different one.
  • Leaving fractions in a final equation when coefficients should be the smallest whole numbers.
  • Balancing oxygen first in combustion, which usually leads to extra rounds of rebalancing.
  • Forgetting that polyatomic ions can be counted as units when they remain unchanged.

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