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1.2.3Physical Chemistry

Balanced Equations — AQA AS Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 10 of 102

📚 AQA AS Chemistry 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 6Balancing Equations

Atoms are conserved, so each side of an equation must contain the same number of each element.

Conservation
  • Conservation Mass is conserved in a reaction, so the number of atoms of each element is the same on both sides.
Coefficients
  • Coefficients Balance by adding large numbers (coefficients) in front of formulae, never by changing the subscripts.
Order
  • Order Balance elements that appear in only one compound on each side first and leave hydrogen and oxygen until last.
Check
  • Check Count the atoms of every element on each side once more after balancing.

Part 2 of 6State Symbols

State Symbols
(s)
  • (s) Solid, as in Mg(s) or CaCO₃(s).
(l)
  • (l) Liquid, as in H₂O(l) or Br₂(l).
(g)
  • (g) Gas, as in CO₂(g) or H₂(g).
(aq)
  • (aq) Aqueous: dissolved in water, as in HCl(aq) or NaOH(aq).

Part 3 of 6Ionic Equations

Only the ions that actually change take part in the ionic equation.

1
Write the full balanced equation with state symbols.
2
Split every soluble ionic compound (aq) into its separate ions.
3
Leave solids, liquids, gases and covalent molecules as whole formulae.
4
Cancel the spectator ions, which are the same on both sides of the equation.
5
Write what is left as the ionic equation, checking that charges as well as atoms balance.

Part 4 of 6Common Examples

Neutralisation
  • Neutralisation H⁺(aq) + OH⁻(aq) → H₂O(l) is the ionic equation for any strong acid with a strong alkali.
Precipitation
  • Precipitation Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) is the ionic equation for the barium chloride test for sulfate ions.
Metal and acid
  • Metal and acid Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g) shows a metal reacting with any dilute acid.
Check charges
  • Check charges The total charge on the left must equal the total charge on the right in every ionic equation.

Part 5 of 6Worked Example

Question Balance the equation Al + HCl → AlCl₃ + H₂, add state symbols and write the ionic equation.

  1. Balance Al and Cl together2AlCl₃ contains 2 Al and 6 Cl, so use 2Al and 6HCl on the left.
  2. Balance hydrogen6HCl supplies 6 H atoms, which form 3H₂ on the right.
  3. Full equation2Al(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂(g).
  4. Remove the spectator ion Cl⁻2Al(s) + 6H⁺(aq) → 2Al³⁺(aq) + 3H₂(g).

Answer

  • 2Al(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂(g);
    • ionic: 2Al(s) + 6H⁺(aq) → 2Al³⁺(aq) + 3H₂(g)

Part 6 of 6Exam Checklist & Tips

1.2.3 – Key Points for the Exam

  • Spectator ion An ion that is present in the reaction mixture but does not change during the reaction.
  • Ionic equation An equation that shows only the particles that actually take part in the reaction.
  • State symbol A symbol showing the physical state of a substance: (s), (l), (g) or (aq).
  • Coefficient The number in front of a formula that shows how many moles of it take part.

Common Mistakes

  • Changing subscripts in formulae to make an equation balance instead of adding coefficients.
  • Splitting solids and gases into ions in an ionic equation.
  • Leaving out state symbols when they are asked for.
  • Balancing atoms but not checking that the charges balance in an ionic equation.

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