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CHEMISTRY · Stage 9 · Chapter 12: Chemical Reactions 3 and Rates

Chemistry 12.2 Writing symbol equations — Balancing Chemical Equations

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12.2 Writing symbol equations — Balancing Chemical Equations — Alhussan International Academy — Chemistry (Stages 7–9) notes: Write balanced symbol equations with formulae and state symbols.Symbol equation — it shows the reactants and products, their chemical formulae, the relati…

  • Precipitation example
  • Balancing numbers
  • Never change subscripts
  • Lowest ratio
  • Zinc and acid
  • Start with

Section 1 of 6What a symbol equation shows

  • Symbol equation
    • it shows the reactants and products, their chemical formulae, the relative numbers of particles and the states.
  • Formulae
    • replace each name in the word equation with its formula, and never guess a formula but look it up.
    • Diatomic elements are H2, N2, O2, F2, Cl2, Br2 and I2.
    • Common formulae: water H2O, carbon dioxide CO2, methane CH4, magnesium oxide MgO, sodium chloride NaCl, hydrochloric acid HCl and sulfuric acid H2SO4.
  • Precipitation example
    • silver nitrate + potassium iodide → silver iodide + potassium nitrate: KI(aq) + AgNO3(aq) → AgI(s) + KNO3(aq).
      • (yellow precipitate)
    • It needs no balancing numbers because each element has the same number of atoms on both sides.

Section 2 of 6State symbols

  • (s)
    • solid.
  • (l)
    • liquid.
  • (g)
    • gas.
  • (aq)
    • aqueous, meaning dissolved in water.
  • Placement
    • write one state symbol to the right of each formula, in lower case.

Section 3 of 6Balancing method

  1. Write the word equation with the chemical formula under each substance.
  2. Count the atoms of each element on both sides.
  3. Add a big number in front of a formula to balance one element, usually leaving hydrogen and oxygen to the end.
  4. Recount all the elements and adjust.
  5. Check the final equation and add state symbols.
  • Balancing numbers
    • a big number applies to every atom in the formula that follows it, so 2MgO means two magnesium atoms and two oxygen atoms.
  • Never change subscripts
    • changing a small number changes the substance, for example MgO to MgO2.
  • Lowest ratio
    • the numbers should be in the lowest whole-number ratio.
  • Brackets
    • in Ca(OH)2 the number outside multiplies everything inside, giving one calcium, two oxygen and two hydrogen atoms.

Section 4 of 6Balanced equations

ReactionBalanced equation
Magnesium + oxygen2Mg(s) + O2(g) → 2MgO(s)
Zinc + hydrochloric acidZn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
Hydrogen + oxygen2H2 + O2 → 2H2O
Methane burningCH4 + 2O2 → CO2 + 2H2O
Magnesium + acidMg + 2HCl → MgCl2 + H2
Sodium + water2Na + 2H2O → 2NaOH + H2
Propane burningC3H8 + 5O2 → 3CO2 + 4H2O
Aluminium + hydrochloric acid2Al + 6HCl → 2AlCl3 + 3H2
Lithium + oxygen4Li + O2 → 2Li2O
Nitrogen + oxygenN2 + O2 → 2NO

Section 5 of 6Balancing magnesium and oxygen step by step

  1. Mg + O2 → MgO has two oxygen atoms on the left and one on the right.
  2. Put 2 in front of MgO: Mg + O2 → 2MgO, which balances oxygen.
  3. Now magnesium needs balancing: 2Mg + O2 → 2MgO.
  4. Add state symbols: 2Mg(s) + O2(g) → 2MgO(s).
  • Zinc and acid
    • one hydrogen on the left and two on the right means 2HCl, which gives one Zn, two H and two Cl on each side.

Section 6 of 6Strategies for harder equations

  • Start with
    • an element that appears in the fewest places, such as carbon in propane.
  • Propane
    • three carbon atoms need 3CO2, eight hydrogen atoms need 4H2O, and 3 × 2 + 4 = 10 oxygen atoms need 5O2.
  • Even numbers
    • if a diatomic molecule such as H2 forces odd and even counts, double the other coefficients.
  • Check table
    • list atom counts for left and right to avoid errors.

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