Balancing Chemical Equations — Fiche d'exercices de Chemistry

Grades 9–11 Difficulté moyenne 10 problèmes

A chemical equation is balanced when each element has the same number of atoms on both sides of the arrow. This reflects the law of conservation of mass: atoms are never created or destroyed in a chemical reaction, only rearranged.

To balance an equation you may only change coefficients, the whole numbers placed in front of each formula. You may never change subscripts, because that changes the substance itself — H2O\mathrm{H_2O} and H2O2\mathrm{H_2O_2} are completely different chemicals.

A reliable strategy is to balance elements one at a time, saving elements that appear as pure substances (like O2\mathrm{O_2}) for last. When a polyatomic ion such as NO3−\mathrm{NO_3^-} or SO42−\mathrm{SO_4^{2-}} appears unchanged on both sides, treat it as a single intact unit to simplify the atom counting.

Finish by recounting every atom on both sides and reducing the coefficients to the lowest whole-number ratio. Fractions can help mid-problem (for example 72O2\tfrac{7}{2}\mathrm{O_2}), but multiply through so the final answer uses whole numbers.

Compétences travaillées

  • Counting atoms of each element on both sides of an equation
  • Balancing equations by adjusting coefficients only
  • Distinguishing coefficients from subscripts
  • Keeping polyatomic ions intact while balancing
  • Connecting balanced equations to conservation of mass
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Fiche d'exercices : Balancing Chemical Equations

Consignes : Résous chaque problème avec soin. Montre clairement tout ton travail. Écris ta réponse finale dans l'espace prévu ou sur une feuille séparée comme indiqué.

  1. 1.Balance the following equation: _____ Fe + _____ O₂ → _____ Fe₂O₃. Write the coefficients in the blanks above.

Balancing Chemical Equations — Fiche d'exercices (suite)

  1. 2.Balance the following equation: _____ C₃H₈ + _____ O₂ → _____ CO₂ + _____ H₂O. Write the coefficients in the blanks above.
  2. 3.Balance the following equation: _____ Al + _____ HCl → _____ AlCl₃ + _____ H₂. Write the coefficients in the blanks above.

Balancing Chemical Equations — Fiche d'exercices (suite)

  1. 4.Balance the following equation: _____ N₂ + _____ H₂ → _____ NH₃. Write the coefficients in the blanks above.
  2. 5.Balance the following equation: _____ KClO₃ → _____ KCl + _____ O₂. Write the coefficients in the blanks above.

Balancing Chemical Equations — Fiche d'exercices (suite)

  1. 6.Balance the following equation: _____ C₂H₆ + _____ O₂ → _____ CO₂ + _____ H₂O. Write the coefficients in the blanks above.
  2. 7.Balance the following equation: _____ Na + _____ H₂O → _____ NaOH + _____ H₂. Write the coefficients in the blanks above.

Balancing Chemical Equations — Fiche d'exercices (suite)

  1. 8.Which of the following is the correctly balanced equation for the reaction of methane with oxygen? Circle the letter of the correct answer.
    1. A. CH₄ + O₂ → CO₂ + H₂O
    2. B. CH₄ + 2O₂ → CO₂ + 2H₂O
    3. C. 2CH₄ + O₂ → 2CO₂ + 2H₂O
    4. D. CH₄ + O₂ → CO₂ + 2H₂O

Balancing Chemical Equations — Fiche d'exercices (suite)

  1. 9.Balance the following equation: _____ Zn + _____ AgNO₃ → _____ Zn(NO₃)₂ + _____ Ag. Write the coefficients in the blanks above.
  2. 10.Balance the following equation: _____ Ca(OH)₂ + _____ H₃PO₄ → _____ Ca₃(PO₄)₂ + _____ H₂O. Write the coefficients in the blanks above.

Corrigé

  1. 1.
    4Fe + 3O₂ → 2Fe₂O₃
    Réponse finale : 4, 3, 2
  2. 2.
    C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
    Réponse finale : 1, 5, 3, 4
  3. 3.
    2Al + 6HCl → 2AlCl₃ + 3H₂
    Réponse finale : 2, 6, 2, 3
  4. 4.
    N₂ + 3H₂ → 2NH₃
    Réponse finale : 1, 3, 2
  5. 5.
    2KClO₃ → 2KCl + 3O₂
    Réponse finale : 2, 2, 3
  6. 6.
    2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O
    Réponse finale : 2, 7, 4, 6
  7. 7.
    2Na + 2H₂O → 2NaOH + H₂
    Réponse finale : 2, 2, 2, 1

Corrigé (suite)

  1. 8.
    CH₄ + 2O₂ → CO₂ + 2H₂O. Check: C: 1=1, H: 4=4, O: 4=4
    Réponse finale : B
  2. 9.
    Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag
    Réponse finale : 1, 2, 1, 2
  3. 10.
    3Ca(OH)₂ + 2H₃PO₄ → Ca₃(PO₄)₂ + 6H₂O. Check: Ca: 3=3, O: 6+8=14, 8+6=14, H: 6+6=12, 12=12, P: 2=2
    Réponse finale : 3, 2, 1, 6
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Erreurs fréquentes à éviter

  • Changing subscripts instead of adding coefficients
  • Forgetting a coefficient multiplies every atom in the formula, including inside parentheses
  • Splitting apart a polyatomic ion that stays together on both sides
  • Leaving coefficients in a ratio that is not reduced to lowest whole numbers
  • Not recounting all atoms after the final coefficient change

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