4.1 Writing and Balancing Chemical Equations - Chemistry 2e

Learning Objectives

• Derive chemical equations from narrative descriptions.
• Write and balance chemical equations in molecular, total ionic, and net ionic forms.

Chemical Equations

• Symbols & Formulas: Represent atoms and molecules.
• Chemical Equation: Represents identities and relative quantities of substances in a chemical change.
  • Reactants: Substances undergoing reaction (left side).
  • Products: Substances generated by the reaction (right side).
  • Coefficients: Numbers indicating the relative quantities of reactants and products.

Example: Methane and Oxygen Reaction

• Reaction: CH₄ + 2 O₂ → CO₂ + 2 H₂O
  • Reactants: Methane (CH₄), Oxygen (O₂)
  • Products: Carbon Dioxide (CO₂), Water (H₂O)
  • Coefficients show a 1:2:1:2 ratio.

Balancing Chemical Equations

• Law of Conservation of Matter: Total number of atoms for each element must be equal on both sides.
• Steps:
  1. Write the unbalanced equation.
  2. Count and compare atoms of each element on both sides.
  3. Adjust coefficients to balance the equation.

Example: Decomposition of Water

• Unbalanced: H₂O → H₂ + O₂
• Balanced: 2 H₂O → 2 H₂ + O₂

Example: Reaction of Nitrogen and Oxygen

• Unbalanced: N₂ + O₂ → N₂O₅

• Balanced: 2 N₂ + 5 O₂ → 2 N₂O₅

• Fractional Coefficients: Sometimes used in balancing (convert to integers in the final equation).

Additional Information in Equations

• State Notations:
  • s (solid), l (liquid), g (gas), aq (aqueous solution).
• Special Conditions: Indicated by symbols (e.g., Δ for heat).

Equations for Ionic Reactions

• Molecular Equation: Does not show ionic dissociation.
• Complete Ionic Equation: Shows all ions as they are in solution.
• Net Ionic Equation: Shows only the ions involved in the chemical change.
  • Spectator Ions: Ions not involved in the reaction are omitted.

Example: Reaction in Aqueous Media

• Molecular: CaCl₂(aq) + 2 AgNO₃(aq) → Ca(NO₃)₂(aq) + 2 AgCl(s)
• Complete Ionic: Ca²⁺(aq) + 2 Cl⁻(aq) + 2 Ag⁺(aq) + 2 NO₃⁻(aq) → Ca²⁺(aq) + 2 NO₃⁻(aq) + 2 AgCl(s)
• Net Ionic: 2 Cl⁻(aq) + 2 Ag⁺(aq) → 2 AgCl(s)

Balancing Guidelines:

• Use smallest whole-number coefficients.
• Retain identity (do not change subscripts).

Summary

• Writing and balancing chemical equations is essential for representing chemical reactions accurately.
• Equations must be balanced to uphold the law of conservation of matter.
• Ionic equations provide a clearer view of reactions in aqueous solutions.

Practice: Use interactive tutorials and exercises to improve balancing skills.