Chemistry Lecture Notes: Chapter 4 - Chemical Reactions

4.1 Writing and Balancing Chemical Equations

Key Concepts

• Chemical Equations: Represent identities and relative quantities of substances in a chemical change.
  • Reactants: Substances undergoing reaction, placed on the left side.
  • Products: Substances generated by the reaction, placed on the right side.
  • Coefficients: Indicate relative number of molecules. Use smallest whole-number coefficients.

Balancing Chemical Equations

• Balanced Equation: Equal numbers of atoms for each element on both sides.
  • Must satisfy the law of conservation of matter.
  • Adjust coefficients to balance the equation.

Example Balancing Chemical Equations

• Methane and Oxygen to Carbon Dioxide and Water
  • Balanced equation: CH4 + 2O2 → CO2 + 2H2O
• Decomposition of Water
  • Balanced equation: 2H2O → 2H2 + O2
  • Balance achieved by adjusting coefficients.

Molecular, Ionic, and Net Ionic Equations

• Molecular Equation: Shows complete formulas of reactants and products.
• Complete Ionic Equation: Shows all ions present in a solution.
• Net Ionic Equation: Shows only the species that actually change during the reaction.

Example: Reaction of CaCl2 and AgNO3

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

4.2 Classifying Chemical Reactions

Types of Chemical Reactions

• Precipitation Reactions: Formation of a solid from ions in solution.
• Acid-Base Reactions: Transfer of H⁺ ions, forming water and salt.
• Oxidation-Reduction (Redox) Reactions: Involve electron transfer.

Solubility Rules

• Soluble Compounds: Group 1 metal cations, NH₄⁺, nitrates, acetates.
• Insoluble Compounds: Carbonates, phosphates, except with Group 1 cations and NH₄⁺.
• Example: 2KI + Pb(NO3)2 → PbI2(s) + 2KNO3 (PbI2 is insoluble)

Acid-Base Reactions

• Strong Acids and Bases: Completely dissociate in water (e.g., HCl, NaOH).
• Weak Acids and Bases: Partially dissociate (e.g., acetic acid, ammonia).
• Neutralization Reaction: Acid + Base → Salt + Water

Redox Reactions

• Oxidation: Loss of electrons.
• Reduction: Gain of electrons.
• Example: 2Na + Cl2 → 2NaCl (Na is oxidized, Cl2 is reduced)

Balancing Redox Reactions

• Half-Reaction Method:
  1. Write separate oxidation and reduction half-reactions.
  2. Balance elements other than O and H.
  3. Balance O by adding H₂O, H by adding H⁺.
  4. Balance charges by adding electrons.
  5. Combine and simplify.
• Example: Reaction of dichromate and iron(II)

Exercises

• Practice balancing equations and identifying reaction types.
• Solve equations for various chemical scenarios, including acid-base and redox reactions.

Study Tips

• Focus on understanding the process of balancing equations.
• Familiarize yourself with solubility rules and common strong/weak acids and bases.
• Practice identifying redox reactions by tracking oxidation number changes.
• Utilize interactive tutorials for additional practice.