Unit 5: Chemical Reactions

Unit 5: Chemical Reactions

Introduction to Chemical Reactions

  • A chemical reaction is a process where atoms rearrange to create a new substance.
  • Represented by a chemical equation, indicating reactants (starting materials) and products (resulting materials).

Categories of Chemical Reactions

  1. Rearrangement of Atoms:

    • Synthesis: Two or more reactants combine to form a single product.
    • Decomposition: A single reactant breaks down into multiple products.
    • Single Replacement: One element replaces another in a compound.
    • Double Replacement: Two compounds exchange components to form two new compounds.
    • Combustion: Fuel reacts with O₂ to produce CO₂ and H₂O.

  2. Mechanics of Reaction:

    • Precipitation Reactions: Formation of an insoluble compound from an aqueous solution.
    • Acid-Base Reactions: Transfer of protons (H+ ions).
    • Oxidation-Reduction (Redox): Transfer of electrons, involving oxidation (loss of electrons) and reduction (gain of electrons).

Balancing Chemical Equations

  • Law of Conservation of Mass: Mass is conserved in a chemical reaction. The total mass of products equals that of reactants.
  • A balanced equation contains equal atoms for each element on both sides, typically achieved by adjusting coefficients (the numbers in front of compounds).

Examples of Chemical Reactions

  • Practice balancing equations:
    • CH₄ + Cl₂ → CCl₄ + HCl
    • KIO₃ → KI + O₂
    • C₂H₆ + O₂ → CO₂ + H₂O
    • CuSO₄ + AgNO₃ → Cu(NO₃)₂ + Ag₂SO₄

Solubility in Chemical Reactions

  • Soluble: Substances that dissolve in liquids.
  • Insoluble: Substances that do not dissolve.
  • Solubility tables help determine whether a compound is soluble or insoluble, particularly for ionic compounds.
  • Example Ions:
    • Mostly Soluble: Li⁺, Na⁺, K⁺, NH₄⁺, NO₃⁻, C₂H₃O₂⁻
    • Mostly Insoluble: OH⁻, S²⁻, CO₃²⁻, PO₄³⁻ (with exceptions)

Ionic Dissolution in Water

  • Ionic compounds dissociate in water, meaning ions separate and become surrounded by water molecules.
  • The polarity of water leads to ion separation:
    • Na⁺ ions are attracted to the negative end of the water molecule.
    • Cl⁻ ions are attracted to the positive end of the water molecule.

Types of Reactions – Precipitation

  1. Precipitation Reaction: Formation of an insoluble product when two solutions are mixed.
    • Example: Pb(NO₃)₂(aq) + KI(aq) → PbI₂(s) + KNO₃(aq)
    • Net Ionic Equation: Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s)
    • Spectator Ions: Ions that remain unchanged in the reaction

Types of Reactions – Redox

  • Redox Reactions: Involves the transfer of electrons.
  • OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
  • Examples include reactions with copper and silver nitrate:
    • Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)
    • Oxidation State Changes: Cu⁰ → Cu²⁺, Ag⁺ → Ag⁰

Acid-Base Reactions

  • Acids: Lower pH, sour taste, H⁺ donors.
  • Bases: Higher pH, bitter taste, OH⁻ acceptors.
  • Typical acid-base reaction:
    • Acid + Base → Salt + Water
    • Example: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

Conjugate Acid-Base Pairs

  • Conjugate pairs differ by one H⁺ ion.
    • Example: H₂O and H₃O⁺
    • Reaction: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

Mixed Reaction Types Review

  • Precipitation: Double replacement forming an insoluble salt.
  • Acid-Base: Double replacement, forming water and possibly a gas.
  • Redox: Changes in oxidation states, often seen in single replacement reactions.

Homework and Practice Problems

  • Identify types of reactions:
    • Given examples, identify which reactions are precipitation, redox, or acid-base.
  • Practice balancing equations, using solubility tables, and writing net ionic equations to strengthen understanding of the concepts discussed.