SC

Chapter 8 Chemical Reactions Notes

Chapter 8 Chemical Reactions

8.1 Chemical Equations: Interpreting and Writing

  • A chemical equation denotes what occurs in a reaction using symbols.

    • Example: NH₃ + HCl → NH₄Cl
    • Reactants are on the left, products on the right.

  • Physical states are labeled:

    • (g) = gas
    • (l) = liquid
    • (s) = solid
    • (aq) = aqueous (dissolved in water)
    • Example: NH₃(g) + HCl(g) → NH₄Cl(s)

8.1 Balancing Chemical Equations

  • Equations must be balanced to obey the conservation of mass.
  • Stoichiometric coefficients are used for balancing.
  • Tips for balancing:
    1. Change coefficients of compounds before elements.
    2. Treat polyatomic ions as units.
    3. Carefully count atoms and polyatomic ions.

Example: Combustion of Propane

  • Unbalanced equation: C₃H₈(g) + O₂(g) → CO₂(g) + H₂O(l)
  • Balanced form: C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l)

8.2 Combustion Analysis

  • Combustion analysis is used to determine empirical formulas.
  • Example: Combustion of glucose:
    • 18.8 g of glucose produces 27.6 g CO₂ and 11.3 g H₂O.
    • Remaining mass is calculated to find oxygen content.

Empirical Formula Calculation

  • Calculate moles of each element based on mass.
  • Divide by the smallest subscript to find whole number ratios.
  • For glucose:
    • Empirical formula: CH₂O

8.3 Calculations with Balanced Chemical Equations

  • Use balanced equations to predict product amounts.
  • Example: For the reaction of CO with O₂:
    • 2 moles CO → 2 moles CO₂
    • Calculate moles of CO₂ or O₂ required based on given moles of reactants.

Example: Synthesis of Urea

  • Balanced equation: 2NH₃ + CO₂ → (NH₂)₂CO + H₂O
  • Calculate amounts produced or needed for urea synthesis.

8.4 Limiting Reactants

  • The limiting reactant is used up first, determining product yield.
  • Example: 5 moles CO reacts with 8 moles H₂ to produce methanol.
    • Determine how much reactant is needed by stoichiometry.

Example: Alka-Seltzer Reaction

  • Reaction: 3NaHCO₃(aq) + H₃C₆H₅O₇(aq) → 3CO₂(g) + 3H₂O(l) + Na₃C₆H₅O₇(aq).
  • Determine limiting reactant, excess, and product yields.

8.5 Reaction Yield and Atom Economy

  • Theoretical yield is the maximum obtainable product; actual yield is what is obtained.
  • Percent yield = (actual yield / theoretical yield) × 100.
  • Atom economy determines how efficiently reactants are converted to products.

8.6 Periodic Trends in Reactivity of Main Group Elements

  • Understanding ionization energy and electron affinity helps predict reactivity and compounds formed.
  • Elements tend to react based on their group properties:
    • Group 1: Very reactive metals (e.g., Na, K).
    • Group 2: Reactive, form alkaline solutions.
    • Group 17: Halogens, react to form salts.
    • Group 18: Noble gases, generally nonreactive except under specific conditions.

General Trends in Reactivity

  • Group comparisons indicate reactivity levels and typical reactions.
    • Group 1 vs Group 11: Group 1 reacts more vigorously; Group 11 contains less reactive, often elemental metals.