Lecture Notes on Moles, Stoichiometry, and Balancing Equations

Balancing Chemical Equations

  • Chemical Equations: Represent chemical reactions on paper. Must obey the Law of Conservation of Mass: Reactants = Products.
  • Requires practice to convert equations into balanced net ionic equations.

Steps to Balance Chemical Equations

  1. Convert Word Equation to Balanced Chemical Equation:
  • Example: Sodium and water reaction.
  • Chemical equation: Na(s) + H₂O(l) → Na⁺(aq) + OH⁻(aq) + H₂(g)
  1. Count Atoms on Each Side:
  • Example:
    • Left Side: 1 Na, 1 O, 2 H
    • Right Side: 1 Na, 2 O, 3 H
  • Uneven hydrogen count indicates unbalanced equation.
  1. Adjust Coefficients:
  • Multiply Na⁺ and OH⁻ by 2 to balance hydrogen and charges.
  • New equation: 2 Na⁺(aq) + 2 OH⁻(aq) + H₂(g)
  1. Final Count of Atoms:
  • Confirm each side now has 2 Na, 2 O, 4 H.

The Mole

  • Unit of Measurement: The mole (mol) is a standard unit in chemistry for quantifying amount of substance.

  • Definition: 1 mole = 6.02214076 x 10²³ entities (Avogadro's Number).

  • Historical Context:

  • Previously based on the mass of carbon-12. 1 mole of carbon = 12 g.

Importance of the Mole

  • Useful for determining simplest formula, calculating reaction quantities, and converting mass to number of moles.

Molecular and Molar Weights

  • Formula Weight: Sum of atomic weights in a chemical formula.
  • Example: H₂SO₄: 98.1 u (2(1.0) + 32.1 + 4(16.0)).
  • Molar Mass: Mass of 1 mole of a substance (g/mol).
  • Molar mass of element = its atomic weight from periodic table.

Stoichiometry

  • Definition: Study of quantities consumed and produced in chemical reactions.
  • Concept: Use balanced equations to determine mole ratios, which indicate the relative amounts of substances.

Mole Ratio Examples

  • Formation of ammonia: N₂ + 3H₂ → 2NH₃ (1:3:2 ratio).
  • Example of NaCl: Na⁺ + Cl⁻ → NaCl (1:1 ratio).

Limiting Reactants and Yield Calculations

  • Limiting Reactant: The reactant present in the smallest stoichiometric amount, dictating the extent of the reaction.
  • Analogy: Peanut butter and jam sandwich example: if limited by bread, that's the limiting reactant.

Steps to Identify Limiting Reactant

  1. Calculate amounts of each reactant.
  2. Compare based on mole ratio.
  3. The one leading to the least product is the limiting reactant.

Predicting Yield

  • Theoretical Yield: Maximum product amount possible from given reactants.
  • Determined from limiting reactant data.
  • Percent Yield Formula:
    [ Percent \, Yield = \frac{actual \, yield}{theoretical \, yield} \times 100 \%
    ]
  • Example Calculation: If produced 4.5 g when theoretical was 6.1 g, the percent yield = (4.5/6.1) × 100% = 73.8%.

Hydrated Crystals Example

  • Example with copper sulfate: CuSO₄·5H₂O indicates water in crystalline structure affecting molar mass calculation.
  • Mass of reactants (CuO and H₂SO₄) can be calculated and compared to determine limiting reactant.

Practical Applications

  • Understanding stoichiometry and limiting reactants is critical for efficient chemical production, e.g., synthesis of drugs like ibuprofen.