Stoichiometry: Key Concepts and Calculations

3.1 Chemical Equations

• Definition: Chemical equations represent chemical reactions.
• Components:
  • Reactants (starting materials) on the left
  • Products (ending materials) on the right
  • Arrows separate reactants from products.
  • Use “+” to separate multiple substances.

Balancing Equations

• Law of Conservation of Mass: Mass is conserved; atom count must be equal on both sides.
• Method:
  1. Choose an element that appears in only one reactant and product to balance first.
  2. Change coefficients (not subscripts, as that alters the substance itself).
  3. Use trial and error starting with the least common multiple (LCM).
  4. Move on to balance other elements, checking all at the end.

Why Coefficients instead of Subscripts?

• Changing subscripts alters the substance. E.g., H$2$O (water) is different from H$2$O$_2$ (hydrogen peroxide).

Required Symbols in Chemical Equations

• Phases of Matter: Denoted in parentheses.
  • (g) = gas
  • (l) = liquid
  • (s) = solid
  • (aq) = aqueous (dissolved in water)
  • Δ = heat applied.

3.2 Simple Patterns of Chemical Reactivity

• Key types of reactions:
  • Combination Reactions: Two or more substances combine to form a single product (e.g., A + B → C).
  • Decomposition Reactions: A single substance breaks down into two or more products (e.g., C → A + B).
  • Combustion Reactions: Rapid reactions producing heat, CO$2$, and H$2$O.

3.3 Formula Weight (FW)

• Definition: Sum of atomic weights in a chemical formula.
• Molecular Weight: Used for molecules; the weight of a compound.
• Example Calculation for H$2$SO$4$:
  • FW = 2(AW${H}$) + AW${S}$ + 4(AW$_{O}$)
  • $FW(H2SO4) = 2(1.0) + 32.1 + 4(16.0) = 98.1$ amu.

3.4 The Mole & Avogadro’s Number

• Definition of a Mole: 1 mole = 6.02 × 10$^{23}$ particles.
• Use of Avogadro’s Number: Bridge between atomic scale and macroscopic quantities.

Molar Mass

• Mass of 1 mol of a substance.
• For elements, referenced directly from the periodic table. For molecules, calculated similarly to FW.

3.5 Empirical Formulas from Analysis

• Steps:
  1. Assume mass % = 100g. Convert % to grams.
  2. Convert grams to moles.
  3. Calculate mole ratios to derive the empirical formula.

3.6 Quantitative Information from Balanced Equations

• Coefficients in equations indicate:
  • Relative numbers of molecules and moles of reactants and products.
  • Used for stoichiometric calculations, establishing mole ratios.

3.7 Limiting Reactants

• Definition: The reactant that is completely consumed first in a reaction that limits product formation.
• Determining the limiting reactant is crucial for accurate calculations.

Theoretical Yield vs. Percent Yield

• Theoretical Yield: Maximum possible amount of product based on stoichiometry.
• Percent Yield:
  • Calculation: Percent Yield = (Actual Yield / Theoretical Yield) × 100%.
  • Indicates the efficiency of a chemical reaction.

Example Stoichiometric Calculation Flow

1. Convert grams of a reactant to moles.
2. Use mole ratio from the balanced equation to find moles of a product.
3. Convert moles of the product to grams using its molar mass.