Ch. 7 - Chemical Reactions and Chemical Quantities

Chapter 7: Chemical Reactions & Chemical Quantities

Introduction

  • Overview of chemical reactions and how they involve changes resulting in the formation of new substances.

Chemical Reactions

  • Definition: Reactions involve a rearrangement and exchange of atoms to produce new molecules.

  • Reactants: Substances that undergo change.

  • Products: New substances formed as a result of the reaction.

Chemical Equations

  • Purpose: Shorthand representation of a reaction that provides vital information about the reaction process.

  • Components:

    • Formulas and states of reactants/products: Indicate the physical state (solid, liquid, gas, aqueous) of compounds.

    • Relative numbers: Show how many molecules of reactants yield how many molecules of products, allowing calculation of masses involved in the reaction.

Balancing Reactions

  • A fundamental law in chemistry is the Law of Conservation of Mass: The mass of reactants must equal the mass of products.

  • Example:

    • Reaction: 2 Na (s) + Cl2 (g) → 2 NaCl (s)

      • Interpretation: Involves 2 moles of sodium reacting with 1 mole of chlorine to produce 2 moles of sodium chloride.

  • Another example:

    • Reaction: C3H8 (g) + O2 (g) → CO2 (g) + H2O (g)

      • Provides a more complex balancing scenario reflecting the conservation principle.

Representing Chemical Reactions

  • Microscopic Level: Includes atomic-level explanations.

    • Example: 2 H2 (g) + O2 (g) → 2 H2O (l) shows the molecular interaction.

  • Macroscopic Level: Describes observable quantities.

    • Example: Amounts in kg for practical measurements, e.g., 0.56 kg of H2 reacting with 4.44 kg of O2.

Molecular and Formula Masses

  • Molecular Mass: Sum of atomic masses within a molecule.

  • Formula Mass: Sum of atomic masses in a formula unit of any compound, molecular or ionic.

  • Example Calculations:

    • C2H4: 2 × 12.0 amu + 4 × 1.0 amu = 28.0 amu

    • HCl: 1.0 amu + 35.5 amu = 36.5 amu

Moles and Mole Calculations

  • One mole of a substance corresponds to its molecular or formula mass expressed in grams.

    • Example: 1 mol C2H4 = 28.0 g; 1 mol HCl = 36.5 g.

  • Basic calculations to determine amounts in reactions:

    • Example: To find moles of Cl2 in 25.0 g:

      • Use ratio: 25.0 g Cl2 × (1 mol/70.9 g) = 0.353 mol Cl2.

Stoichiometry Overview

  • Relates the amounts of reactants and products within a chemical reaction.

  • General formula:

    • aA + bB → cC + dD

    • Use coefficients to determine molar ratios and mass conversions.

Examples of Stoichiometry Calculations

  • Example with NaOCl formation:

    • Given 25g Cl2: Reactant relation and molar masses determine grams of NaOH needed.

    • Through various calculations, determine final grams needed: 28.2 g NaOH.

Yield Calculations

  • Theoretical Yield: Maximum amount of product expected from given reactants.

  • Actual Yield: Amount obtained from the reaction.

  • Percent Yield: (Actual Yield / Theoretical Yield) × 100.

    • Example: Production of C2H6O and calculating yields based on provided data.

Limiting Reactants

  • Definition: The reactant that limits the amount of product formed, causing a reaction to stop.

  • Example: Water used in excess compared to ethylene oxide for glycol production.

  • Important in efficiency calculations in reactions, particularly in industrial settings.

Practical Application: Li2O Reaction

  • Examines the reaction of Li2O with water for removing moisture.

  • Determine limiting reactants and excess reactant amounts through calculations based on mass and moles.

  • Detailed calculations reveal how much LiOH is produced, demonstrating stoichiometric principles.