Thermochemistry Notes

Thermochemistry

• Unit Overview: Thermochemistry focuses on the energy changes during chemical reactions and includes understanding limiting reactants and thermochemical equations.

Learning Objectives

• By the end of this section, you should be able to:
  • Calculate the limiting reactant using thermochemical equations.

Key Concepts

• Limiting Reactant:

  • Definition: The limiting reactant is the reactant that is completely consumed first in a chemical reaction, thereby determining the amount of product formed.
  • Importance: Identifying the limiting reactant is crucial in stoichiometry as it will dictate how much of the product can be formed based on the reactants available.

• Thermochemical Equations:

  • These are balanced chemical equations that include the enthalpy change (energy) associated with the reaction. For instance, a thermochemical equation reflects how much energy is absorbed or released when the reactants are converted to products.

Examples

Example 1: Hydrosulfuric Acid and Oxygen Gas Reaction

• Reaction:
  • 2 H₂S + 3 O₂ → 2 SO₂ + 2 H₂O + 175 kJ
• Given Data:
  • 14.3 moles of H₂S
  • 17.1 moles of O₂
• Energy Calculations:
  • Calculate energy for H₂S:
  • 14.3 mol H₂S × (175 kJ / 2 mol H₂S) = 1250 kJ
  • Calculate energy for O₂:
  • 17.1 mol O₂ × (175 kJ / 3 mol O₂) = 1000 kJ
• Conclusion: Since O₂ limits the reaction, it is the limiting reactant.

Example 2: Tin and Nitrogen Reaction

• Reaction:
  • Sn + N₂ → Sn₃N₄ + 632 kJ
• Given Data:
  • 13.11 g of Sn
  • 2.715 g of N₂
• Molar Masses:
  • Molar mass of Sn = 118.71 g/mol
  • Molar mass of N₂ = 28.02 g/mol
• Energy Calculations:
  • Convert grams to moles:
  • Sn: 13.11 g Sn × (1 mol Sn / 118.71 g Sn)
  • N₂: 2.715 g N₂ × (1 mol N₂ / 28.02 g N₂)
  • Calculate energy produced:
  • Based on stoichiometric ratios:
    • For Sn: (1 mol Sn produces 632 kJ)
    • For N₂: (3 mol Sn produces 632 kJ)
• Conclusion: Determine which reactant is limiting by calculating amount of energy produced based on the moles of each reactant.

Worksheets

• Questions?
  • Begin Worksheet #4 after this session to practice identifying limiting reactants and using thermochemical equations in calculations.