Thermochemistry Notes

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

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

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.

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.

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.

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