Lecture on Molar Enthalpy and Calorimetry

Introduction

• Molar Enthalpy: Enthalpy change per mole of a chemical that reacts.
• Calorimetry: Central to understanding both enthalpy and molar enthalpy.

Key Concepts

Change in Enthalpy (ΔH)

• ΔH increases as more limiting reagent is added.
• Limiting reagent is measured in moles.

Enthalpy to Moles Ratio

• The ratio of change in enthalpy to moles is constant.
• Formula: ΔH/n = ΔHᵐ (Molar Enthalpy)
  • ΔHᵐ expressed in kilojoules per mole (kJ/mol).
  • ΔH in kilojoules (kJ) and n in moles.
• Example: Methane releases constant kJ/mol under reaction conditions.

Steps to Calculate Molar Enthalpy

1. Identify the Limiting Reagent:

   • Focus on the chemical that runs out.
   • No calculations for limiting/excess reagents required (Chemistry 20 level).

2. Calculate Thermal Energy (q):

   • Formula: q = m × c × ΔT
   • Assumes isolated environment; heat comes from chemicals.

3. Determine Sign of ΔH:

   • Positive if reaction is endothermic (surroundings cool down).
   • Negative if exothermic (surroundings warm up).

4. Calculate Moles of Limiting Reagent:

   • For pure substances: moles = mass/molar mass.
   • For solutions: moles = concentration × volume.

5. Compute Molar Enthalpy:

   • ΔHᵐ = ΔH/n

Example Calculation

• Scenario: Combustion of ethanol.
• Surroundings: 3.63 L of water (converted to grams for calculations).
• Temperature Change: ΔT = 26.18°C - 19.88°C = 6.30°C.

Thermal Energy Calculation

• q = m × c × ΔT:
  • m = 3630 grams of water.
  • c = 4.19 J/g°C (specific heat capacity of water).
  • ΔT = 6.30°C.
• Result: 95,821 J or 95.821 kJ (not rounded yet).

Moles of Ethanol

• Given: 3.5 grams of ethanol.
• Molar Mass of Ethanol: 46.08 g/mol.
• Moles of Ethanol: Mass/Molar Mass = 3.5 g / 46.08 g/mol = 0.0760 mol.

Molar Enthalpy Calculation

• ΔHᵐ = ΔH/n:
  • ΔH = 95.821 kJ (with sign considered: negative for exothermic).
  • n = 0.0760 mol.
• Result: ΔHᵐ ≈ -1260 kJ/mol (rounded to three significant figures).

Conclusion

• Molar enthalpy values are constants for given reactions and can be referenced from data tables.
• The steps outlined provide a framework for solving multi-step questions involving calorimetry and molar enthalpy.

These notes summarize the key points and steps from the lecture on molar enthalpy and calorimetry, offering a guide for future reference and study.