Lecture Notes: Energetics and Enthalpy Changes

Key Concepts

• Enthalpy Change: Heat energy transferred in a reaction at constant pressure.
  • Exothermic Reactions: Energy transferred from system to surroundings; products have less energy than reactants; ΔH is negative.
  • Endothermic Reactions: Energy transferred from surroundings to system; products have more energy than reactants; ΔH is positive.

Standard Conditions

• Conditions: 100 kPa, 298 K (25°C), solutions at 1 mol/dm³.
• Standard Enthalpy Change of Formation (ΔfH): Change when 1 mole of compound is formed from its elements.
• Standard Enthalpy Change of Combustion (ΔcH): Change when 1 mole of substance combusts completely in oxygen.

Measurement of Enthalpy Change

• Calorimetry: Method for measuring heat transition in a reaction.
  • Equation: Q = m × cp × ΔT
  • General Method:
    • Use insulated containers.
    • Measure temperature changes regularly.
    • Calculate energy change per mole.
• Errors/Considerations:
  • Energy loss to surroundings.
  • Approximation in specific heat capacity.
  • Incomplete reactions.

Hess's Law

• Principle: Total enthalpy change is independent of reaction path.
• Application: Used to calculate enthalpy changes not directly measurable.
• Equations: ΔH = ΔfH (products) - ΔfH (reactants) or ΔH = ΔcH (reactants) - ΔcH (products).

Bond Energies

• Definition: Average energy required to break covalent bonds.
• Usage: Calculate enthalpy changes using mean bond energies.
• Limitations: Less accurate than formation or combustion data.

Practical Applications

• Experimental Procedures:
  • Measure reactant temperatures, calculate energy using calorimetry, and analyze using Hess's Law.
• Errors in Experiments:
  • Energy loss, incomplete combustion, non-standard conditions.
• Comparisons:
  • Enthalpies of combustion in homologous series increase with more bonds broken/made.
  • Experimental vs. calculated values often show discrepancies due to heat loss and incomplete reactions.

These summarized points provide an overview of the essentials in studying energetics and enthalpy changes in chemistry.