Lecture on Enthalpy and Standard State

Standard State in Thermodynamics

• Standard State Definition:
  • Indicated with a Delta H (ΔH) and a "not" (°) sign above it.
  • As per IUPAC:
    • Standard state is 1 bar of pressure and 1 molar concentration.
    • Temperature typically room temperature (25°C or 298.15 K).
  • Commonly seen in lab tables as:
    • 1 atmosphere (close to 1 bar).
    • 1 molar concentrations.

Importance of Standard State

• Used in thermodynamic calculations to compare data reliably.
• Non-standard conditions require careful reading of problems, as these conditions can vary.

Types of Standard Enthalpies

• Standard Enthalpy of Combustion (ΔH°_c):
  • Indicates the energy change when a substance combusts under standard conditions.
  • Combustion reactions produce CO₂ and H₂O.
  • Example: Enthalpy of combustion for ethanol is -13668 K.

Calculating Energy from Combustion

• Example Calculation:
  • For 1.0 liters of ethanol, convert to mass using density (789 g/L).
  • Convert mass to moles using molecular weight (46.07 g/mol).
  • Calculate energy using combustion enthalpy (-13668 K/mol).
  • Result: -2.34 × 10⁴ K of energy produced.
  • Comparison with isooctane (gasoline): -3.31 × 10⁴ K per liter, indicating higher energy density.

Standard Enthalpy of Formation (ΔH°_f)

• Used for one mole of compound formed from pure elements.
• Example: Formation of CO₂ from carbon and oxygen:
  • Reaction: C (solid) + O₂ (gas) → CO₂ (gas).
  • Enthalpy of formation for CO₂: -313.5 K.
• Key Point: For pure elements in their standard state, ΔH°_f = 0.

Practical Applications

• Knowing these values helps understand reactions, energy consumption, and efficiencies of different fuels.
• Important in fields like chemistry, engineering, and environmental science for designing processes and evaluating energy use.