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Enthalpy of Formation

Enthalpy of formation (ΔH_f°) is the change in enthalpy when 1 mole of a compound is formed from its elements in their standard states.

Key Concepts​

  • Standard State: Most stable form of an element at 1 atm and 25°C
  • Exothermic Formation: Releases energy (ΔH_f° < 0)
  • Endothermic Formation: Absorbs energy (ΔH_f° > 0)

Using Enthalpies of Formation​

  • Calculate Reaction Enthalpy (ΔH_rxn) using Hess’s Law:
    ΔH_rxn = Σ ΔH_f°(products) − Σ ΔH_f°(reactants)

Example​

For H₂ + ½ O₂ → H₂O

  • ΔH_f°(H₂O) = −286 kJ/mol
  • ΔH_rxn = ΔH_f°(H₂O) − [ΔH_f°(H₂) + ½ ΔH_f°(O₂)] = −286 − 0 = −286 kJ/mol

Importance​

  • Predicts energy changes in chemical reactions
  • Essential for thermodynamic calculations
  • Helps in designing energy-efficient chemical processes

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