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Bond Enthalpies

Bond enthalpy (or bond dissociation energy) is the energy required to break one mole of a particular bond in a molecule in the gas phase.

Key Concepts​

  • Endothermic Process: Breaking bonds requires energy
  • Exothermic Process: Forming bonds releases energy
  • Average Bond Enthalpy: Average energy needed to break a specific bond type in different molecules

Calculating Reaction Enthalpy Using Bond Enthalpies​

ΔH_rxn ≈ Σ(Bonds Broken) − Σ(Bonds Formed)

Example​

For the reaction: H₂ + Cl₂ → 2 HCl

  • Bonds broken: H–H, Cl–Cl
  • Bonds formed: 2 H–Cl
  • ΔH_rxn ≈ (D_H–H + D_Cl–Cl) − 2 D_H–Cl

Importance​

  • Predicts reaction energetics
  • Useful for estimating exothermic or endothermic nature of reactions
  • Provides insight into chemical stability and bond strengths

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