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Bond Enthalpies: Flashcards

Revise bond enthalpy, bond order and length, and estimating ΔH from bonds broken and formed.

10 cardsIntermediate

10 cards

  1. Question
    What is bond enthalpy?
    Answer

    The energy needed to break one mole of a bond in gaseous molecules, forming gaseous atoms.

  2. Question
    Is breaking a bond endothermic or exothermic? And forming one?
    Answer

    Breaking: endothermic (energy in). Forming: exothermic (energy out).

  3. Question
    Give the formula for estimating ΔH from bond enthalpies.
    Answer

    ΔH ≈ Σ(bonds broken) − Σ(bonds formed)

  4. Question
    How do bond length and strength change with bond order?
    Answer

    Higher order: shorter and stronger. C–C 154 pm, 348 kJ/mol; C=C 134 pm, 614 kJ/mol; C≡C 120 pm, 839 kJ/mol.

  5. Question
    Why is a C=C bond less than twice as strong as C–C?
    Answer

    The second bond is a π bond, which is weaker than a σ bond.

  6. Question
    Why are bond-enthalpy calculations only estimates?
    Answer

    Most values are averages over many compounds, and they apply only to gases.

  7. Question
    Estimate ΔH for H₂ + Cl₂ → 2HCl (H–H 436, Cl–Cl 242, H–Cl 431 kJ/mol).
    Answer

    (436 + 242) − 2 × 431 = −184 kJ/mol

  8. Question
    Why is nitrogen gas so unreactive?
    Answer

    The N≡N triple bond is very strong (945 kJ/mol).

  9. Question
    When is a reaction exothermic, in terms of bonds?
    Answer

    When the bonds formed are stronger in total than the bonds broken.

  10. Question
    How many O–H bonds are formed in CH₄ + 2O₂ → CO₂ + 2H₂O?
    Answer

    Four (two in each water molecule).