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Chemistry Clarity · chemistryclarity.com

Lattice Enthalpy and Born–Haber Cycles: Flashcards

Revise lattice enthalpy, the steps of a Born–Haber cycle and what makes an ionic lattice strong.

11 cardsIntermediate

11 cards

  1. Question
    Define lattice enthalpy (formation convention).
    Answer

    The enthalpy change when 1 mol of an ionic solid forms from its gaseous ions under standard conditions. It is always exothermic (negative).

  2. Question
    Why can't lattice enthalpy be measured directly?
    Answer

    You cannot collect a mole of separate gaseous ions and combine them. It is calculated with a Born–Haber cycle (Hess's law).

  3. Question
    What is the enthalpy of atomization of chlorine?
    Answer

    The enthalpy change to form 1 mol of Cl(g) atoms from Cl₂(g): half the Cl–Cl bond enthalpy, +121.3 kJ/mol.

  4. Question
    Is a first electron affinity usually exothermic or endothermic?
    Answer

    Exothermic (negative): the nucleus attracts the incoming electron. For Cl, EA₁ = −348.6 kJ/mol.

  5. Question
    Why is the second electron affinity of oxygen endothermic?
    Answer

    The electron is being added to an ion that is already negative (O⁻), so it is repelled; energy must be put in.

  6. Question
    Write the Born–Haber equation for lattice enthalpy.
    Answer

    ΔH(latt) = ΔHf − (sublimation of the metal + atomization of the non-metal + ionization energies + electron affinities).

  7. Question
    Which ionization energies are needed to form Mg²⁺(g)?
    Answer

    Both the first and the second: IE₁ + IE₂ = 737.7 kJ/mol + 1450.7 kJ/mol.

  8. Question
    Name two factors that make a lattice enthalpy more negative.
    Answer

    Higher charges on the ions, and smaller ions (which can get closer together).

  9. Question
    Put NaF, NaCl, NaBr and NaI in order of decreasingly negative lattice enthalpy.
    Answer

    NaF (−923) > NaCl (−787) > NaBr (−747) > NaI (−704) kJ/mol: the halide ion gets larger down the group.

  10. Question
    What does it mean if a Born–Haber lattice enthalpy is more negative than the ionic-model value?
    Answer

    The bonding has some covalent character (for example in AgI), so it is stronger than pure ionic attraction predicts.

  11. Question
    What is the enthalpy of sublimation of sodium?
    Answer

    The enthalpy change for Na(s) → Na(g), +107.5 kJ/mol: the energy to break up the metallic lattice into free gaseous atoms.