1.5.6

Born-Haber Cycle Calculations

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Calculations from Born-Haber Cycles

A Born-Haber cycle can be used to calculate any of the enthalpy changes involved in it, as long as you measure the rest.

Lattice enthalpies

Lattice enthalpies

  • To calculate a lattice enthalpy from the Born-Haber cycle, you treat it like a large Hess cycle.
    • One route to the products is the enthalpy of formation.
    • The other route is the sum of every other enthalpy change.
    • You can rearrange this to find the lattice enthalpy.
Theoretical differences

Theoretical differences

  • The calculated lattice enthalpies are rarely correct.
    • They’re close, but not exact.
  • This is because the ionic model assumes ions are perfect spheres and have even charge distribution.
    • This is not the case - ions usually have some degree of covalent bonding.
Polarisation

Polarisation

  • Positive ions can be said to polarise negative ions, if they cause distortion of the charge cloud. They can pull electrons to one side of the ion.
    • There is, in fact, a spectrum between ionic and covalent bonding.
    • If the lattice enthalpy you calculate is very different from an experimental value, that’s a hint that there is a relatively large degree of covalent bonding.

Advanced Uses of Born-Haber Cycles

You can use a Born-Haber cycle to calculate enthalpies of hydration or solution.

Enthalpy of hydration

Enthalpy of hydration

  • The enthalpy change of hydration is the enthalpy change when one mole of gaseous ions dissolves to form one mole of aqueous ions.
  • The enthalpy of solution is the enthalpy change when one mole of a solid crystal is dissolved in a large excess of solvent.
Born-Haber cycles

Born-Haber cycles

  • You can draw a cycle involving only the enthalpies of hydration, solution, and lattice formation.
Calculations

Calculations

  • Image: Born-Haber cycle and pictorial calculation.
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