Born-Haber Cycle Calculations (A2 Only)
Chemistry: AQA A Level, topic 2.1.2
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.
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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
- 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
- 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
- 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
- You can draw a cycle involving only the enthalpies of hydration, solution, and lattice formation.
Calculations
- Image: Born-Haber cycle and pictorial calculation.
Test yourself on Born-Haber Cycle Calculations (A2 Only)
Which of these often suggests a large degree of covalent bonding?
Which of the following best described the enthalpy change of hydration?