- Lattice enthalpy represents the enthalpy change when the ions in one mole of a solid ionic compound are broken apart (lattice enthalpy of dissociation) or brought together (lattice enthalpy of formation).
- The lattice enthalpy of a compound is a indication of the strength of the ionic bonding the greater the magnitude of the lattice enthalpy, the stronger the bonding.
- Generally speaking, compounds with smaller ions and/ or ions with higher charges have stronger attractions and so greater lattice enthalpy.
- eg. NaCl has a higher lattice enthalpy (stronger ionic bonding) than KCl as the Na+ ion is smaller than the K+ ion
- using Born-Haber cycle, this value is often called the 'experimental value' as the data used in the Born-Haber cycle is determined by experiments.
- A Born-Haber Cycle is a cycle that included all the enthalpy changes in the formation of an ionic compound.
- Draw a separate step for every enthalpy change (eg. for atomisaton of the metal atoms separately from the non-metal atoms, for each individual ionisation enthalpy, for each individual electron affinity)
- second and third electron affinities are endothermic and show be drawn going up 👆not down
- In AS/A-level Chemistry exam, it is best to write the numerical values of the enthalpy changes on each step to prevent mistakes! 👩🏫
- The arrows pointing upwards represent endothermic changes👆🏻, those pointing downwards show exothermic changes👇🏻.
- From the Born-Haber Cycle, the enthalpy change associated with the route depicted by the blue arrow is equal to the enthalpy change of the route shown by the red arrow.
Example: Find the lattice enthalpy formation of NaCl.
Na+(g) + Cl-(g) ➔ NaCl(s)
Input the appropriate values to the corresponding enthalpy change. The Born-Haber cycle for NaCl is shown below:
So, from the cycle:
This is the end of the topic!
Drafted by Cherry (Chemistry)