AS/A-level Chemistry - Born-Haber Cycle

Lattice Enthalpy

• 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 types of question will definitely appear in AS/A-level Chemistry exam, it may look difficult but practice makes perfect! You can do it!🙌

This is the end of the topic!

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