Lattice Enthalpy and Born-Haber Cycles
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Define lattice enthalpy of formation
Lattice enthalpy of formation is the enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions under standard conditions
Define lattice enthalpy of dissociation
Lattice enthalpy of dissociation is the enthalpy change when 1 mole of a solid ionic compound is completely dissociated into its gaseous ions under standard conditions
What can you use to calculate lattice enthalpies?
Born-Haber cycles
Name the steps in a Born-Haber cycle
• Start with enthalpy of formation • Then put the enthalpies of atomisation and ionisation energy above that • Next put electron affinity • Finally put lattice enthalpy of formation
Why are Born-Haber cycles involving Group 2 elements different?
Group 2 elements form 2+ ions so you have to include the second ionisation energy
How do you work out theoretical lattice enthalpy?
You can work out theoretical lattice enthalpy by doing some calculations based on the purely ionic model of a lattice. The purely ionic model of a lattice assumes that all the ions are spherical, and have their charge evenly distributed around them
Why when you work out lattice enthalpy experimentally does it differ from theoretical lattice enthalpy?
The value that you get differs because most ionic compounds have some covalent character. The positive and negative ions aren’t exactly spherical. Positive ions polarise neighbouring negative ions to different extents, and the more polarisation there is, the more covalent the bonding will be