Lattice Enthalpy and Born-Haber Cycles

Question 1

Define lattice enthalpy of formation

Answer 1

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

Question 2

Define lattice enthalpy of dissociation

Answer 2

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

Question 3

What can you use to calculate lattice enthalpies?

Answer 3

Born-Haber cycles

Question 4

Name the steps in a Born-Haber cycle

Answer 4

1. Start with enthalpy of formation
2. Then put the enthalpies of atomisation and ionisation energy above that
3. Next put electron affinity
4. Finally put lattice enthalpy of formation

Question 5

Why are Born-Haber cycles involving Group 2 elements different?

Answer 5

Group 2 elements form 2+ ions so you have to include the second ionisation energy

Question 6

How do you work out theoretical lattice enthalpy?

Answer 6

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

Question 7

Why when you work out lattice enthalpy experimentally does it differ from theoretical lattice enthalpy?

Answer 7

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