Factors Affecting Lattice Enthalpy:
Add up ionic radius/ charge if given a range of elements and compounds.
When ions in two different groups, need to talk about both ionic charge and ionic size e.g. Mg2+
and Na+.
Don’t write Na, write Na+. Don’t forget the charges.
Effect of Ionic Size: Ionic size increases down a group.
– MN+ is has a smaller ionic radius than Y+.
– Stronger attraction between M+ and N- ions (of compound MN).
– More energy needed to overcome attraction between ions and break ionic bonds.
– Lattice energy more exothermic (negative).
– Melting point increases.
Effect of Ionic Charge:
– M+ has greater charge than Y+. (Group charge).
– So M+ smaller.
– Stronger attraction between M+ and N- than…
– More energy needed to overcome attraction between ions and break ionic bonds.
– Lattice energy more exothermic (negative).
– Melting point increases.
Ionic charge outweighs the fact that Na in Na2O has more moles than Ca in CaO.
Across Period 3: For orange, both ionic radius and charge support an increase in melting point.
For green, ionic radius and charge oppose each other so hard to predict melting point.