Covalent character of ionic bonds
We know, ionic bonds have some covalent character. When talking about lattice enthalpy, the covalent character increases lattice enthalpy. However, when talking about melting points, an increase in covalent character decreases the meltinf point. How and why does this happen? How does it increase lattice enthalpy but decrease melting point? Isn't this contradictory?
Lattice enthalpy is defined as the energy (at constant pressure) required separate the ions in a crystal lattice into individual gaseous ions. This involves breaking all interatomic bonds (either covalent or ionic), and so generally increases as covalent character increases, since covalent bonds are usually stronger and more directional than ionic bonds.
By contrast, melting involves dissociating the solid (crystal) into individual atomic or molecular units. As covalent character of bonding increases, this is more likely to result in a molecular liquid held together by weak intermolecular bonds. A purely ionic crystal would be more resistant to melting as the bonding is less directional and stronger than the equivalent intermolecular bonding for a mixed covalent/ionic crystal. So, the melting point tends to decrease with increasing covalent character.
By contrast, melting involves dissociating the solid (crystal) into individual atomic or molecular units. As covalent character of bonding increases, this is more likely to result in a molecular liquid held together by weak intermolecular bonds. A purely ionic crystal would be more resistant to melting as the bonding is less directional and stronger than the equivalent intermolecular bonding for a mixed covalent/ionic crystal. So, the melting point tends to decrease with increasing covalent character.
(edited 1 year ago)
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