Compare Melting Point of Ionic Compounds

In this video lesson let us compare the melting point of ionic compounds.

The strength of ionic bonds in ionic compounds is related to lattice energy via the following expression:

230417 lattice energy

where
q+ is the charge of cation
q- is the charge of anion
r+ is the radius of cation
r- is the radius of anion

Notice the denominator term (r+ + r-) is simply the distance between the cation and anion.

So in general, the greater the charges of the ions and the shorter the distance between the ions, the magnitude of lattice energy will be greater.

This means more energy is required to overcome the stronger ionic bonds and that ionic compound will have a higher melting point.

In most questions, we only need to consider one of the 4 terms in the lattice energy expression.

Let's compare the melting points between Na2O and MgO as an illustration.

The difference will be in the cations Na+ and Mg2+.

Mg2+ has a higher charge and smaller radius than Na+, but the 2x increase in charge is more significant than the small difference in radius so we can just focus on the q+ term.

The difference in radius is small because both Na+ and Mg2+ are isoelectronic with 10 electrons, so they will both have 2 filled principal quantum shells with roughly the same size.

230417 Na2O vs MgO

The lattice energy of MgO will be greater, ionic bonds in MgO are stronger and hence MgO will have a higher melting point than Na2O.

Topic: Chemical Bonding, Physical Chemistry, A Level Chemistry, Singapore

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