Answer to Question #153895 in Inorganic Chemistry for judth

Question #153895

1) Lithium chloride, LiCl.

Theoretical lattice energy in kJmol: -845

Experimental lattice energy in kJmol: -848

2) Magnesium iodide, MgI2.

Theoretical lattice energy in kJmol: -1944

Experimental lattice energy in kJmol: -2327

Comment on the theoretical and experimental lattice energy values, giving reasons for any differences and similarities.

Expert's answer

First of all, it is important to note that lattice energy (enthalpy) is closely related to bond strength. Thus the greater the bond strength the higher the lattice enthalpy. Therefore, LiCl has lower lattice enthalpy (less exothermic) because of low bond strength, as compared to MgI2 which has higher lattice enthalpy (more exothermic), because of the greater bond strength.

When we compare between theoretical lattice energy and experimental values, it is also important to not e that; theoretical lattice enthalpy values are based on the ionic model (electrostatic theory), which assumes that the only bonding between oppositely charged ions is ionic bonds. On the other hand, experimental lattice energy values are based on the Born-Haber Cycle, which suggests additional covalent character, that adds additional strength to the ionic bonds.

Therefore for both ions (LiCl and MgI2), although the values are quite similar, normally the experimental value is greater (more exothermic) than the theoretical value, because of the additional covalent character discussed above.

Lastly, we notice that MgI2 has a greater difference between experimental value and theoretical value. This is because MgI2 has got more covalent character than LiCl. The explanation for that is based on the difference in electronegativity between the ions forming the substances. The closer the electronegativity of the ions, the more the covalent character the bonds will have which provides additional strength to the bonding. As the difference in the electronegativity decreases the difference between the theoretical and experimental lattice enthalpy decreases.

It is also important to note that small positive ions with big charges (high charge density) are more polarising. Also, big negative ions with small charges (low charge density) are polarisable. Therefore, MgI2 consist of a smaller postive ion with high charge density and a bigger negative ion with low charge density, hence has more covalent character than LiCl with a small positive ion but with low charge density and a smaller negative ion with low charge density, hence a lower covalent character.

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