Question

Based on lattice energy and other considerations, which one of the following alkali metal chlorides has the highest melting point?
A.) $KCl$
B.) $RbCl$
C.) $LiCl$
D.) $NaCl$

Answer 1

We can solve this question by considering that as we go down the group, the melting point decreases. This is because as we go down the group, the ionic radii increases and hence their melting point decreases.

Complete step by step answer:
As given in question, by lattice energy we mean the energy that is required to separate a mole of an ionic solid into gaseous ions. Also lattice energy depends on the size of ions. As the size of anion (negative ion) or cation (positive ion) increases then we can say that the distance between the nuclei of ions decreases. This will result in less attraction between ions and finally the lattice energy will be less. Thus, lattice energy decreases with increases in size of ions.
Also melting point depends on lattice energy. As the lattice energy increases that is more energy is required to separate an ionic solid into gaseous solid. Thus, it becomes difficult to melt it.
Hence, melting point is directly proportional to lattice energy and in turn lattice energy is inversely proportional to the ionic size.
As we can see in the question that in all given compounds the anionic part is chloride that is the same in all of them. So, we can see the cationic parts that are elements of the alkali group. As we go down the group, we know the ionic size will increase so the lattice energy will decrease down the group.
Hence, according to it melting point should decreases down the group as:
$LiCl > NaCl > KCl > RbCl$
But in practical terms, the $LiCl$ has a lower melting point than $NaCl$. This is because of the covalent nature of $LiCl$. $NaCl$ has strong Coulombic attraction between ions of opposite charges because it is ionic in nature.
Thus, the correct decreasing order of melting point will be:
$NaCl > LiCl > KCl > RbCl$
Hence, we can conclude that $NaCl$ has the highest melting point among them.
Hence option (D) is correct.

Note:
In this question we may think that $LiCl$ has the highest melting point just by considering the concept of lattice energy. But we should always consider other factors to check the answer as ionic bonds are more stronger than covalent bonds due to electrostatic attraction.