Question

Which of the following substances have the highest melting point?
A. $NaCl$
B. $KCl$
C. $MgO$
D. $BaO$

Answer 1

Hint : The amount of energy required to dissociate one mole of an ionic compound into its respective gaseous state ions is known as lattice energy. In simple words, it is a measure of binding forces which exist between oppositely charged ions. It has many practical applications like comparing solubility, hardness and volatility.

Complete Step By Step Answer:
In solid molecules, smaller the size of cation and anion, larger is the lattice enthalpy of molecules. That means the ions are bonded with strong electrostatic forces in a crystal lattice and require a high amount of energy or heat to dissociate into constituent ions. Therefore, we can conclude that the higher the lattice enthalpy of a molecule, the greater will be its melting point.
Now, the dissociation of given ionic compounds takes place as follows:
Dissociation of $NaCl$ :
$NaCl \rightleftharpoons N{a^ + } + C{l^ - }$
Dissociation of $KCl$ :
$KCl \rightleftharpoons {K^ + } + C{l^ - }$
Dissociation of $MgO$ :
$MgO \rightleftharpoons M{g^{2 + }} + {O^{2 - }}$
Dissociation of $BaO$ :
$BaO \rightleftharpoons B{a^{2 + }} + {O^{2 - }}$
The ions formed after dissociation of given ionic compounds are $N{a^ + }$ , ${K^ + }$ , $M{g^{2 + }}$ and $B{a^{2 + }}$ respectively, in which $M{g^{2 + }}$ ion is the smallest in size and therefore will have the highest lattice energy and highest melting point.
Hence, the substance which has the highest melting point is magnesium oxide i.e., $MgO$ .

Note :
It is important to note that the melting point of the ionic solids also depends on the ratio of ions in which the solid gets dissociated. The smaller ratio of ions indicates that the greater number of cations or anions will be surrounded by the central ion dividing the electrostatic force of attraction between oppositely charged ions. Thus, a decrease in melting point of the solid is observed. For example: Although $M{g^{2 + }}$ ion is smaller than $N{a^ + }$ ion but the melting point of $NaCl$ is greater than $MgC{l_2}$ due to the larger ratio of ions.