Chapter 7, Problem 74A

Interpretation Introduction

Interpretation:

The compounds from MgO, KI, AgCl which has highest melting points needs to be determined.

Concept introduction:

Molecules are bound together by different types of bonds namely ionic and covalent. Ionic bonds are formed when there is complete transfer of electrons from one atom to another. These atoms either lose or gain electrons to become negatively or positively charged ions. The forces of attraction between these ions causes the ionic bond formation and are strong bonds.

Answer to Problem 74A

The compound MgO has highest lattice energy and hence is one having highest melting point as more energy is required to break the bonds.

Explanation of Solution

Ionic bonds involve the transfer of electrons from one atom to the other. The metal atoms, an electrons donor, loses electrons to form a positively charged ion while the non-metal atoms, electron acceptor, gains electrons to form negatively charged ions. During the process of donating or gaining electrons, the atoms follow octet rule to attain a stable noble gas configuration.

ConsideringMgO, the force of attraction between the ${\text{Mg}}^{2+}$ and ${\text{O}}^{2-}$ is strong. The lattice energy is dependent on size of the ions and is directly proportional to product of the charges. The ${\text{Mg}}^{2+}$ and ${\text{O}}^{2-}$ will have smaller diameter ion as both are found first in the series and is not shielded by the outermost electronic shells. The nucleus holds the outer electrons closer giving a smaller diameter. Hence, their lattice energy is -3795KJ/mol.

Considering KI, it can be seen that K belongs to Group 1 and Ibelongs to Group 17. The ion ${\text{K}}^{+}$ is the fourth element in group and ${\text{I}}^{-}$ is the fourth element in the group. We know that as we go down the group, the number of shells increases and outermost electrons gets farther away from nuclear force of attraction. Thereby the diameter of these ions are more. Hence, their lattice energy is -632 kJ/mol

Considering AgCl it can be seen that Ag belongs to Group 11 and Cl belongs to Group 17. The ion ${\text{Ag}}^{+}$ is second element in its group and ${\text{Cl}}^{-}$ is second element in its group. As it is second element, the number of shells are not more. Hence, diameter of the ion is not so much. The lattice energy of AgCl compound is -910 kJ/mol.

When comparing all the compounds, the one with highest lattice energy is the one having highest melting point because more energy is required to break the bonds so that the compound will melt. As MgO has highest lattice energy, it is the compound with highest melting point.

Conclusion

The compound MgO has highest lattice energy and hence is one having highest melting point as more energy is required to break the bonds.