Question

In which of the following cases, breaking of covalent bonds takes place?
(A) Boiling of ${{H}_{2}}O$
(B) Melting of KCN
(C) Boiling of $C{{F}_{4}}$
(D) Melting of $Si{{O}_{2}}$

Answer 1

Hint: The covalent bond can be broken by applying a desired amount of energy to overcome the strength of the covalent bond. As this is a stronger bond we need a higher amount of energies to make the processes possible.

Complete Solution :
Here, we would see each process one by one to conclude an answer;
1. Boiling of ${{H}_{2}}O$-
Boiling of water is a simple process in which simpler intermolecular forces act and distinct ${{H}_{2}}O$ molecules are separated. But the covalent bonds in between the ${{H}_{2}}O$ molecules do not break i.e. no intramolecular forces act.

2. Melting of KCN-
In the given molecule i.e. KCN contains both the ionic and covalent bonds. Also, it is a simpler molecule in which intermolecular forces can overcome during melting. But the covalent bonds cannot be broken in this case as well.

3. Boiling of $C{{F}_{4}}$-
Carbon tetrafluoride is a covalent compound as a water molecule, in which carbon forms a single bond with each fluorine atom. So, when we boil this compound, it is impossible to break this covalent bond due to its high stability.

4. Melting of $Si{{O}_{2}}$ -
In silica, silicon is $s{{p}^{3}}$ hybridised and is thus linked to four oxygen atoms forming a three-dimensional structure. This giant molecule is highly stable and hence, requires a high amount of energy to break this crystal. But at a higher melting point, this can be made possible.
Therefore, breaking of covalent bonds can take place while we melt $Si{{O}_{2}}$.
So, the correct answer is “Option D”.

Note: The overall analysis can be done easily when we observe the structures of given molecules. More the complexity, more possibility to break the bonds.