Problem 61 - Chemistry The Central Science

Question

Classify each of the following statements as true or false: (a) Although both molecular solids and covalent-network solids have covalent bonds, the melting points of molecular solids are much lower because their covalent bonds are much weaker. (b) Other factors being equal, highly symmetric molecules tend to form solids with higher melting points than asymmetrically shaped molecules.

Step-by-Step Solution

Answer

Statement (a) is false - The difference in melting points between molecular solids and covalent-network solids is due to the strength of the interactions holding the solids together, not a difference in bond strength. Statement (b) is true - Highly symmetric molecules tend to have stronger intermolecular interactions and higher melting points due to their efficient packing in the solid state, compared to asymmetric molecules.

Step 1: Statement (a)

Molecular solids are held together by weak intermolecular forces, such as dipole-dipole interactions, dispersion forces, and hydrogen bonds. In covalent network solids, atoms are bonded by strong covalent bonds that form a large network structure throughout the entire solid. The melting points of molecular solids are lower because the weak intermolecular forces that hold them together can be overcome more easily compared to the strong covalent bonds in covalent-network solids. However, this statement asserts that the covalent bonds in molecular solids are weaker than the covalent bonds in covalent-network solids, which is incorrect. The difference in melting points is due to the strength of the interactions holding the solids together, not a difference in bond strength. Hence, this statement is false.

Step 2: Statement (b)

The symmetry of a molecule plays a role in determining its melting point. Highly symmetric molecules tend to have more efficient packing in the solid state, which results in stronger intermolecular interactions and higher melting points. Asymmetric molecules, on the other hand, don't pack as efficiently, resulting in weaker intermolecular interactions and lower melting points. So, the given statement is true. Other factors, such as molecular weight, can also be influential in determining the melting points of substances, but when comparing molecules with similar structures and molecular weights, symmetry will play a significant role.