Problem 63 - 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

(a) False. The lower melting points of molecular solids compared to covalent-network solids are due to the weaker intermolecular forces in molecular solids, not weaker covalent bonds. (b) True. Highly symmetric molecules generally form solids with higher melting points due to their more efficient packing in the crystalline lattice, leading to stronger molecular interactions.

Step 1: Statement (a) Analysis

To classify statement (a) as true or false, we need to understand the difference between molecular solids and covalent-network solids, as well as the effect of bond strength on melting points. Molecular solids are formed by weak intermolecular forces (such as dipole-dipole, London dispersion, and hydrogen bonding), while covalent-network solids have a continuous network of covalent bonds between their atoms. The key point is that molecular solids have weaker intermolecular forces, whereas covalent-network solids have strong covalent bonds. Let's analyze the given statement. (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.

Step 2: Statement (a) Classification

The given statement (a) is false. The melting points of molecular solids are much lower than those of covalent-network solids, but this is not because their covalent bonds are weaker. The difference in melting points is due to the weak intermolecular forces in molecular solids compared to the strong covalent bonds in covalent-network solids.

Step 3: Statement (b) Analysis

To classify statement (b) as true or false, we need to understand the effect of molecular symmetry on the melting points of solids. Highly symmetric molecules can pack more efficiently in a crystalline lattice, leading to stronger interactions between the molecules and higher melting points. Asymmetric molecules generally cannot pack as efficiently, resulting in weaker interactions and lower melting points. Let's analyze the given statement. (b) Other factors being equal, highly symmetric molecules tend to form solids with higher melting points than asymmetrically shaped molecules.

Step 4: Statement (b) Classification

The given statement (b) is true. When other factors are equal, highly symmetric molecules do tend to form solids with higher melting points because they can pack more efficiently in the crystalline lattice, leading to stronger molecular interactions.