Problem 64 - Chemistry: The Central Science

Question

Classify each of the following statements as true or false: (a) For molecular solids, the melting point generally increases as the strengths of the covalent bonds increase. (b) For molecular solids, the melting point generally increases as the strengths of the intermolecular forces increase.

Step-by-Step Solution

Answer

(a) False - Covalent bond strength does not directly determine the melting point of molecular solids. (b) True - Stronger intermolecular forces generally result in higher melting points for molecular solids.

Step 1: Statement (a) Analysis:

To determine the truthfulness of statement (a), let's consider the definition of molecular solids and the effect of covalent bonds on their properties. Molecular solids consist of molecules held together by weak intermolecular forces such as van der Waals forces, dipole-dipole forces, and hydrogen bonds. Covalent bonds are strong intramolecular forces that hold atoms within each molecule. Considering this, we can conclude that the strength of covalent bonds would predominantly affect the individual molecules' stability and their properties, but not directly determine the melting point of the molecular solid. Instead, the melting point will mostly depend on the strength of intermolecular forces as they are the ones holding the molecules in the solid lattice at low temperatures.

Step 2: Statement (b) Analysis:

For statement (b), let's consider the relationship between the intermolecular forces in molecular solids and their melting points. As mentioned earlier, molecular solids are held together by weak intermolecular forces like van der Waals forces, dipole-dipole forces, and hydrogen bonds. These forces are responsible for keeping the molecules together, forming a molecular solid at low temperatures. When the temperature increases, the thermal energy that the molecules gain competes with these intermolecular forces. If these forces are strong enough to resist the thermal energy, the solid will maintain its lattice structure and remain in a solid state. However, if the intermolecular forces are weak, then the thermal energy will eventually overpower these forces, causing the molecules to break free from the lattice structure and transform into a liquid state, which in turn corresponds to the melting point. Thus, the melting point of a molecular solid is generally expected to increase with the strength of intermolecular forces, as it requires more thermal energy to break these stronger forces and convert the solid into a liquid state.

Step 3: Conclusion:

Based on the analysis of both statements: (a) For molecular solids, the melting point generally increases as the strengths of the covalent bonds increase. - This statement is FALSE, as the melting point depends on the intermolecular forces and not the covalent bond strength. (b) For molecular solids, the melting point generally increases as the strengths of the intermolecular forces increase. - This statement is TRUE, as stronger intermolecular forces generally result in higher melting points.