Problem 67 - Chemistry: The Central Science in SI Units, Global Edition

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 because the lower melting points of molecular solids are due to weaker intermolecular forces, not weaker covalent bonds. Statement (b) is true because highly symmetric molecules tend to form solids with higher melting points due to their efficient packing and stronger intermolecular forces.

Step 1: Statement (a) Analysis:

We need to determine if it's true that molecular solids have lower melting points because their covalent bonds are much weaker than those in covalent-network solids. Molecular solids are made of molecules held together with forces such as van der Waals, hydrogen bonding, or dipole-dipole interactions. Covalent-network solids, on the other hand, consist of a giant framework of covalently bonded atoms, forming a continuous network. The covalent bonds themselves are actually of similar strengths in both molecular solids and covalent-network solids. However, the key difference lies in the forces between the molecules, which are weaker in molecular solids. While the covalent bonds are not the cause of the lower melting points of molecular solids, the result is still true – molecular solids melt at lower temperatures than covalent-network solids.

Step 2: Statement (a) Classification:

Since the cause for lower melting points in molecular solids is due to weaker intermolecular forces rather than weaker covalent bonds, statement (a) is false.

Step 3: Statement (b) Analysis:

We need to determine if it's true that, other factors being equal, highly symmetric molecules tend to form solids with higher melting points than asymmetrically shaped molecules. Higher symmetric molecules pack more efficiently into a crystalline lattice, leading to stronger intermolecular forces and therefore higher melting points compared to asymmetric molecules. Less efficient packing results in weaker forces between the molecules, leading to lower melting points when the overall molecular shape is not symmetrical.

Step 4: Statement (b) Classification:

Since highly symmetric molecules can pack more efficiently in a solid, leading to stronger intermolecular forces and higher melting points, statement (b) is true.

Key Concepts

Molecular Solids

Molecular solids are composed of individual molecules held together by intermolecular forces, which include Van der Waals forces, dipole-dipole interactions, and hydrogen bonding. These forces are weaker than covalent bonds, which are prevalent in covalent-network solids.

As a result of these weaker intermolecular forces, molecular solids tend to have lower melting points. The molecules in the solid form are bound by relatively weak attractions, making it easier for them to move apart and melt. This is different from covalent-network solids, where a strong, continuous lattice of covalent bonds holds the solid together, requiring significantly more energy to break apart and resulting in higher melting points.

Molecular solids are common in everyday substances like ice (solid water), dry ice (solid carbon dioxide), and many organic compounds. Each of these substances exemplifies the characteristics of molecular solids, especially their low melting points.

Covalent-Network Solids

Covalent-network solids feature atoms connected in a vast web of covalent bonds that create a continuous and rigid structure. This extended network results in very high melting points due to the strong covalent bonds that must be broken for melting to occur.

In contrast to molecular solids, where discrete molecules are held together by relatively weak forces, each part of a covalent-network solid is bonded directly to its neighbors. This means that to melt a covalent-network solid, you'd have to input enough energy to break many covalent bonds across the structure, not just separate a few molecules.

Examples of covalent-network solids include diamond and quartz (SiO₂). These substances are known for their durability and high melting points. The exceptional strength of the covalent bonds in these structures contributes to their rigidity and requires much more energy (and therefore a higher temperature) to transition from solid to liquid.

Intermolecular Forces

Intermolecular forces are the forces that exist between molecules, as opposed to intramolecular forces, which are the forces holding atoms together within a molecule. These forces significantly affect the physical properties of molecular solids, such as their melting and boiling points.

There are several types of intermolecular forces that play roles of varying importance:

• Van der Waals Forces: These are weak forces that arise from temporary dipoles formed when electron clouds slightly shift. They are present in all molecular solids but are primarily significant in nonpolar molecules.
• Dipole-Dipole Interactions: These forces occur between polar molecules where positive ends of one dipole attract the negative ends of another. They are stronger than Van der Waals forces but weaker than hydrogen bonds.
• Hydrogen Bonds: A type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms like fluorine, oxygen, or nitrogen. These are the strongest intermolecular forces among the three and highly influence the melting points, making substances like water have unusually high melting points for their size.

Intermolecular forces are key in determining the melting points and hardness of molecular solids, with weaker forces leading to lower melting points and softer structures. Recognizing these forces and their effects is vital in predicting the behavior of substances under different conditions.