Problem 41 - Chemistry The Molecular Science

Question

Which of these substances has the highest melting point? The lowest melting point? Explain your choices briefly. (a) \(\mathrm{SiC}\) (b) I (c) \(\mathrm{Rb}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\)

Step-by-Step Solution

Answer

Highest: \( \mathrm{SiC} \), Lowest: Butane \( (\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{CH}_{3}) \).

Step 1: Identify the Type of Bonds

Analyze the chemical bonds present in each compound to understand the forces governing their melting points. \( \mathrm{SiC} \) is a covalent network solid with strong bonds, elemental iodine \((\mathrm{I}_2)\) is a molecular solid, \( \mathrm{Rb} \) is a metallic solid, and \( \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{CH}_{3} \) (butane) is a molecular solid with weak van der Waals forces.

Step 2: Compare the Strengths of Intermolecular and Intramolecular Forces

Assess the intermolecular and intramolecular forces. \( \mathrm{SiC} \) has strong covalent bonds throughout the solid, \( \mathrm{Rb} \) has metallic bonds, \( \mathrm{I}_2 \) has van der Waals forces between molecules, and butane also mainly has van der Waals forces.

Step 3: Rank Based on Melting Points

Generally, covalent network solids like \( \mathrm{SiC} \) have the highest melting points due to strong covalent bonds. Metallic bonds in \( \mathrm{Rb} \) usually result in moderate melting points. Molecular solids like \( \mathrm{I}_2 \) and butane, which rely on van der Waals forces, tend to have lower melting points, with butane being lower due to its lighter molecular structure.

Key Concepts

Covalent Network Solids

Covalent network solids are materials where atoms are bonded covalently in a continuous network. This means there are no discrete molecules, and each atom is covalently bonded to several others in a hard, rigid structure. Properties of covalent network solids include:

• Very high melting points due to the strong covalent bonds, which require a lot of energy to break.
• Generally, poor conductors of electricity, as there are no free electrons or ions to carry charge.
• Typically hard and durable, making them resistant to scratching and wear.

An example is silicon carbide (\(\mathrm{SiC}\)), which illustrates these properties very well. It has a high melting point and is exceptionally hard, making it useful as an abrasive and in high-temperature applications.

Intermolecular Forces

Intermolecular forces are forces of attraction or repulsion which act between neighboring particles (atoms, molecules, or ions). These forces are generally weaker than the intramolecular forces holding a molecule together.Types of intermolecular forces include:

• Van der Waals forces: Weak attractions caused by temporary dipoles. They include London dispersion forces and dipole-dipole interactions.
• Hydrogen bonding: A stronger type of dipole-dipole attraction occurring when hydrogen is bonded to a highly electronegative element like nitrogen, oxygen, or fluorine.

Understanding intermolecular forces helps explain why molecular solids like iodine (\(\mathrm{I}_2\)) and butane (\(\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_3\)) have lower melting points. The weak van der Waals forces in these substances require far less energy to overcome compared to strong covalent or metallic bonds.

Molecular Solids

Molecular solids are composed of molecules held together by relatively weak forces compared to ionic or covalent bonds. These solids often have low melting points because the forces that keep the molecules together, like van der Waals interactions, are easily overcome with heat.Key characteristics of molecular solids include:

• Low-to-moderate melting points due to the weak intermolecular forces.
• Typically soft and easily deformed as the individual molecules can shift relative to each other.
• Poor electrical conductors because they lack free-moving charged particles.

In the examples provided, both iodine (\(\mathrm{I}_2\)) and butane are considered molecular solids. Butane, being a smaller, lighter molecule, has weaker van der Waals forces, leading to an even lower melting point compared to iodine.

Metallic Bonds

Metallic bonds are found in metals, where atoms are in a sea of shared electrons. This type of bonding gives metals their characteristic properties.Features of metallic bonds include:

• Good electrical and thermal conductivity due to free electrons that move easily throughout the metal.
• Malleability and ductility, as layers of atoms can slide over each other without breaking the bond.
• Varied melting points depending on the strength of metallic bonding and the metal involved.

Rubidium (\(\mathrm{Rb}\)) is an example of a metallic solid with a moderate melting point. While it does not have the highest melting point like covalent network solids, its metallic bonds are stronger than the van der Waals forces found in molecular solids, giving it a higher melting point compared to substances like iodine or butane.