Question

At room temperature, Si is a solid, \(\mathrm{CCl}_{4}\) is a liquid, and Ar is gas. List these substances in order of (a) increasing intermolecular energy of attraction and (b) increasing boiling point.

Short answer

a) Increasing intermolecular energy of attraction: Ar < CCl4 < Si b) Increasing boiling point: Ar < CCl4 < Si

Step-by-step solution

Identify the intermolecular forces

For Si: Since it's a solid and a covalent network, it has covalent bonding. For CCl4: It is a nonpolar molecule due to the equal distribution of charge, and thus has London dispersion forces (LDF). For Ar: It is an inert noble gas, and also has London dispersion forces. Step 2: Compare the strengths of intermolecular forces

Compare the intermolecular forces

Covalent bonding in Si is stronger than LDFs. Therefore, Si will have a greater energy of attraction and a higher boiling point than both CCl4 and Ar. Between CCl4 and Ar, CCl4 has stronger LDFs than Ar because it has more electrons which make the LDFs stronger. So, CCl4 has a greater energy of attraction and a higher boiling point than Ar. Step 3: List the substances in order of increasing intermolecular energy of attraction

Arrange the substances based on intermolecular energy of attraction

From our comparison in step 2, we can list the substances in the following order: Ar < CCl4 < Si Step 4: List the substances in order of increasing boiling point

Arrange the substances based on boiling point

Since boiling point is directly related to the strength of intermolecular forces, we can arrange the substances in the same order as in step 3: Ar < CCl4 < Si Final Answer: a) Increasing intermolecular energy of attraction: Ar < CCl4 < Si b) Increasing boiling point: Ar < CCl4 < Si

Key concepts

Covalent Bonding

Covalent bonding is a type of chemical bond where atoms share pairs of electrons. These bonds occur when the electronegativity difference between two atoms is not substantial. Instead of donating or accepting electrons, the atoms involved hold onto a shared electron pair, ensuring they complete their outer electron shell.
Covalent bonding can occur between atoms of the same element or between different elements, often leading to the formation of molecules. In its network form, as seen in silicon (Si), covalent bonding results in a structure where each atom is bonded to several others, creating a strong, rigid, three-dimensional matrix.
Here are a few key points about covalent bonds:

• Strong and directional: The strength and directionality of covalent bonds result in high melting and boiling points for covalent network solids like silicon.
• Stable energy states: Atoms in covalent bonds reach a more stable energy state due to shared electron pairs.
• Formation of complex structures: Covalent bonding enables the formation of complex and robust molecular structures found in organic compounds and certain crystalline materials.

Understanding covalent bonding is essential for explaining why substances like silicon exhibit high boiling points, unlike those experiencing weaker forces such as London dispersion forces.

London Dispersion Forces

London dispersion forces (LDF) are a type of weak intermolecular force that occur due to temporary fluctuations in electron distribution around atoms or molecules. You can think of them as instant, fleeting attractions that arise because of movement of electrons, which creates a temporary dipole that induces another dipole in an adjacent atom or molecule.
These forces exist between all atoms and molecules, regardless of polarity, but they are the only type of intermolecular force acting between nonpolar entities such as Ar and \(\mathrm{CCl}_{4}\). As the size of the atom or the number of electrons increases, so does the strength of the London dispersion forces.
Key points to consider about LDF:

• Present in all molecules: These forces affect both polar and nonpolar substances, though they are the sole forces in nonpolar.
• Increase with molecular size: As the electron cloud becomes more easily polarized, the dispersion forces become stronger.
• Weaker compared to covalent or ionic bonds: They are significant only in large or heavy atoms and molecules, which explains why small gas molecules, like Ar, can exist as gases at lower temperatures.

This understanding helps explain why \(\mathrm{CCl}_{4}\), with more electrons, exhibits higher boiling points than Ar, despite both utilizing London dispersion forces.

Boiling Point

A substance's boiling point is the temperature at which it changes from liquid to gas. It is the point where the vapor pressure of the liquid matches the atmospheric pressure surrounding it. The boiling point directly correlates with the strength of a substance's intermolecular forces; stronger forces result in a higher boiling point.
Silicon's high boiling point relative to \(\mathrm{CCl}_{4}\) and Ar is attributable to its covalent bonding structure. In contrast, \(\mathrm{CCl}_{4}\) and Ar have lower boiling points due to the weaker London dispersion forces.
Essential aspects to remember about boiling points include:

• Indicator of intermolecular force strength: A higher boiling point reflects stronger intermolecular attractions within the substance.
• Environmental dependency: Boiling points can vary based on changes in ambient pressure and conditions.
• Comparison tool: When comparing substances, boiling point gives a quick snapshot of potential intermolecular forces at play.

These points help in understanding why the boiling points, and thereby the order of Ar < \(\mathrm{CCl}_{4}\) < Si, are reflective of the differing interatomic and intermolecular forces in action.