Problem 12 - Chemistry The Central Science In Si Units

Question

At room temperature, \(\mathrm{CO}_{2}\) is a gas, \(\mathrm{CCl}_{4}\) is a liquid, and \(\mathrm{C}_{60}\) (fullerene) is a solid. List these substances in order of (a) increasing intermolecular energy of attraction and (b) increasing boiling point.

Step-by-Step Solution

Answer

(a) CO2, CCl4, C60 (b) CO2, CCl4, C60

Step 1: Understanding Intermolecular Forces

First, analyze the types of intermolecular forces present in each substance. CO2 is nonpolar and experiences London dispersion forces; CCl4 is also nonpolar with London dispersion forces but has larger molecular size, resulting in stronger London forces; C60, being a large molecule, has even stronger London dispersion forces due to increased surface area.

Step 2: Order by Intermolecular Forces

Based on the strength of the intermolecular forces, we order the substances as follows, from weakest to strongest force: 1. CO2 (gas) 2. CCl4 (liquid) 3. C60 (solid)

Step 3: Understanding Boiling Points

Boiling point increases with the strength of intermolecular forces. A weaker intermolecular attraction corresponds to a lower boiling point, and a stronger intermolecular attraction leads to a higher boiling point.

Step 4: Order by Boiling Point

Based on the strength of intermolecular forces determined in Step 2, order the substances from lowest to highest boiling point: 1. CO2 (gas) 2. CCl4 (liquid) 3. C60 (solid)

Key Concepts

London dispersion forces

London dispersion forces, often known as van der Waals forces, are the weakest type of intermolecular forces that occur between molecules. They arise due to the momentary fluctuations in electron density within a molecule, creating temporary dipoles. These temporary dipoles induce similar dipoles in adjacent molecules, leading to an attraction between them. The strength of London dispersion forces is influenced by several factors:

• Molecular Size: Larger molecules have more electrons, which means a greater chance of creating temporary dipoles, resulting in stronger dispersion forces.
• Surface Area: Molecules with larger surface areas have more space to interact with neighboring molecules, increasing the strength of these forces.

The substances in our exercise,

• CO₂, being the smallest, has the weakest London dispersion forces, which is why it exists as a gas at room temperature.
• CCl₄, being larger, experiences stronger London forces and is found as a liquid.
• C₆₀, with a very large surface area, has the strongest London forces, maintaining its solid state at room temperature.

boiling point

The boiling point of a substance is the temperature at which its vapor pressure equals the atmospheric pressure. At this temperature, the substance transitions from a liquid to a gas. The boiling point is closely tied to the strength of intermolecular forces present within the substance. Stronger forces mean more energy is required to separate the molecules, leading to a higher boiling point. Consider the examples from the exercise:

• CO₂: Contains weak London dispersion forces, so less energy is required to separate the molecules, resulting in a low boiling point and gaseous state at room temperature.
• CCl₄: With stronger dispersion forces than CO₂, its boiling point is higher, thus it exists as a liquid under the same conditions.
• C₆₀: Due to its substantial molecular size and strong London forces, it has the highest boiling point. This requires significant energy to overcome the attractive forces, making it a solid at room temperature.

molecular polarity

Molecular polarity refers to the distribution of electrical charge over the atoms joined by the bond in a molecule. The overall polarity of a molecule is determined by the shape of the molecule and the polarity of its individual bonds. A molecule is considered nonpolar if:

• It has a symmetrical shape such that polar bonds cancel out.
• It does not have polar bonds, to begin with.

In our example:

• CO₂: The linear and symmetrical structure of CO₂ means that any polar bonds effectively cancel out, making it nonpolar.
• CCl₄: The tetrahedral symmetry results in nonpolarity as the polarities of the carbon-chlorine bonds cancel out.
• C₆₀: This molecule, although comprising nonpolar bonds, has a large structure which primarily influences the strength of dispersion forces it experiences.

While molecular polarity affects intermolecular interactions such as dipole-dipole forces, the nonpolar nature of these molecules means London dispersion forces are their primary intermolecular attraction.