Problem 5 - Chemistry and Chemical Reactivity

Question

Considering intermolecular forces in the pure substance, which of these substances exists as a gas at \(25^{\circ} \mathrm{C}\) and \(1 \mathrm{atm} ?\) (a) Ne (c) \(\mathrm{CO}\) (b) \(\mathrm{CH}_{4}\) (d) \(\mathrm{CCl}_{4}\)

Step-by-Step Solution

Answer

Neon (Ne) exists as a gas at 25°C and 1 atm.

Step 1: Identify the Intermolecular Forces

First, understand the types of intermolecular forces (IMFs) present in each substance. Neon (Ne) is a noble gas and experiences London dispersion forces. Methane ( CH_4 ) and carbon tetrachloride ( CCl_4 ) are nonpolar, experiencing London dispersion forces. Carbon monoxide ( CO ) is polar and experiences dipole-dipole interactions.

Step 2: Consider the Strength of Intermolecular Forces

In general, the strength of intermolecular forces from weakest to strongest is: London dispersion < dipole-dipole < hydrogen bonding < ionic. Gases tend to have the weakest IMFs. Neon ( Ne ), with only London dispersion forces, has weaker IMFs than the others.

Step 3: Relate Intermolecular Forces to Physical State

Substances with weaker intermolecular forces tend to exist as gases at room temperature and 1 atm pressure. Since London dispersion forces are the weakest, a substance with only these forces is most likely to be gaseous at these conditions.

Step 4: Determine the Substance in Gas Form

As Neon ( Ne ) experiences only weak London dispersion forces, it exists as a gas at room temperature and 1 atm. The other substances, which have stronger intermolecular attractions (either because of polarity or larger size), are likely to be in the liquid or solid state under these conditions.

Key Concepts

London Dispersion Forces

London dispersion forces are the weakest type of intermolecular forces. They are temporary attractions that occur when the electrons in two adjacent atoms form temporary dipoles. This happens because the electrons are constantly moving, creating short-lived polarizations.

These forces are present in all molecules, whether polar or nonpolar, but they are the only intermolecular force acting in noble gases like Ne. The strength of London dispersion forces depends on the size of the molecule and the number of electrons. Generally, larger molecules with more electrons have stronger dispersion forces.

• Temporary and weak
• Present in all molecules
• Stronger in larger, heavier atoms

Understanding these forces helps explain why some substances remain gases at room temperature, as they result in low boiling and melting points.

Dipole-Dipole Interactions

Dipole-dipole interactions are stronger than London dispersion forces and occur between polar molecules. A polar molecule has a region with a slight positive charge and another with a slight negative charge.

This occurs due to differences in electronegativity between atoms in a molecule, causing electron density to shift. The positive end of one polar molecule is attracted to the negative end of another. This attraction is what we call dipole-dipole interaction.

• Present in polar molecules
• Stronger than dispersion forces
• Influences boiling and melting points

In the exercise, carbon monoxide (CO) exhibits dipole-dipole interactions due to its polar nature, differentiating it from Ne and others that are nonpolar.

Noble Gases

Noble gases like neon (Ne) are unique elements found in Group 18 of the periodic table. They are characterized by a complete outer electron shell, which makes them highly stable and unreactive under normal conditions.

Because noble gases have fully-filled electron configurations, they do not easily form compounds. Their primary interaction is through London dispersion forces, as they are nonpolar and have no permanent dipoles.

• Stable and unreactive
• Exist naturally as gases
• Experience only London dispersion forces

This is why noble gases like neon are often found as gases at room temperature and standard pressure, exemplifying the minimal impact of weak intermolecular forces.