How to identify the types of intermolecular forces present in CH3CH3

Identifying Intermolecular Forces in CH$_3$CH$_3$

Molecular Structure and Bonding

Ethane (CH$_3$CH$_3$), is a non-polar molecule composed of two carbon atoms single-bonded to each other and each carbon atom is bonded to three hydrogen atoms. The Lewis structure of ethane is shown below:

Intermolecular Forces Present

London Dispersion Forces

London Dispersion Forces (LDF) are the primary type of intermolecular force present in ethane (CH$_3$CH$_3$). These forces arise due to temporary dipoles that occur as a result of electron movement within the molecule, which leads to a momentary uneven distribution of electron density. This can be represented as:

Where:
$F_{\text{LDF}}$ is the force of London dispersion
$\alpha$ is the polarizability of the molecule
$A$ is a proportionality constant
$r$ is the distance between two interacting atoms or molecules

Given that ethane is a non-polar molecule, it does not exhibit other types of intermolecular forces such as dipole-dipole interactions or hydrogen bonding.

Summary

To summarize, due to its non-polar nature and relatively simple structure, the intermolecular forces present in ethane (CH$_3$CH$_3$) are solely London Dispersion Forces (LDF). These forces dominate due to the interaction of temporary dipoles in the molecules.

Understanding the Non-Polar Nature of Ethane

The compound ethane (C$_2$H$_6$) is an alkane and falls under the category of saturated hydrocarbons. The non-polar nature of ethane can be fundamentally understood by examining the following aspects:

Molecular Structure and Symmetry

Ethane consists of two carbon atoms single-bonded to each other, with each carbon atom also single-bonded to three hydrogen atoms:

This symmetrical arrangement means that any slight dipole moments created by the C-H bonds are canceled out, resulting in a net dipole moment of zero.

Electronegativity

Electronegativity is the tendency of an atom to attract shared electrons. In ethane:

1. Carbon and Hydrogen: Both carbon (C) and hydrogen (H) have relatively similar electronegativities.
2. The electronegativity difference between C and H is minimal (carbon: 2.55, hydrogen: 2.20 on the Pauling scale).

Due to this small difference, the C-H bonds are considered non-polar covalent bonds, meaning:

Lack of Permanent Dipole

Since ethane is made up primarily of non-polar covalent bonds and symmetrical molecular geometry, no permanent dipole is formed in the molecule. This results in:

where $\mu$ is the dipole moment, and D (Debye) is the unit of measurement.

Intermolecular Forces

Ethane exhibits London dispersion forces (a type of Van der Waals force), which are the only type of intermolecular force at play. These forces are typically found in non-polar molecules and are weaker compared to other intermolecular interactions like hydrogen bonds or dipole-dipole interactions.

Boiling and Melting Points

Due to its non-polar nature, ethane has relatively low boiling and melting points. The weak London dispersion forces mean that less energy is required to overcome these forces, resulting in a:

• Boiling point of $-88.6 ^\circ\text{C}$
• Melting point of $-182.8 ^\circ\text{C}$

Conclusion

The non-polar nature of ethane can be attributed to its symmetrical molecular structure, minimal electronegativity difference, lack of permanent dipole, and the presence of weak London dispersion forces. This non-polarity is a key feature that defines the physical and chemical properties of ethane.

Explanation of London Dispersion Forces (LDF)

London dispersion forces, also known as LDF, are a type of intermolecular force that exists between atoms and non-polar molecules. These forces are the weakest type of Van der Waals forces and arise due to the temporary fluctuations in electron density in a molecule or atom.

How LDF Works

The electron density around a molecule or atom is constantly in motion. At any given instant, the electrons might be unevenly distributed, creating a temporary dipole moment. These instantaneous dipoles can induce a similar temporary dipole in a neighboring molecule, leading to attractive forces between them.

Dependence on Size and Shape

The strength of London dispersion forces increases with:

• Molecular Size: Larger atoms or molecules have more electrons, leading to greater fluctuations in electron density. This results in stronger dispersion forces.

$F \propto \frac{Z^2}{r^6}$

where $F$ is the force, $Z$ is the number of electrons, and $r$ is the distance between particles.

• Surface Area: Molecules with a larger surface area provide more area for interactions, which strengthens the dispersion forces.

Boiling and Melting Points

The boiling and melting points of non-polar substances are significantly influenced by London dispersion forces. Substances with stronger LDF have higher boiling and melting points because more energy is required to overcome these forces.

Examples

Even in noble gases such as He, Ne, and Ar, London dispersion forces are present. The temporary dipoles created in these atoms lead to liquefaction and solidification at low temperatures.

Understanding London dispersion forces is critical in the study of material properties, especially in organic chemistry, polymer science, and physical chemistry. These forces, albeit weak, play a crucial role in the condensation of gases and the behavior of complex molecular systems.