Question

Define each of the following cohesive forces and order them in terms of increasing strength: a. dispersion force b. dipole force c. hydrogen bonding

Short answer

Dispersion forces are the weakest, caused by temporary dipoles. Dipole forces are stronger, occurring in permanent polar molecules. Hydrogen bonding is the strongest, involving a hydrogen atom and highly electronegative atoms.

Step-by-step solution

Define dispersion force

Dispersion forces, also known as London dispersion forces, are a type of van der Waals force that exist between all molecules, regardless of polarity. They arise from the fluctuations in the electron distribution within molecules, which create a temporary dipole. These forces are generally the weakest of all the intermolecular forces.

Define dipole force

Dipole forces, or dipole-dipole interactions, occur between polar molecules where a permanent dipole moment exists. These forces result from the electrostatic attraction between the positive end of one polar molecule and the negative end of another. They are stronger than dispersion forces but weaker than hydrogen bonds.

Define hydrogen bonding

Hydrogen bonding is a strong type of dipole-dipole interaction, but it occurs specifically when a hydrogen atom is covalently bonded to a highly electronegative atom such as nitrogen, oxygen, or fluorine, and is attracted to another electronegative atom in a different molecule. These bonds are generally stronger than normal dipole forces due to the high polarity and the small size of the hydrogen atom.

Order the cohesive forces by strength

To order the cohesive forces by strength, we start with the weakest and move to the strongest. Following this, the order is: dispersion forces, dipole forces, and hydrogen bonding. Each force is progressively stronger than the previous one.

Key concepts

Dispersion Forces

Dispersion forces, familiar as London dispersion forces, are the fundamental form of intermolecular forces that appear between all molecules, including nonpolar ones. The essence of these forces lies in the momentary fluctuations of electron density within any given molecule. Your intuitive notion of stable electron clouds is, in practice, a simplification. Electrons are constantly in motion, leading to an instantaneous dipole, a temporary uneven distribution of electrons.

Consider comparing dispersion forces to a fleeting encounter; they are brief and may seem insignificant, but collectively, they can exert a considerable influence. Specifically, they affect properties like boiling points, melting points, and solubilities. These forces are relatively weak and more pronounced in larger molecules or atoms, as a bigger electron cloud involves greater fluctuations. Hence, larger molecules generally have stronger dispersion forces.

Understanding the relative strength of dispersion forces is critical. Although they are the weakest intermolecular force, they are always present and become more relevant when other types of forces are not in effect. This means that substances made up solely of nonpolar molecules will predominantly rely on dispersion forces to maintain cohesion.

Dipole-Dipole Interactions

Dipole-dipole interactions arise in molecules that are polar, bearing a permanent dipole moment wherein there's a distinct division of positive and negative charges. Unlike dispersion forces, which are temporary, dipole moments persist due to the permanent separation of charge across the molecule.

Imagine dipole-dipole interactions like magnets at the molecular level: the positive end of one polar molecule is attracted to the negative end of another, similar to the north and south poles of a magnet drawing towards each other. This attraction is the dipole-dipole interaction, and it significantly influences the physical properties of the compounds involved.

Real-World Relevance of Dipole-Dipole Interactions

The impact of these interactions is evident in the higher boiling and melting points of polar molecules compared to nonpolar ones with similar molar masses. This is crucial when considering solubility, as polar solvents are better at dissolving polar substances, thanks to the strong dipole-dipole interactions.

Hydrogen Bonding

Hydrogen bonding stands apart as a specialized case of dipole-dipole interactions, elevated due to its impressive strength and specificity. It is not a bond in the classical sense, like a covalent or ionic bond; rather, it's a particularly strong form of intermolecular attraction.

This form of bonding is highly specific, happening when hydrogen is covalently bonded to one of the highly electronegative elements—typically nitrogen (N), oxygen (O), or fluorine (F). When juxtaposed with other dipole-dipole interactions, hydrogen bonds are comparatively much stronger, owing to the high electronegativity of N, O, or F and the small size of the hydrogen atom which allows for closer proximity between molecules.

Significance of Hydrogen Bonding

Hydrogen bonding is the hero behind vital characteristics of water—high boiling point, high surface tension, and the solid form (ice) being less dense than liquid water—features that are critical to life as we know it. They also play a pivotal role in the structure and function of biological molecules, such as DNA's double helix and the secondary and tertiary structures of proteins. By understanding hydrogen bonding, you gain insights into biological processes and can anticipate the behavior of molecules in various conditions.