Question

What types of intermolecular forces exist in a crystal of ice? How do these forces differ from the types of intermolecular forces that exist in a crystal of solid oxygen?

Short answer

In a crystal of ice, the intermolecular forces present are strong hydrogen bonds, which occur due to the difference in electronegativity between oxygen and hydrogen, creating a strong electrostatic attraction between hydrogen and oxygen atoms of adjacent water molecules. In contrast, a crystal of solid oxygen exhibits weaker London dispersion forces and dipole-dipole interactions, resulting from temporary electron imbalances and the small, permanent dipole moment due to oxygen's double bond. The presence of hydrogen bonding in ice gives it unique properties and makes it more stable at higher temperatures than solid oxygen, which relies mainly on weaker forces.

Step-by-step solution

Intermolecular forces

Intermolecular forces are the forces that hold molecules together in a solid or liquid. They play a crucial role in determining the physical properties of substances.

Types of intermolecular forces

Broadly, there are three types of intermolecular forces: 1. Dispersion forces or London forces: These are weak and temporary forces caused due to random electron movement leading to instantaneous dipoles in the molecules. 2. Dipole-dipole interactions: These are stronger than dispersion forces and occur between molecules with permanent dipoles. 3. Hydrogen bonds: Hydrogen bonds are a special type of strong dipole-dipole interaction that occurs between hydrogen atoms bonded to electronegative atoms such as fluorine, oxygen, or nitrogen.

Intermolecular forces in ice

Ice is a solid form of water and has a crystalline structure. In ice, the molecules are held together by strong hydrogen bonds. Oxygen atoms in the water molecules have a partial negative charge, and hydrogen atoms have a partial positive charge due to the difference in electronegativity between oxygen and hydrogen. This difference in charge creates a strong electrostatic attraction between hydrogen and oxygen atoms of adjacent water molecules, leading to hydrogen bonding.

Intermolecular forces in solid oxygen

Solid oxygen has a crystalline structure with oxygen molecules arranged in a lattice pattern. In contrast to ice, there are no hydrogen atoms present in solid oxygen molecules. The intermolecular forces present in solid oxygen are London dispersion forces and dipole-dipole interactions. London dispersion forces arise due to instantaneous dipoles created by temporary electron imbalances, while dipole-dipole interactions occur because oxygen molecules have a small, permanent dipole moment as a result of their double bond.

Differences between ice and solid oxygen

The primary difference between the intermolecular forces present in a crystal of ice and a crystal of solid oxygen is the presence of hydrogen bonding in ice. Hydrogen bonds are stronger than both dispersion forces and dipole-dipole interactions, giving ice unique properties when compared to solid oxygen. Substances with hydrogen bonding tend to have higher melting and boiling points compared to similar molecules without hydrogen bonding. In contrast, solid oxygen relies mainly on weaker London dispersion forces and dipole-dipole interactions, making it less stable at higher temperatures than ice.

Key concepts

Hydrogen Bonds

In the world of molecules, hydrogen bonds stand out as a special type of intermolecular force. They occur when a hydrogen atom is directly bonded to an electronegative element such as nitrogen, oxygen, or fluorine. This creates a significant difference in electronegativity that results in hydrogen gaining a partial positive charge. This positively charged hydrogen is then attracted to the negative dipoles of nearby electronegative atoms.

A good example of hydrogen bonding can be found in water molecules, especially in ice. Each water molecule forms a close bond with neighboring water molecules through these strong hydrogen bonds. They are crucial because they determine many properties of water. For instance:

• Ice has a high melting point in comparison to other substances with similar molecular weights.
• The strong hydrogen bonding gives ice its open hexagonal crystalline structure, which is why ice floats on water.

Hydrogen bonds are relatively strong compared to other types of intermolecular forces. This makes substances like ice uniquely stable under conditions where other molecular solids may not be.

Dispersion Forces

Dispersion forces, also known as London dispersion forces, are considered to be the weakest of intermolecular forces. They arise due to temporary fluctuations in electron density within atoms or molecules. These fluctuations create transient, instantaneous dipoles, which can induce similar dipoles in neighboring molecules, leading to an attractive force.

Despite being weak, dispersion forces are universal. They occur between all molecules, regardless of whether they are polar or non-polar, and increase with the size of the molecule. This is why larger molecules with more electrons exhibit stronger dispersion forces. Some key characteristics of dispersion forces include:

• They are the primary intermolecular force in noble gases and non-polar molecules like solid oxygen.
• Dispersion forces contribute to the liquefaction of gases and the condensation of vapors.

These forces play an important role in the properties of molecular solids, even though they are not as significant in magnitude as hydrogen bonds or dipole-dipole interactions.

Dipole-Dipole Interactions

Dipole-dipole interactions occur in molecules that possess permanent dipoles. These are molecules with a clear separation of positive and negative charges. Such interactions are stronger than dispersion forces but weaker than hydrogen bonds, forming when the positive end of one polar molecule attracts the negative end of another.

Solid oxygen demonstrates dipole-dipole interactions. Although the oxygen molecule itself is nonpolar, it can exhibit dipole moments because of its bent molecular structure and the double bond present. These permanent dipoles result in moderate attractive forces between oxygen molecules. Characteristics of dipole-dipole interactions include:

• Their strength depends on the magnitude of the dipole moment of the interacting molecules.
• These interactions impact the boiling and melting points of substances, making them higher than those with only dispersion forces but lower than those with hydrogen bonds.

Dipole-dipole interactions are important for understanding physical properties like solubility and melting points in various substances, providing insight into why some molecules behave the way they do under certain conditions.