Problem 15 - Chemistry The Central Science In Si Units

Question

(a) Which type of intermolecular attractive force operates between all molecules? (b) Which type of intermolecular attractive force operates only between polar molecules? (c) Which type of intermolecular attractive force operates only between the hydrogen atom of a polar bond and a nearby small electronegative atom?

Step-by-Step Solution

Answer

(a) London dispersion forces, (b) Dipole-dipole interactions, (c) Hydrogen bonds.

Step 1: Understanding Intermolecular Forces

Intermolecular forces are forces of attraction or repulsion which act between neighboring particles (atoms, molecules, or ions). They are weaker than the intramolecular forces that hold a molecule together.

Step 1: Identifying Forces Present in All Molecules

The type of intermolecular force present in all molecules, regardless of their polar or nonpolar nature, is the London dispersion force. These forces arise due to fluctuations in the electron distribution within molecules or atoms, creating a temporary dipole moment.

Step 2: Determining Forces Specific to Polar Molecules

The type of intermolecular force that operates only between polar molecules is the dipole-dipole interaction. These occur when the positive end of a polar molecule is attracted to the negative end of another polar molecule.

Step 3: Identifying Forces Involving Hydrogen Bonds

Hydrogen bonding is a special type of dipole-dipole interaction that occurs when the hydrogen atom, which is covalently bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine, interacts with another electronegative atom nearby.

Key Concepts

London Dispersion Force

London dispersion forces are incredibly fascinating because they occur in all molecules, no matter if they are polar or nonpolar. These forces are the unsung heroes of molecular interactions!

Here's how they work: electrons are always moving around the nucleus of an atom. Sometimes, by random chance, the electrons are unevenly distributed around the nucleus, creating a temporary dipole moment. This means that one part of the atom becomes slightly negative while the other side is slightly positive.

• These temporary dipoles can induce similar dipoles in neighboring atoms or molecules.
• The result is a domino effect of attraction between various molecules.

Impressively, the strength of London dispersion forces increases with the number of electrons in a molecule. Therefore, larger and heavier atoms or molecules will exhibit stronger dispersion forces. These forces may be weak individually, but collectively they play a crucial role in phenomena like boiling points of compounds.

Dipole-Dipole Interaction

Dipole-dipole interactions are the specialized team members that work specifically with polar molecules. A polar molecule is one where there is an uneven distribution of charges, meaning one end is slightly positive, and the other is slightly negative.

Think about how magnets work. The opposite charges attract each other, and so too do polar molecules.

• In a dipole-dipole interaction, the positive end of one molecule is attracted to the negative end of another.
• This interaction increases the forces of attraction between molecules, which influences properties such as boiling and melting points.

These interactions are particularly stronger in substances with high polarity. Because of this unique feature, dipole-dipole interactions are exclusive to molecules that have a permanent dipole moment.

Hydrogen Bonding

Hydrogen bonding is like the king of dipole-dipole interactions, holding immense sway due to its unique characteristics. This type of interaction occurs specifically when a hydrogen atom is bonded to a highly electronegative atom — typically nitrogen, oxygen, or fluorine.

What makes hydrogen bonding so special is its strength. Although it is just a type of dipole-dipole interaction, it is much stronger than other types of dipole-dipole forces.

• This is because the hydrogen atom, being small and light, allows the partial charges to come close together.
• The strong attraction results because of the high difference in electronegativity between hydrogen and the electronegative partner atom.

Hydrogen bonding is responsible for many important biological and chemical properties, such as the unique structure of water and DNA's double helix.