10.1 Intermolecular Forces - Chemistry 2e _ OpenStax

By the end of this section, you will be able to:
- Describe the types of intermolecular forces possible between atoms or molecules in condensed phases (dispersion forces, dipole-dipole attractions, and hydrogen bonding).
- Identify the types of intermolecular forces experienced by specific molecules based on their structures.
- Explain the relation between the intermolecular forces present within a substance and the temperatures associated with changes in its physical state.

The kinetic molecular theory can help explain the behavior of solids and liquids:
- Particles in a Solid: Tightly packed in a regular pattern; vibrate about fixed positions.
- Particles in a Liquid: Close together, no regular arrangement; move past each other while remaining in contact.
- Particles in a Gas: Far apart, no regular arrangement; move independently until they collide.

The phase of a substance depends on the balance between intermolecular forces (IMFs) and kinetic energy (KE) of the particles.
IMFs are attractive forces that hold particles close together, while KE provides the energy to overcome these forces and increase distance between particles.
Changes in temperature can induce state changes by altering the average KE of a substance.

Transitions between solid, liquid, and gaseous states occur when temperature or pressure conditions favor associated changes in IMFs.
- Example: Water vapor condensing into liquid water on a cold surface.
- Example: Gaseous butane condensing to liquid within a lighter due to increased pressure.

Intermolecular Forces: Attractions between molecules, affecting physical properties.
Intramolecular Forces: Forces within a molecule that keep the molecule intact (bonding between atoms).
- Example: Breaking intramolecular covalent bonds requires significantly more energy compared to influencing intermolecular forces.

Present in all condensed phases, known as London dispersion forces.
Results from the formation of temporary dipoles due to electron motion.
Larger and heavier molecules exhibit stronger dispersion forces.
Factors Affecting Strength:
- Polarizability: How easily an electron cloud can be distorted to form a temporary dipole.

Occur between polar molecules with positive and negative dipoles.
Example: Hydrogen chloride (HCl) exhibits dipole-dipole attractions, resulting in a liquid phase at lower temperatures compared to nonpolar substances.

A strong type of dipole-dipole interaction that occurs when H is bonded to F, O, or N.
Hydrogen bonds are significantly stronger than other types of dipole-dipole attractions and dispersion forces, influencing boiling points.
Examples: H2O exhibits hydrogen bonding, leading to a higher boiling point compared to molecules of comparable mass, such as nitrosyl fluoride (ONF).

Geckos can adhere to surfaces due to microscopic hairs (setae) creating a large surface area that facilitates dispersion forces.
The ability to switch between adhesion and non-adhesion via changes in spatula orientation highlights intermolecular force applications.

Predictions of boiling points based on IMFs:
- Ethanol (CH3CH2OH) exhibits the strongest hydrogen bonding compared to dimethyl ether (CH3OCH3) and propane (C3H8), reflecting high boiling points due to stronger IMFs.
Comparative analysis between different compounds reveals trends in boiling points attributed to varying strengths of intermolecular forces.

In DNA, hydrogen bonding between complementary base pairs (adenine-thymine, cytosine-guanine) plays a crucial role in maintaining the double helical structure, facilitating easy unzipping for replication.