10.1 Intermolecular Forces - Chemistry_ Atoms First 2e _ OpenStax

Learning Objectives

• Types of Intermolecular Forces: At the end of this section, students should be able to:

  • Describe the types of intermolecular forces present in condensed phases, including dispersion forces, dipole-dipole attractions, and hydrogen bonding.

  • Identify the intermolecular forces experienced by specific molecules based on their structures.

  • Explain the relationship between intermolecular forces and the temperatures related to physical state changes.

Kinetic Molecular Theory in Solids and Liquids

• The kinetic molecular theory helps explain the behavior of solids and liquids.

• Particles in solids:

  • Tightly packed and arranged in regular patterns.

  • Vibrate in fixed positions but do not move relative to one another.

• Particles in liquids:

  • Close together but lack a fixed arrangement.

  • Move past each other, remaining in constant contact.

• Particles in gases:

  • Spread far apart with no regular arrangement.

  • Move independently except during collisions.

• The phase of a substance depends on:

  • The strength of the attractive intermolecular forces (IMFs).

  • The kinetic energy (KE) of the molecules.

Intermolecular Forces and Kinetic Energy

• Intermolecular forces (IMFs): forces of attraction between particles (atoms, molecules, or ions).

• These forces keep particles close together, while kinetic energy provides the energy that allows particles to overcome these forces during temperature changes.

• Changes in physical state occur when temperature or pressure conditions favor shifts in intermolecular forces.

Phase Transitions of Water

• When water vapor cools sufficiently, the attractions between water molecules allow condensation from gas to liquid.

• Example applications:

  • Cold glass causing moisture to condense from air, forming liquid water on its surface.

• Many gases can liquefy under high pressure, compressing molecules closer together to allow intermolecular attractions to dominate over their kinetic energy.

  • For instance, Butane (C₄H₁₀) is gas at standard conditions but condenses into liquid in a lighter's fuel compartment.

Strength of Intermolecular Forces

• Attractive forces between molecules (intermolecular forces) differ from those within molecules (intramolecular forces).

• Intramolecular forces: bonds within molecules (e.g., covalent bonds); much stronger than IMFs.

• To break IMFs, relatively low energy is needed, while breaking covalent bonds requires significantly more energy.

  • Example: To convert 1 mole of liquid HCl to gas, only about 17 kJ is needed compared to approximately 430 kJ to break H-Cl bonds.

Types of Intermolecular Forces

Dispersion Forces

• London dispersion forces are present in all condensed phases.

• Temporary dipoles form when electron distributions in atoms or molecules change, leading to weak attractions.

• Larger, heavier atoms have stronger dispersion forces than smaller, lighter ones due to more substantial electron cloud distortions.

Dipole-Dipole Attractions

• Polar molecules have a distinct positive and negative end, creating a dipole.

• Dipole-dipole attractions occur between the positive end of one molecule and the negative end of another, as seen in molecules such as HCl.

Hydrogen Bonding

• Particularly strong dipole-dipole interactions occur when hydrogen is bonded to highly electronegative atoms such as F, O, or N.

• This results in significant attractions evident in substances like water (H₂O), leading to higher boiling points compared to nonpolar counterparts.

Effect of Molecular Size and Shape on Boiling Points

• As the molecular size increases, boiling and melting points generally increase due to stronger dispersion forces.

• The shape of molecules affects dispersion strength—elongated shapes have more surface area for intermolecular contact than compact shapes, enhancing attraction and boiling points.

• Example: n-pentane has a higher boiling point due to its larger contact area compared to neopentane.

Everyday Examples: Geckos and Intermolecular Forces

• Geckos can adhere to surfaces due to intermolecular forces acting through their feet, which are covered with tiny hairs (setae).

• These setae maximize surface area contact, enabling the gecko to stick to various surfaces without adhesive substances.

• Their ability to toggle adhesion is due to the change in angles between the spatulae on their toes when under force, illustrating the practical application of dispersion forces.