Intermolecular Forces

Definition

Intermolecular forces are the attractive or repulsive forces that exist between molecules, which determine the physical and chemical properties of substances. These forces play a crucial role in understanding the behavior of fluids, the surface tension and capillary action of liquids, as well as the phase changes that occur in matter.

5 Must Know Facts For Your Next Test

1. Intermolecular forces determine the physical state of a substance at a given temperature and pressure, affecting properties such as boiling point, melting point, and surface tension.
2. Stronger intermolecular forces, such as hydrogen bonding, result in higher boiling points and melting points, while weaker forces, like van der Waals interactions, lead to lower boiling and melting points.
3. Cohesion, the attraction between like molecules, and adhesion, the attraction between different molecules, are both influenced by intermolecular forces and contribute to surface tension and capillary action in liquids.
4. The phase changes that occur in matter, such as the transition from a liquid to a gas (evaporation) or from a solid to a liquid (melting), are driven by changes in the strength of intermolecular forces.
5. Intermolecular forces play a crucial role in the structure and function of biological molecules, such as proteins and DNA, which are held together by a complex network of these forces.

Review Questions

• Explain how intermolecular forces determine the physical state of a substance.
  • The strength of intermolecular forces between molecules directly affects the physical state of a substance at a given temperature and pressure. Stronger intermolecular forces, such as hydrogen bonding, lead to higher boiling and melting points, resulting in substances that are more likely to exist as liquids or solids at room temperature. Conversely, weaker intermolecular forces, like van der Waals interactions, correspond to lower boiling and melting points, allowing substances to exist as gases at ambient conditions. The balance of these attractive and repulsive forces between molecules determines whether a substance will be a solid, liquid, or gas.
• Describe the role of intermolecular forces in the phenomena of cohesion, adhesion, surface tension, and capillary action in liquids.
  • Intermolecular forces are responsible for the cohesive and adhesive properties of liquids, which in turn govern surface tension and capillary action. Cohesion, the attraction between like molecules, is driven by strong intermolecular forces, such as hydrogen bonding or dipole-dipole interactions. This cohesion leads to the formation of a liquid's surface, which exhibits surface tension. Adhesion, the attraction between different molecules, occurs at the interface between a liquid and a solid, and is also influenced by intermolecular forces. The balance between cohesive and adhesive forces determines the extent of capillary action, where liquids can rise or be drawn into narrow spaces against the force of gravity.
• Analyze the role of intermolecular forces in the phase changes of matter, and explain how changes in these forces drive the transitions between solid, liquid, and gaseous states.
  • Intermolecular forces are the driving force behind the phase changes that occur in matter. As the strength of these forces changes, the physical state of a substance can transition between solid, liquid, and gaseous states. For example, when a solid is heated, the increased kinetic energy of the molecules overcomes the strong intermolecular forces holding them in a fixed position, causing the substance to melt and become a liquid. Further heating can then overcome the weaker intermolecular forces in the liquid, allowing the molecules to escape into the gaseous state through evaporation. Conversely, cooling a gas can increase the strength of intermolecular forces, causing the molecules to condense into a liquid, and further cooling can result in the molecules arranging into a solid crystalline structure. The delicate balance of these attractive and repulsive intermolecular forces is the key to understanding the phase changes that occur in matter.

Related terms

Van der Waals Forces: Weak intermolecular forces that arise from the temporary or induced dipoles between neutral atoms or molecules.

Hydrogen Bonding: A special type of intermolecular force that occurs between a hydrogen atom covalently bonded to a highly electronegative atom, such as nitrogen, oxygen, or fluorine.

Dipole-Dipole Interactions: Attractive forces that occur between polar molecules, where the positive end of one molecule is attracted to the negative end of another molecule.