Cohesive Forces

Definition

Cohesive forces are the attractive intermolecular forces that hold molecules together within a substance, contributing to the physical and chemical properties of that substance. These forces are particularly relevant in the context of noncovalent interactions between molecules.

5 Must Know Facts For Your Next Test

1. Cohesive forces are responsible for the condensed state of matter (liquid and solid) and contribute to the surface tension and viscosity of liquids.
2. The strength of cohesive forces depends on the size and polarizability of the molecules, as well as the presence of functional groups capable of forming hydrogen bonds.
3. Cohesive forces are weaker than covalent bonds but stronger than the intermolecular forces that hold gases together, allowing liquids and solids to maintain their shape and volume.
4. Cohesive forces play a crucial role in the structure and stability of biological macromolecules, such as proteins and nucleic acids, which rely on noncovalent interactions for their folding and function.
5. Understanding cohesive forces is essential for predicting and explaining the physical and chemical properties of substances, as well as for designing new materials with desired characteristics.

Review Questions

• Explain how cohesive forces contribute to the physical properties of liquids.
  • Cohesive forces are responsible for the condensed state of liquids and contribute to their physical properties, such as surface tension and viscosity. The attractive forces between molecules in a liquid help hold the substance together, allowing it to maintain a defined volume and shape. Stronger cohesive forces result in higher surface tension, which can be observed in the formation of droplets, and greater viscosity, which affects the flow and resistance of the liquid. The specific strength of cohesive forces in a liquid depends on factors like the size and polarity of the molecules, as well as the presence of functional groups capable of hydrogen bonding.
• Describe the role of cohesive forces in the structure and stability of biological macromolecules.
  • Cohesive forces play a crucial role in the structure and stability of biological macromolecules, such as proteins and nucleic acids. These noncovalent interactions, including hydrogen bonding, van der Waals forces, and ionic interactions, help stabilize the three-dimensional folding of these large, complex molecules. The specific arrangement of cohesive forces between different regions of the macromolecule determines its overall shape and function. For example, the secondary and tertiary structures of proteins are maintained by a network of cohesive forces that hold the polypeptide chain in its functional conformation. Disrupting these cohesive forces can lead to changes in the macromolecule's structure and, consequently, its biological activity.
• Analyze how an understanding of cohesive forces can be applied to the design of new materials with desired properties.
  • By understanding the principles of cohesive forces, scientists and engineers can design new materials with specific physical and chemical properties. The strength and nature of cohesive forces between molecules or atoms can be manipulated to create substances with tailored characteristics. For example, the incorporation of functional groups capable of forming strong hydrogen bonds can increase the cohesive forces within a material, leading to higher melting and boiling points, increased viscosity, or enhanced mechanical strength. Conversely, the inclusion of bulky, nonpolar groups can weaken cohesive forces and result in materials with lower surface tension or reduced intermolecular interactions. This knowledge allows for the rational design of new materials, such as advanced polymers, ceramics, or composites, with applications in areas like energy storage, aerospace engineering, and biomedical technology.

Related terms

Intermolecular Forces: Intermolecular forces are the attractive or repulsive forces that exist between molecules, which determine the physical and chemical properties of a substance.

Hydrogen Bonding: Hydrogen bonding is a special type of dipole-dipole intermolecular force that occurs when a hydrogen atom covalently bonded to a highly electronegative element (such as nitrogen, oxygen, or fluorine) is attracted to another highly electronegative element.

Van der Waals Forces: Van der Waals forces are weak intermolecular forces that arise from the temporary or induced dipoles that can form between molecules, even in the absence of permanent dipoles.