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Intermolecular Interactions Effect on Properties

Intermolecular forces are the attraction forces between molecules that hold them together. Know about dispersion forces, dipole-dipole forces, Van der Waal forces, and hydrogen bonds.

Introduction

Intermolecular forces are the forces that attract molecules together. They determine various bulk properties, including melting points, temperatures, boiling points, and solubilities (miscibilities) of substances. Compared to a low-boiling-point liquid, such as hexane (C6H14, b.p. 68.73 °C), a high-boiling-point liquid, such as water (H2O, b.p. 100 °C) exhibits stronger intermolecular forces. Ion–dipole forces, dipole-dipole interactions, and Van der Waals forces, including London dispersion forces, are among the types of intermolecular interactions. The electrostatic attraction between an ion and a dipole is known as ion-dipole forces. They’re common in solutions and play a key role in the dissolving of ionic compounds in water, such as KCl. The strength of ion-dipole interactions are related to i) the ion’s charge and ii) the magnitude of polar molecules’ dipole.

Properties of intermolecular interactions

Weak interactions between particles are known as intermolecular (or interparticle) forces. As you progress from solid to liquid to gas, they decrease. Remember that in a gas, particles have the greatest degree of freedom of movement and intermolecular interactions are low or weak. The vapour pressure (the pressure of a vapour in equilibrium with its liquid) drops as the intermolecular attraction increases As the temperature rises, the boiling point (the temperature at which the vapour pressure equals the pressure imposed on the liquid’s surface) rises Surface tension (a liquid’s resistance to spreading out and increasing its surface area) rises Viscosity (a liquid’s resistance to flow) rises

What effect do intermolecular forces have on matter states?

  1. The balance between the kinetic energy of individual particles (molecules or atoms) and intermolecular forces determines the state of a substance. When you apply more pressure to a substance, the molecules are forced closer together, which strengthens the intermolecular forces.
  2. The higher the intermolecular interactions between liquid particles, the more difficult it is for it to escape into the vapour phase, i.e., more energy is required to convert it from liquid to vapour phase, resulting in a higher boiling point.
  3. Stronger physical properties, such as higher melting or boiling temperatures, will come from stronger intermolecular forces, which necessitate breaking molecules apart. Because it is easier to evaporate a compound with greater vapour pressure, fewer intermolecular tensions result in higher vapour pressure.
As previously stated, the properties of a compound are determined by the intermolecular forces present. When you think about it, this isn’t all that surprising. The intermolecular interactions between water molecules decrease as a liquid turns into a gaseous liquid or steam. Intermolecular forces are strongest in the solid-state and weakest in the gaseous form for any particular chemical. After all, if the molecules in one liquid are held together firmly by a strong intermolecular force, this liquid would behave differently than a second liquid in which the molecules are bonded together weakly. Intermolecular forces influence the characteristics of a liquid in the following two ways:

  • Melting and boiling points: 

Hydrogen bonding causes compounds to melt and boil at higher temperatures than dipole-dipole forces or London dispersion forces. Consider the following three molecules as an example.

  • Surface tension:

Surface tension is higher in liquids with stronger intermolecular forces. The shape of liquid droplets is determined by surface tension. The imbalance in cohesive forces of the surface layer tends to draw droplets of water into a spherical shape, despite their ease of deformation. Drops of almost all liquids would be roughly spherical in the absence of additional forces.

  • Viscosity:

Viscosity is the resistance of a fluid (liquid or gas) to changing shape or moving neighbouring portions relative to one another. The term “viscosity” refers to the resistance to flow. Fluidity is defined as the reciprocal of viscosity, which is a measure of flow ease. Molasses, for example, has a viscosity that is higher than water.

Conclusion

Bulk properties such as melting points of solids and boiling points of liquids are determined by intermolecular forces. When the molecules in a liquid have enough thermal energy to overcome the intermolecular attraction interactions that hold them together, the liquid boils, generating vapour bubbles. Solids melt when the molecules gain enough thermal energy to overcome the intermolecular interactions that keep them locked in place. Intermolecular forces are electrostatic in nature, meaning they are caused by the interaction of positively and negatively charged species.