Intermolecular Forces

Intermolecular forces (IMFs) are the attractive or repulsive forces between molecules. These forces determine the physical and chemical properties of substances and play an important role in many natural and synthetic processes. This study guide will cover the types of IMFs, their properties, and how to predict their strength.

Types of Intermolecular Forces

There are four main types of intermolecular forces:

1. Van der Waals Forces: These are the weakest type of IMFs and result from temporary or induced dipoles in molecules. There are two types of Van der Waals forces:

• London Dispersion (or Dispersion) Forces: These occur between nonpolar molecules and are caused by temporary dipoles that arise due to fluctuations in electron distribution.

• Dipole-Induced Dipole Forces: These occur between polar and nonpolar molecules and are caused by the polar molecule inducing a temporary dipole in the nonpolar molecule.

2. Dipole-Dipole Forces: These forces occur between polar molecules and are caused by the alignment of partial charges in the molecules. The positive end of one molecule is attracted to the negative end of another molecule, creating a net attractive force.

3. Hydrogen Bonding: This is a special type of dipole-dipole interaction that occurs when a hydrogen atom bonded to a highly electronegative atom (such as nitrogen, oxygen, or fluorine) interacts with another highly electronegative atom in a different molecule. The resulting interaction is stronger than a regular dipole-dipole interaction.

Properties of Intermolecular Forces

• IMFs increase as molecular weight increases.
• IMFs increase as polarity increases.
• IMFs decrease as distance between molecules increases.
• IMFs determine the boiling point and melting point of substances.
• IMFs affect the solubility of substances in solvents.

Predicting Strength of Intermolecular Forces

1. Determine the polarity of the molecules involved. If the molecules are nonpolar, London Dispersion Forces will be the only IMFs present. If the molecules are polar, dipole-dipole forces and/or hydrogen bonding may be present. H-bonds > dipole-dipole forces > Van der Waals Forces

2. Determine the molecular weight of the molecules involved. Heavier molecules will have stronger London Dispersion Forces.

3. Determine if hydrogen bonding is possible. Hydrogen bonding occurs when hydrogen is bonded to nitrogen, oxygen, or fluorine.

4. Consider the shape of the molecules involved. If the molecules are linear, the IMFs will be stronger than if the molecules are bent.

5. Consider the effect of temperature. Higher temperatures will weaken IMFs.

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