Intermolecular forces (IMFs) are the forces of attraction or repulsion between molecules, and they play a huge role in determining how substances behave chemically and physically. Here’s a clear breakdown:
1. Boiling and Melting Points
- Stronger intermolecular forces → molecules stick together more → more energy is needed to separate them.
- Example: Water has hydrogen bonds, so it boils at a higher temperature than molecules of similar size without hydrogen bonds.
2. Solubility
- “Like dissolves like” is determined by intermolecular forces.
- Polar molecules dissolve well in polar solvents because they can interact via dipole forces.
- Nonpolar molecules dissolve in nonpolar solvents because of dispersion forces.
3. Viscosity and Surface Tension
- Strong IMFs make liquids thicker (high viscosity) and more resistant to breaking the surface (high surface tension).
- Example: Honey flows slowly because its molecules interact strongly, while water flows more easily.
4. Chemical Reactions
- Intermolecular forces can affect reaction rates and mechanisms.
- Molecules that stick together strongly may react more slowly because they don’t move freely.
- Solvents with specific forces can stabilize or destabilize reactants or transition states, changing reaction speed.
5. Phase Behavior
- IMFs determine whether a substance is solid, liquid, or gas at a given temperature.
- Strong forces → solid or liquid at room temperature.
- Weak forces → gas at room temperature.
6. Other Properties
- Vapor pressure: Strong IMFs → low vapor pressure because fewer molecules escape into the gas phase.
- Capillary action: Liquids with strong adhesion to a surface can climb narrow tubes.
In short:
Intermolecular forces are like invisible “glue” between molecules. They control how molecules stick, move, dissolve, and react, which in turn shapes almost every physical and chemical property of a substance.