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Simple Explanation:
Intermolecular forces (IMFs) are the forces of attraction or repulsion between molecules. They are responsible in determining the physical properties of substances, such as boiling points, melting points, and solubility.
Types of Intermolecular Forces
London Dispersion Forces (LDFs)
Caused by brief, random moments of uneven electron distribution in a particle’s electron cloud. This creates a temporary dipole, inducing polarity in nearby particles and leading to a weak, momentary attraction. LDFs are present in all particles, and the effects of it are more significant on larger molecules because they have a higher likelihood of uneven electron distribution resulting from larger electron clouds. However, this effect is relatively weak.
Dipole-Dipole Forces
Due to the uneven charge distribution of polar molecules, the (permanent) dipoles of those molecules cause intermolecular attraction, as the partially charged ends of the molecules are attracted to the oppositely-charged ends of neighboring molecules. This effect is usually stronger than LDFs.
Hydrogen Bonding: A stronger dipole-dipole force that occurs when hydrogen is bonded to highly electronegative elements: Nitrogen, Oxygen, or Fluorine. The large polarity difference and small size of these atoms create strong dipoles, leading to powerful intermolecular attractions.
Dipole-Induced Dipole
When polar molecules induce temporary, weak polarity in the electron clouds of nonpolar molecules, creating temporary dipoles in the nonpolar molecules.
Ion-Dipole Forces
Caused by attractions between an ion and a polar molecule. Because the ions carry a full charge, these forces are stronger than dipole-dipole forces.
Depending on the strength of the ionic bond, polar solvents could dissociate some ionic compounds to form aqueous solutions. A popular example is the dissolving of table salt (NaCl) into water (H₂O).
How Can IMFs Influence Properties of Substances?
Boiling/Melting Points: Stronger IMFS require more energy to overcome the attractions between molecules, meaning more energy must be put in to endothermically change a substance's state of matter. This in turn also affects evaporation rate and vapor pressure.
Solubility: Molecules with similar IMFS dissolve better in each other.* Polar molecules most effectively dissolve polar molecules, whereas nonpolar molecules most effectively dissolve nonpolar molecules.
Viscosity and Surface Tension: Higher IMFs increase the viscosity and surface tension of a substance because the molecules are more strongly attracted to each other.
Structure in Solid Phase: Polar compounds rather form crystal lattice structures than being amorphous due to electrostatic attraction between particles. Oppositely-charged ends link in a way that maximizes attraction while minimizing repulsion.
Molecular Speed: In conditions with the same temperature (where particles have equal average kinetic energy), molecules with more IMFs move more slowly than molecules with less IMFs, as these forces create resistance.
*Like Dissolves Like
Polar molecules can dissolve other polar molecules, while nonpolar molecules can dissolve other nonpolar molecules. However, nonpolar and polar molecules usually cannot mix.
What determines the result of two substances interacting are:
Thermodynamic Favorability
Compatibility of IMFs
This explanation requires knowledge of spontaneity and heat of solution:
Nonpolar molecules interact via LDFs. The interactions between polar molecules aren't limited to LDFs, as they also include dipole-dipole interactions and hydrogen bonding.
During the dissolving process, bonds that hold particles of the substances together need to be broken in order for the substances to mix, which requires energy. Substances with similar polarity need to break up interactions that are similar in strength. However, if we tried to mix polar with nonpolar, polar molecules need to break up strong dipole-dipole forces to separate themselves, only to be integrated with nonpolar molecules that have weaker interactions. This process would be endothermic, as the energy spent to break the interactions exceed the energy released from interactions formed after mixing, and if the increase in entropy from the process isn't enough, the dissolving process won't happen spontaneously. It's like mixing magnetic and plastic material together.
For example, this is why water (polar) and oil (nonpolar) don't mix, but water and alcohol (polar) can mix.
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