Intermolecular Forces:
Scenario: As liquid is heated, the kinetic energy of its particles increases. The particles pull away from each other and enter into the gas phase. Boiling point is therefore a good measure of the force of attraction between particles of a liquid. The higher the boiling point, the stronger the forces between particles.
- Intermolecular Forces: the forces of attraction between molecules.
- Vary in strength, but are generally weaker than ionic bonds or metallic bonds.
* Molecular Polarity and Dipole-Dipole Forces:
- Strongest intermolecular forces exist between polar molecules.
o Polar molecules have an uneven charge distribution, which make them act as tiny dipoles.
o Dipole = created by equal but opposite charges that are separated by a short distance
§ Arrow = head towards neg. end of molecule and tail towards pos. end of molecule.
H-Cl
- When a negative region ion one polar molecule attracts the positive region in adjacent molecules it is known as a dipole-dipole force.
o Short-range force between nearby polar molecules.
- Polarity of diatomic molecules is determined by just one bond.
- Polarity of molecules containing more than two atoms depends on both the polarity and the orientation of each bond.
o EX: explain water
- Bond dipoles can cancel in some molecules, resulting in zero polarity.
o CCl4 and CO2
- A polar molecule can induce a dipole in a nonpolar molecule by temporarily attracting its electrons.
o The result is a short-range intermolecular force that is somewhat weaker than the dipole-dipole force.
o Induced dipoles account for the solubility of nonpolar compounds, such as oxygen, in polar compounds, such as water.
* Hydrogen Bonding:
- Higher the boiling point the stronger the dipole-dipole force.
- Great electronegativity difference between hydrogen and electronegative atoms causes the molecule to by highly polar.
- Also, the small size of the hydrogen atom allows the atom to come very close to an unshared pair of electrons on adjacent molecules.
o EX: water, ammonia, hydrogen fluoride
- Hydrogen bond: an intermolecular force in which a hydrogen atom that is bonded to a highly electronegative atom is attracted to an unshared pair of electrons of an electronegative atom in a nearby molecule.
o How does hydrogen sulfide, H2S (-61 °C) compare with strongly hydrogen-bonded water, H2O (100 °C)?
* London Dispersion Forces:
- Electrons are in constant motion.
- In result, at any moment the electron distribution may be slightly uneven.
o Pos. pole in one part and neg. pole in another part of the molecules. Temporary dipole can induce a dipole in an adjacent atom or molecule. Weak attraction of temp. dipoles hold the atoms together temporarily.
- London Dispersion Forces: Intermolecular attraction resulting from the constant motion of electrons and the creation of instantaneous dipoles.
o Act between all atoms and molecules
o The only intermolecular force between noble-gas atoms and nonpolar molecules.
§ Low boiling point
o Strength increases with the number of electron in the interacting atoms or molecules.
§ London forces increase with increasing atomic or molar mass.
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