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Intermolecular Forces. Forces Between Molecules. Intermolecular Forces. Electrical forces between molecules causing one molecule to influence another Heats of vaporization give a measure of the strength of attractions present between molecules

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intermolecular forces

Intermolecular Forces

Forces Between Molecules

intermolecular forces2
Intermolecular Forces
  • Electrical forces between molecules causing one molecule to influence another
  • Heats of vaporization give a measure of the strength of attractions present between molecules
    • the energy required to separate molecules when changing from liquid to gas state
ionic compounds
Ionic Compounds
  • The forces of attractions in ionic compounds are the electrostatic force between ions
    • A relatively strong force
    • Hvap /100 kJ/mol
molecular compounds

Molecular Compounds

Polar Molecules

polar molecules
Polar Molecules
  • Force of attraction between molecules is a dipole-dipole attraction
  • Dipole-dipole forces are smaller than ion-ion forces
    • Hvap.20 kJ/mol





Molecules are electrically neutral overall but organize themselves by attractions of head to tail dipole orientation






hydrogen bonding

Hydrogen Bonding

A Special Dipole-Dipole Interaction

hydrogen bonding7
Hydrogen Bonding

The energy of the H-bond

depends on the electronegativity

of the X-atom

F > O > N . Cl





:X-H....... :X-H

heats of vaporization
Heats of Vaporization

non-polar molecule

molecular compounds9

Molecular Compounds

Non-Polar Molecules

non polar molecules
Non-Polar Molecules
  • Non-polar molecules do not possess permanent dipoles
  • Force of attraction between molecules is a London Force
principles of solubility

Principles of Solubility

Solubility is dependent on intermolecular forces

liquid liquid
  • “like dissolves like”
  • liquids with similar structures (similar type & magnitude intermolecular forces) will be soluble in each other in all proportions.
  • Both are held together by London Forces
  • When a pentane molecule passes into a volume of hexane molecules, there is no significant environment change



oil slicks
Oil Slicks
  • Non-polar substances have little water solubility
    • Water molecules are held together by H-bonds
    • Non-polar are held together by London Forces
  • H-bonds must be broken to dissolve appreciable quantities of non-polar substances in water
oil slicks16
Oil Slicks
  • For substances to be soluble, there must be compensation for any forces broken in the dissolution process.
  • Since there is no compensating force between a non-polar molecule and a water molecule, enough energy is not available to break the H-bonds
water solubility of polar molecules
Water Solubility of Polar Molecules
  • Water will dissolve some polar molecules
  • CH3OH and CH3CH2OH are capable of forming H-bonds
  • Intermolecular forces between these alcohols and water are similar to those forces in pure alcohol and pure water.
water solubility of alcohols
Water Solubility of Alcohols
  • Solubility decreases as length of carbon chain increases
  • As the chain gets longer, more H-bonds in the water must be broken to make room for the alcohol.
  • Not enough H-bonds can be reformed to compensate
non polar slightly polar substances
Non-Polar & Slightly Polar Substances
  • Most soluble in solvents of low polarity
  • Least soluble in H-bonding solvents
the ddt story
The DDT Story
  • Soluble in non-polar or slightly polar solvents
  • Concentrates in fatty tissue of fish, birds & game
  • Quite water insoluble
    • isn’t washed out of contaminated soil
solid liquid
  • Solids always have limited solubility in liquids
    • due to differences in the magnitudes of intermolecular forces in solid vs. liquid state
    • at 25oC a solid has much stronger intermolecular forces than a liquid
solid liquid22
  • The closer a solid is to its mp, the better its intermolecular forces will match up with a liquid
  • Typically, solubility increases as the temperature increases
  • Low mp solids tend to exhibit greater solubility than high mp solids