47 intermolecularforces
TRANSCRIPT
Intramolecular vs. Intermolecular
• An intramolecular bond is the chemical bond within a molecule
• An intermolecular force is a force that occurs betweenmolecules
3 Types of Intermolecular Forces
1. London Dispersion Force
2. Dipole-Dipole Force
3. Hydrogen Bonds
London Dispersion Force
Attraction resulting from a random momentary non symmetrical electron distribution
You can also think of it as the negative electrons from one molecule being attracted to the positive nucleus of a adjacent molecule
London Dispersion Force
• All molecules experience London Dispersion Forces, but they have the greatest impact in non-polar molecules
• London Dispersion Force is the weakest intermolecular force
• The strength of the London force increases with:– Increasing size
– Increasing surface area of contact or proximity
Dipole-Dipole Force
• The dipole-dipole force is the electrostatic attraction that is caused when the dipoles of polar molecules position their positive and negative ends near each other
Dipole-Dipole Force
• Only polar molecules experience Dipole-Dipole forces
• Dipole-Dipole forces are stronger than London Dispersion forces (but are still only 1% as strong as covalent or ionic bonds)
• The strength of the Dipole-Dipole force increases with:– Increasing polarity
– Decreased distance between molecules
Hydrogen Bonds
• A hydrogen bond is the strong dipole-dipole interaction that occurs when a hydrogen atom bonded to a small highly electronegative atom (oxygen, nitrogen, or fluorine) is attracted to a partially negative atom on a nearby molecule
Hydrogen Bonds
• Only molecules that have O, N, or F attached to H experience hydrogen bonding
• Hydrogen bonds are the strongest type of intermolecular force (but they are still 10-20 times weaker than covalent bonds)
Intermolecular Forces and Physical Properties
• The stronger the intermolecular forces are, the more energy it will require to pull the molecules apart
• Molecules that have strong intermolecular forces also have high melting points and boiling points
• Intermolecular forces can also affect solubility
Intermolecular Forces and Physical Properties
trimethylamine, b.p. 3.5°C
N CH3H3C
CH3
propylamine, b.p. 49°C
CH3CH2CH2 N
H
H
ethylmethylamine, b.p. 37°C
N CH3CH3CH2
H
explain the trend
explain the trend
Physical Properties of Liquids
• Surface Tension is the resistance of a liquid to increase its surface area
• The stronger the intermolecular forces, the greater the surface tensoin
OBSERVATION EXPLANATION
Physical Properties of Liquids
• Capillary action is the spontaneous rising of a liquid in a narrow tube due to adhesive and cohesive forces
Glass (SiO4)
Physical Properties of Liquids
• Viscosity is the measure of a liquid’s resistance to flow
• Liquids with strong intermolecular forces tend to be more viscous
• Ex: Vegetable oil is more viscous than water