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Intermolecular Forces
• Why do some solids dissolve in water but others do not?
• Why are some substances gases at room temperature, but others are liquid or solid?
• What gives metals the ability to conduct electricity, what makes non-metals brittle?
• The answers have to do with …
Intermolecular forcesIntermolecular forces
Questions
Intermolecular Forces(IMFs)Intermolecular forces are attractive forces between molecules.
Intramolecular forces hold atoms together in a molecule
Intermolecular vs Intramolecular
• 41 kJ to vaporize 1 mole of water (inter)
• 930 kJ to break all O-H bonds in 1 mole of water (intra)
• Generally, intermolecular forces are much weaker than intramolecular forces.
Intermolecular force• Ion-dipole• dipole - dipole • H-bonding • London dispersion forces
Intramolecular force• Ionic• covalent (network solids)• metallic
Properties of Liquids
Surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area.
Strong intermolecular
forces
High surface tension
Capillary ActionCapillary Action: : Spontaneous rising Spontaneous rising
of a liquid in a of a liquid in a narrow tube.narrow tube.
Properties of Liquids
Cohesion is the intermolecular attraction between like molecules
Adhesion is an attraction between unlike molecules
Adhesion
Cohesion
attracted to glass
attracted to each other
Properties of WaterSurface Tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area.
The stronger the IMFs the molecules have with each other, the higher is its surface tension.
This is because of the cohesive forces inside the liquid (Polar-Polar forces) become much greater than the adhesive forces between the liquid (polar) and the air (non-polar) molecules outside.
Properties of Liquids
Viscosity is a measure of a fluid’s resistance to flow.
Strong intermolecular
forces
High viscosity
• Nonpolar Cl2
(Electronegativity
difference is zero)
• Polar HCl (Electronegativity difference
is not zero)
• Ionic NaCl
Bond Polarity
Types of Intermolecular Forces
1. Ion-Dipole Forces
Attractive forces between an ion and a polar molecule
Ion-Dipole Interaction
Types of Intermolecular Forces
2. Dipole-Dipole Forces
Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid
Types of Intermolecular Forces3. Hydrogen Bond (strongest)
The hydrogen bond is a special dipole-dipole interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom. IT IS NOT A BOND.
A H…B A H…Aor
A & B are N, O, or F
Hydrogen Bond
Why is the hydrogen bond considered a “special” dipole-dipole interaction?
Decreasing molar massDecreasing boiling point
Types of Intermolecular Forces4. Dispersion Forces – van der Walls forces/London forces (weakest)
Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules
ion-induced dipole interaction
dipole-induced dipole interaction
Non-polar molecules do not have dipoles like polar molecules.So London dispersion force exists in non polar molecule.
Dispersion forces usually increase with molar mass.
Intermolecular Forces4. Dispersion Forces Continued
Polarizability is the ease with which the electron distribution in the atom or molecule can be distorted.
Polarizability increases with:
• greater number of electrons
• more diffuse electron cloud
Dispersion forces usually increase with molar mass.
SO
O
What type(s) of intermolecular forces exist between each of the following molecules?
HBrHBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules.
CH4
CH4 is nonpolar: dispersion forces.
SO2
SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules.
Types of IMF• The hydrogen bonds in water explain its relatively high boiling
point, considering that it is a small molecule. The H-bonds hold the water molecules together as a liquid, so you have to heat it a lot before it will change to a gas. Compare boiling points of these molecules:
Molecule IMF (s) present Molar Mass (g/mol)
Boiling Point (oC)
CH4 16.05 - 164
HCl 36.46 - 85
H2O 18.02 100
London Disp.
London Disp.
Dipole-DipoleLondon Disp./Dipole-Dipole/Hydrogen Bonding
Water, H20
Which is related to…
And Glycerine (Glycerol)
Compare the three structures
Which of these molecules has the greatest polarity?
Which will exhibit the strongest intermolecular forces?
Ethanol. Polar or Non-Polar?
Methanol. Polar or Non-Polar?
Methane. Polar or Non-Polar?
Chloro-Methane. Polar or Non-Polar?
Sulfur Hexafluoride. Polar or Non-Polar?
Nitrogen Trihidride, aka Ammonia. Polar or Non-Polar?