Intermolecular Forces

Chapter 41

Intermolecular Forces

Intermolecular forces are attractive forces between molecules.

Intramolecular forces hold atoms together in a molecule.

Generally, intermolecular forces are much weaker than intramolecular forces.

“Measure” of intermolecular force

boiling point

melting point

Hvap

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OverviewOverview• There are 2 types of attraction in molecules:

intramolecular bonds & intermolecular forces• We have already looked at intramolecular bonds

(ionic, polar, non-polar)

• Intermolecular forces (IMF) have to do with the attraction between molecules (vs. the attraction between atoms in a molecule)

• IMFs come in six flavours: 1) ionic, 2) dipole - dipole, 3) H-bonding, 4) London forces, 5) covalent (network solids), 6) metallic

Intermolecular forcesIntermolecular forces

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Intermolecular forcesIntermolecular forces (cont..) (cont..)

• IMFs come in six flavours: 1) ionic, 2) dipole - dipole, 3) H-bonding, 4) London forces, 5) covalent (network solids), 6) metallic

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Intermolecular Forces

Dipole-Dipole Forces

Attractive forces between polar molecules

Orientation of Polar Molecules in a Solid

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Intermolecular Forces

Ion-Dipole Forces

Attractive forces between an ion and a polar molecule

Ion-Dipole Interaction

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Intermolecular ForcesDispersion Forces

Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules

ion-induced dipole interaction

dipole-induced dipole interaction

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Intermolecular ForcesDispersion 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.

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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.

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Intermolecular ForcesHydrogen Bond

The hydrogen bond is a special dipole-dipole interaction between they hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom.

A H…B A H…Aor

A & B are N, O, or F

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Hydrogen Bond

11.212

Why is the hydrogen bond considered a “special” dipole-dipole interaction?

Decreasing molar massDecreasing boiling point

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• 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

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Ionic, Dipole - Dipole attractionsIonic, Dipole - Dipole attractions•We have seen that molecules can have a separation of charge

•This happens in both ionic and polar bonds (the greater the EN, the greater the dipoles)

H Cl

+ –

• Molecules are attracted to each other in a compound by these +ve and -ve forces

+–

+ –

+ –

+ –

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H - bondingH - bonding• H-bonding is a special type of dipole - dipole

attraction that is very strong

• It occurs when N, O, or F are bonded to HQ- Calculate the EN for HCl and H2O

A- HCl = 2.9-2.1 = 0.8, H2O = 3.5-2.1 = 1.4

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• The high EN of NH, OH, and HF bonds cause these to be strong forces (about 5x stronger than normal dipole-dipole forces)

• They are given a special name (H-bonding) because compounds containing these bonds are important in biological systems

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London forcesLondon forces• Non-polar molecules do not have dipoles

like polar molecules. How, then, can non-polar compounds form solids or liquids?

• London forces are named after Fritz London (also called van der Waal forces)

• London forces are due to small dipoles that exist in non-polar molecules

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London forces (cont..)London forces (cont..)

• Because electrons are moving around in atoms there will be instants when the charge around an atom is not symmetrical

• The resulting tiny dipoles cause attractions between atoms/molecules

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• Because electrons are moving around in atoms there will be instants when the charge around an atom is not symmetrical

• The resulting tiny dipoles cause attractions between atoms/molecules

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London forcesLondon forces

Instantaneous dipole: Induced dipole:

Eventually electrons are situated so that tiny dipoles

form

A dipole forms in one atom or molecule, inducing a dipole in the

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