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Intermolecular

Forces

Liquids

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A. Definition of IMF

• Attractive forces between molecules.

Much weaker than

chemical bondswithin molecules.

van der Waals forces

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Intermolecular Forces of Attraction

(IMF)

• Dipole-dipole Forces 

• Ion- Dipole Forces 

• Ion/Dipole-Induced Dipole

• Dispersion Forces 

• Hydrogen Bonding 

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Dipole-Dipole Forces

• Attractive forces between polar molecules(molecules with large EN difference, thus havedipole moments)

+ - + -

  +  -   +  -H Cl +  - 

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Ion-Dipole Forces

• Attraction between an ion (anion / cation) & a

polar molecule to each other.

+   +  -

- + -cation polar molecule

anion polar molecule

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Ion-Induced Dipole & Dipole Induced

Dipole

• Induced Dipole- due to the separation of

positive & negative charges in an atom (or anon-polar molecule) caused by the proximity of

an ion or a  polar  molecule.

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Fluctuating Dipole in a Non-polar Molecule 

These instantaneous dipoles may be induced and stabilized as an

ion or a polar molecule approaches the non-polar molecule.

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+ +

-

ion non-polar molecule

  +  -+

  +  -

+

-

polar non-polar molecule

Induced

dipole

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Dispersion Forces

• Attractive forces that arise as a result of

temporary dipoles induced in atoms or

molecules• Exists in all types of species (neutral, charged,

polar, non-polar)

• Dispersion forces are very weak until themolecules or ions are almost touching each

other, as in the liquid state.

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Dispersion Forces

In general, stronger interactions allow the solidand liquid states to persist to higher

temperatures.

However, non-polar molecules show similarbehavior, indicating that there are some types of

intermolecular interactions that cannot beattributed to simple electrical attractions. Theseinteractions are generally called dispersionforces.

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Hydrogen Bonding

• Special type of dipole-

dipole interaction

between Hydrogen atomin a polar bond, such as

N-H, O-H or F-H & an

electronegative O, N, or Fatom.

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HYDROGEN BONDING

• To recognize the possibility of hydrogen bonding,examine the Lewis structure of the molecule.

•The electronegative atom must have one or moreunshared electron pairs as in the case of oxygenand nitrogen, and has a negative partial charge.

• The hydrogen, which has a partial positive chargetries to find another atom with excess electrons toshare and is attracted to the partial negativecharge.

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3. HYDROGEN BONDING

The hydrogen bond is reallya special case of dipoleforces. A hydrogen bond isthe attractive force betweenthe hydrogen attached to anelectronegative atom of onemolecule and anelectronegative atom of adifferent molecule. Usuallythe electronegative atom is

oxygen, nitrogen, or fluorine.

In other words - Thehydrogen on one moleculeattached to O or N that is

attracted to an O or N of a

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Hydrogen Bonds in liquid water

• Hydrogen bonds aremuch weaker thancovalent bonds.

• However, when a largenumber of hydrogenbonds act in unisonthey will make a strongcontributory effect.This is the case inwater.

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