Section 5.5—Intermolecular Forces

Intra- versus Inter-molecular Forces

So far this chapter has been discussing intramolecular forcesIntramolecular forces = forces within the

molecule (chemical bonds)

Now let’s talk about intermolecular forcesIntermolecular forces = forces between

separate molecules

Breaking Intramolecular forces

Breaking of intramolecular forces (within the molecule) is a chemical change2 H2 + O2 2 H2O

Bonds are broken within the molecules and new bonds are formed to form new molecules

Breaking Intermolecular forces

Breaking of intermolecular forces (between separate molecules) is a physical changeBreaking glass is breaking the intermolecular

connections between the glass molecules to separate it into multiple pieces.

Boiling water is breaking the intermolecular forces in liquid water to allow the molecules to separate and be individual gas molecules.

London Dispersion Forces

All molecules have electrons.

Electrons move around the nuclei. They could momentarily all “gang up” on one side

This lop-sidedness of electrons creates a partial negative charge in one area and a partial positive charge in another.

+ Positively charged nucleus - Negatively charged electron

+-

-

-

-

Electrons are fairly evenly dispersed.

+--

- -As electrons move, they “gang up” on one side.

+

-

London Dispersion Forces

Once the electrons have “ganged up” and created a partial separation of charges, the molecule is now temporarily polar.

The positive area of one temporarily polar molecule can be attracted to the negative area of another molecule.

+ - + -

Strength of London Dispersion Forces

Electrons can gang-up and cause a non-polar molecule to be temporarily polar

The electrons will move again, returning the molecule back to non-polar

The polarity was temporary, therefore the molecule cannot always form LDF.

London Dispersion Forces are the weakest of the intermolecular forces because molecules can’t form it all the time.

Strength of London Dispersion Forces

Larger molecules have more electrons

The more electrons that gang-up, the larger the partial negative charge.

The larger the molecule, the stronger the London Dispersion Forces

Larger molecules have stronger London Dispersion Forces than smaller molecules.

All molecules have electrons…all molecules can have London Dispersion Forces

Dipole Forces

Polar molecules have permanent partial separation of charge.

The positive area of one polar molecule can be attracted to the negative area of another molecule.

+ - + -

Strength of Dipole Forces

Polar molecules always have a partial separation of charge.

Polar molecules always have the ability to form attractions with opposite charges

Dipole forces are stronger than London Dispersion Forces

Hydrogen Bonding

Hydrogen has 1 proton and 1 electron.There are no “inner” electrons. It bonds with the only

one it has.When that electron is shared unevenly (a polar

bond) with another atom, the electron is farther from the hydrogen proton than usual.This happens when Hydrogen bonds with Nitrogen,

Oxygen or FluorineThis creates a very strong dipole (separation of

charges) since there’s no other electrons around the hydrogen proton to counter-act the proton’s positive charge.

Strength of Hydrogen Bond

Hydrogen has no inner electrons to counter-act the proton’s charge

It’s an extreme example of polar bonding with the hydrogen having a large positive charge.

This very positively-charged hydrogen is highly attracted to a lone pair of electrons on another atom.

This is the strongest of all the intermolecular forces.

Hydrogen Bond

N

H H

N

H H

Hydrogen bond