Intermolecular Forces... explaining the properties of liquids

Intermolecular Forces

. . . explaining the properties of liquids

Let’s get one thing straight . . .

Intra*molecular forces include:

• Non-polar covalent bonds

• Polar covalent bonds

• Ionic bonds (not technically within a molecule, but anyway . . .)

• Metallic Bonding

*intra = within/between (pick one)

within

Intermolecular forces exist

between molecules

Intermolecular forces are the glue that holds molecules together

Consider the following data:

Compound Formula Molar mass (g)

mp (oC) bp (oC)

methane CH4 16 -183 -164

ammonia NH3 17 -77.7 -33.3

water HOH 18 0 100

neon Ne 20 -249 -246

methanol CH3OH 32 -93.9 64.9

hydrogen sulfide

HSH 34 -85.5 -60.7

Why is the boiling point of water (and its mp) so much higher than that of other small molecules?

Due to the high strength of intermolecular forces.

Types of Intermolecular Forces

1. van der Waals Forces include:Dipole-dipole

Ion-dipole

Induced Intermolecular Forces• Ion-induced dipole• Dipole-induced dipole

Dispersion (London) Forces

2. Hydrogen Bonding

Let’s look more closely at each type …

Dipole-Dipole Forces

• exist between polar molecules

--or represet as--

• polar molecules will have a stronger attraction for one another than similarly-sized non-polar molecules

• this results in ____________ boiling points for polar molecules compared to similarly-sized non-polar molecules

(higher)

Ion-Dipole Forces

exist between a polar solvent and an ionic solute

eg. NaCl dissolved in water

Remember: Ion-dipole attractions only occur with soluble ions.

Induced Intermolecular Forces

What does induced mean?

1. To lead or move, as to a course of action, by influence or persuasion.

Synonym: ______________.

2. To bring about or stimulate the occurrence of; cause: a drug used to induce labour. (thanks to dictionary.com)

Ion-induced dipole

• Occurs when an ion induces a nearby non-polar molecule to become polar

• ie. The ion distorts the electron density of the nearby non-polar molecule

eg of Ion-induced dipole

Bonding between Fe2+ ion and O2 in hemoglobin

Vital for O2

transport in

the body.

dipole-induced dipole

Similar to

ion-induced dipole.

A polar molecule

induces a nearby

molecule to become

polar by distorting its

electron cloud.

example of dipole-induced dipole

Explains the—limited, but significant—solubility of O2 in water.

Dispersion (London) Forces

Seems like nonpolar molecules have no way of attracting each other.

BUT: nonpolar gases (O2, N2, CH4) can be liquefied.

This indicates that if kinetic energy of molecules reduced, some type of attractive force can predominate.

Fritz London (1930) suggested that the motion of electrons within an atom or non-polar molecule can result in a transient dipole moment.

(transient = short-lived)

A Model To Explain London Dispersion Forces:

Consider two Helium atoms (each with 2 electrons)

Explanation of Dispersion Forces

At a given instant, theremay be an uneven distribution of e-s aroundan atom or molecule.

Think electron “pile-up”.

How does the strength of dispersion forces correlate to molecular size?

• as the size of a non-polar molecule increases, so does its electron cloud

• this will increase the strength of the temporarily induced dipole

• ie. as non-polar molecules get larger, their bp increases

Consider the following bp data

What’s with the unusually high bp of

• HF

• NH3

• H2O ?

What does this suggest about the strength of intermolecular bonding in these compounds?

Very strong . . .

Hydrogen Bonding

• HF, HOH, NH3 all have significantly higher bp than predicted from the trend in each family.

(bp of H2O ca. 200oC greater than expected)

• due to the very polar

H-F, H-O, H-N bonds

Simple molecules which exhibit H-bonding:

For Hydrogen Bonding to occur . . .

Hydrogen must be attached to one of the most electronegative elements (N,O,F)

very polar bonds

dipole vectors don’t cancel

In these v polar bonds H acquires a significant amount of positive charge.

Remember: O has 2 lone pairs of e-s

The H must be bonded to either N, O, F.

These atoms each have at least one "active" lone pair.

“Active” lone pairs in second E level are contained in a relatively small volume results in high density of negative charge.

(Lone pairs at higher levels are more diffuse and not so attractive to positive charges.)

Consider five water molecules close together

Review of Intermolecular Bonding

Another Review

HomeworkFrom Chemistry 12 (Mustoe et al)This is posted as “intermolecular forces explanation”From this handout: P 208 SR 1, 2, 3abc

(omit “network”) 4, 5 (omit “network”), 6, 7 Also: Illustrate the intermolecular forces that

exist in NH3(aq). Draw a pseudo 3-D picture of NH3, decide on molecular polarity and intermolecular forces.