Outline for Today Wednesday, Nov. 28 Chapter 11: Intermolecular - - PowerPoint PPT Presentation

Outline for Today

Wednesday, Nov. 28

• Chapter 11: Intermolecular Forces and Liquids
• Intermolecular Foces
• Properties of Liquids
• Vapor Pressure
• Phase Changes

1

Intermolecular Forces

• London Dispersion: Attraction between momentary or

instantaneous dipoles in molecules.

• Dipole-Dipole: Attraction between partial charges due to

permanent dipoles

• Hydrogen-Bonding: Attraction between partial charges on

H-F , H-O, or H-N bonds.

• Ion-Dipole: Attraction between ions and partial charges of

dipoles

Review of London Dispersion

Dipole-Dipole

• Dipole-Dipole: Attraction between partial charges due to

permanent dipoles.

• Example: Is there a dipole on CH3CN?
• How do the dipoles interact?

Dipole-Dipole Interactions in Solids

Dipole Dipole Interactions in Liquids

The larger the dipole, the stronger the dipole-dipole interactions!

O O N

Increasing Polarity Increasing Dipole-Dipole Interactions Increasing Boiling Point

Hydrogen-Bonding is a special type of dipole interaction

• Hydrogen-Bonding: Attraction between partial charges
• n H-F

, H-O, or H-N bonds.

• H-F

, H-N, and H-O are especially polar bonds!

• They interact strongly with small electronegative ions or

atoms (like F , O, or N of another molecule).

• That interaction is called hydrogen bonding.

NOT A Hydrogen Bond

C H H H H

Yes, there are hydrogens bonded to stuff… BUT there are no intermolecular hydrogen bonding

• interactions. There is no hydrogen bond.

C H H H H

Hydrogen Bonding Is Critical for Understanding the Properties of Water!

Predict the relative strengths of the dipole-dipole interactions

• Order the following by increasing boiling point (based on

strength of the intermolecular interactions)

a) Butane c) 2-Methylpropane O d) Isopropyl alcohol b) Propyl alcohol O

Intermolecular Forces with Charged Particles

• Ion-Dipole Forces: Attraction (or Repulsion) between an

ion and the partial charge on the end of a molecule

Try this chart with: Ammonia, Methane, Acetic Acid, Hydrochloric Acid

Comparing IMFs for different molecules

• When molecules of two substances have similar molecular

weights:

• Their dispersion forces are about equal.
• Use dipole-dipole interactions to determine IMF strength.
• Example: CH3COOH vs. CH3CH2CH2OH

C C C O H H H H H H H H

C C O O H H H H vs. 118 oC 97 oC Boiling Temperature:

C C O O H H H H 118 oC Boiling Temperature: Acetic Acid can form 2 hydrogen bonds It can be a hydrogen bond donor and a hydrogen bond acceptor

C C O O H H H H C C O O H H H H

Hydrogen Bond #1 Hydrogen Bond #2

C C C O H H H H H H H H

97 oC Propanol can only form 1 hydrogen bond. Its IMFs are weaker than acetic acid’s therefore its boiling point is lower.

C C C O H H H H H H H H C C C O H H H H H H H H

Just 1 Hydrogen Bond

Comparing IMFs for different molecules

• When molecules of two substances have very different

molecular weights and there is no hydrogen bonding:

• Dispersion forces tend to be more important in

determining strength of IMF .

• The higher the molecular weight the stronger the IMFs.
• Example: CCl4 vs. C8H18

CCl4 C8H18 Molecular Mass

154 g/mol 257 g/m

Boiling Point

76oC 135oC

Comparing IMFs for different molecules

• Because C8H18 has a higher molecular mass, it has more

electrons.

• More electrons means it is more polarizable than the CCl4
• Thus the London Dispersion Forces will be larger.

Properties of Liquids: Viscosity

• Viscosity: A substance’s

resistance to flow

• Stronger intermolecular

forces = More Viscous

• Example: Pitch Drop

Experiment

Properties of Liquids: Surface Tension

• Surface Tension: Energy

required to increase surface area by a given amount

• The stronger the

intermolecular forces, the greater the surface tension.

Properties of Liquids: Capillary Action

• Cohesive Forces: intermolecular forces that bind a

substance to itself

• Adhesive Forces: intermolecular forces that bind a

substance to a surface

• Capillary Action occurs when adhesive force is greater

than cohesive force.

Examples of Capillary Action

• Shape of a meniscus in a glass test tube