Intermolecular Forces(Chapter 11)

Intermolecular Forces

Forces that exist between molecules(IMFs). Depend on Molecular Polarity and size(molar mass()

4 Van der Waals IMFs

• Induced dipole-induced dipole(London Dispersion Forces)

• Dipole-induced dipole(Debye Forces)

• Dipole-Dipole

• Hydrogen Bonding

IMFs in Pure Substances

• London Dispersion forces

• Dipole-Dipole

• Hydrogen Bonding

London Dispersion Forces

Also called induced dipole-induced dipole are generally the weakest interactions

• All molecules exhibit LDF

• Non-polar molecules only have LDF(in a pure sample)

London Dispersion forces get stronger with:

• Increasing Polarizability

• Increasing atomic or molecular size

• Increasing molecular weight

• Increasing Surface area of a molecule

Dipole-Dipole Forces

Exist in polar substances

• Stronger as the net dipole increases

• Generally stronger than LDFs

Hydrogen Bonding

A special type off dipole-dipole interaction, where an H atom is directly bonded to an N, O, or F atom

Effects of IMFs: Stronger IMFs lead to

Higher boiling points

Higher melting points

Low Vapor Pressure

IMFs in Mixtures:

Debye Forces and Ion dipole

Debye Forces

When a polar molecule interacts with a non-polar molecule, it induces a dipole in the non-polar molecule

Ion-dipole

When an ionic compound dissolves in a polar solvent, the individual ions interact with the partial charges on polar molecules

• These are generally the strongest IMFs

What keeps matter together?

• Intermolecular forces bring molecules closer

• Kinetic energy keeping molecules apart

At the same time temperature:

• Solids have the strongest IMFs

• Gases have the weakest IMFs

• Liquids are in the middle

Physical properties of Liquids

• Boiling and Freezing point

• Vaporatization and vapor pressure

• Surface Tension

• Capillary action

• Viscoscity

Vaporization

Even when a sample of liquid is below its boiling point, a portion of the molecules will have enough kinetic energy to escape as a gas

Vaporization(rate) increases with:

• Increasing temperature

• Increasing surface area

• Weakening IMFs

Volatile

Liquid that evaporate easily. Ex: alcohol, nail polish remover

Nonvolatile

Liquids that don’t evaporate easily ex: water, motor oil

Vapor Pressure

The molecules in the gas phase will exert pressure on their container

Vapor Pressure depends on:

• The IMFs

• The temperature

• Does not depend on volume

• Higher temps will cause Higher vapor pressure

The normal boiling point is the temperature at which the vapor pressure equals normal atmospheric pressure:

760 torr or 1 atm or 1 bar

Surface Tension

The amount of energy(work) required to increase the surface area of a liquid(or break through)

Two factors affect surface area of a liquid:

IMFs and temperature

• Weak IMFs cause low surface tension

• High temperatures, lower surface tension

Capillary Action

Is the ability of a liquid to flow against gravity into a narrow tube

Cohesive forces

Attraction between like molecules in a liquid (convex)

Adhesive

Attraction between the liquid molecules and the walls of the tube (concave)

Viscosity

A liquids resistance to flow

Factors that affect Viscosity

IMFs and temperature

• Weaker IMFs mean less cohesive forces mean lower viscosity(like molecular size matters)

• Higher temps. Means lower viscosity

Which properties in a liquid indicate strong intermolecular forces?

• High surface tension

• Low Vapor Pressure

• Low Volatility