Lecture 34 Chapter 11 Sections 1 – 4

• Intermolecular interactions

• Dispersive

• Dipolar

• Hydrogen-bonding

• Phases: gas, liquid solid

• Solids

Ideal Gas Law vs. van der Waals Eq.

• Remember Ideal Gas Law

• Empirically explained by the van der Waals equation

1or ==nRTpVnRTpV

( ) nRTnbVV

anp =−

+ 2

2

attraction molecular size

Melting and Boiling Points

• Both are indicators of the strengths of intermolecular forces:– Normal freezing point (fp): the temperature at which a solid

and liquid coexist at equilibrium under a pressure of 1 atm– Normal boiling point (bp): the temperature at which a liquid

and vapor coexist at equilibrium under a pressure of 1atm• Vaporization: l g• Condensation: g l

Types Intermolecular Interactions

1. Dispersion – attraction between the negatively charged electron cloud of one molecule and the positively charged nuclei of a neighbor molecule.

2. Dipolar – attraction between negatively charged portions of one molecule with the positively charged portions of another molecule.

3. Hydrogen bonding – attraction between lone pair electrons on an O, N or F atom with a partially positive hydrogen atom.

Dispersion Forces• Dispersion forces are found in all molecular substances• Such forces are electrostatic in nature and arise from

attractions between neighboring nuclei and electrons. Sometimes called induced dipoles.

• The magnitude of dispersion forces depends on how easy it is to polarize the electron cloud of a molecule.

• A larger molecule (more electrons) has a larger polarizability.

Boiling Point vs Molecular Size

Dipolar Forces

• Occur when one polar molecule encounters another polar molecule.

• The positive parts are attracted to negative parts.• Dipolar forces are usually stronger than dispersion forces.• Dipolar forces increase with an increase in the polarity of

the molecule.

Models of acetone and 2-methylpropane:

What is the big difference?

Acetone is a polar molecule, 2-methylpropane is not.

Therefore, acetone will experience dipolar forces, but 2-methylpropane will not.

2-methylpropane acetone

Order three molecules from lowest to highest boiling point.

54321

A B C

25%25%25%25% 1. lowest A B C highest

2. lowest C B A highest3. lowest A C B highest4. lowest B C A highest

Hydrogen Bonding Forces

1. One molecule has a hydrogen atom attached by a covalent bond to an atom of oxygen, nitrogen, or fluorine, i.e. a polar H-X bond

2. The other molecule has a slightly negative oxygen, nitrogen, or fluorine atom.

This is mostly just the attraction between slightly positive H and slightly negative O, N, or F

NOT REALLY A BOND

This is the strongest of all intermolecular forces

Many of water’s unusual properties are due to H-bonding

Solid vs. Liquid vs. Gas

• Gas– little intermolecular interaction– take on any shape– expand to fill any volume

• Liquid– strong intermolecular interaction– take on any shape– very resistant to compression (fixed volume)– exhibit surface tension, capillary action, viscosity, vapor pressure

• Solid– very strong intermolecular interaction– fixed volume and shape– extensive variety of molecular packing

Surface Tension

• The resistance of a liquid to an increase in its surface area. ORA liquid tends to minimize its surface area.

• The surface molecules of a liquid have a net inward force of attraction, forming a “skin”.

• The toughness of the skin is called surface tension.

Capillary Action– The upward movement of water up a capillary against

the force of gravity.– Due to attractive forces between the glass (polar) and

the water (also polar)

Viscosity– Resistance to flow– For example, water vs. oil

• Dependent on intermolecular forces• Dependent on size & shape of molecules

Vapor Pressure

Vapor pressure: the partial pressure of a vapor in dynamic equilibrium with its liquid (in a closed container).

Forces in solids

• In liquids and gases, molecules are free to move continually and randomly.

• In solids, particles are constrained to fixed positions.• Particles can only vibrate and occasionally rotate.• There are four major types:

– molecular solids– network solids– metallic solids– ionic solids

Forces in Solids

Network Solids of Carbon

Today• Finish reading Chapt 11• Finish CAPA• Take last year’s exam

Wednesday• Problems, problems, problems

Remember: You are done with the homework when you understand it!