Intermolecular Forces of Attraction

AP Chemistry 2

SNSH

Spring 2000

Kinetic Molecular Theory

All matter is composed of atoms that are in constant motion

Kinetic Theory Facts

All phases of matter express the degree that they reflect the kinetic theory through their kinetic energy

kinetic energy is measured by temperature phase changes involve changes in

temperature due to the existence threshold temperature of each phase (i.e. ice naturally is found at cold not hot temperatures)

Solids Liquids Gases

Definitesize

Definite size No Definitesize

Definiteshape

No Definiteshape

No Definiteshape

LowKineticEnergy

More KineticEnergy thanSolids, but lessthan Gases

High KineticEnergy

While gases have a great deal of random motion, solids and liquids exist at lower temperatures, thus allowing other forces of attraction to act upon them

these forces are the van der Waals forces

Definitions

Bonds are intramolecular forces of attraction

Forces of attraction between molecules are called intermolecular forces of attraction

intermolecular forces of attraction are commonly called van der Waals forces

The Condensed Phases

Solids and Liquids Physical properties of the condensed phases

reflect the degree of intermolecular forces (i.e. boiling point)

nod ipo le-d ipo le fo rces

yesH ydrogen bond ing

YesIs H bond ing to N , O , or F?

noLondon d ispersion fo rces

(induced d ipo les)

N Ovan der W aals

Are po lar m olecues invo lved?

yesIon-d ipo le forces

noIon ic bond ing

YesAre po lar m olecu les and ions present?

In teracting substancesAre ions present?

Dipole-dipole forces

Exist between neutral polar molecules work best the closer the molecules are to

each other the greater the polarity of the molecules, the

greater the force of attraction

H bonding

Special case of dipole-dipole interaction specifically between H of one polar molecule with N, O or F and an unshared electron pair of another nearby small electronegative ion (usually N, O, or F on another molecule)

VERY STRONG

London dispersion forces

Induced dipoles not really dipoles on the AVERAGE, but

instantaneously dipole conditions can exist thus allowing for pseudopolar regions to occur

No matter how strong the van der Waal force of attraction is, it is still not stronger than attractions involving ions

Ion-dipole forces

Attraction between ions and the partial charge on the end of a polar molecule

ex. NaCl in water solution