Aim: What attraction forces exist among neighboring molecules? I.O.SWBAT: 1)Use the KMT to explain the differences in motion of particles in the gas, liquid, and solid phase. 2)Explain the various type of intermolecular forces of attraction. 3)Identify the expected intermolecular force of attraction given the compounds molecular structure. MOTIVATIVATION:

Do-Now: Compare the physical properties of gases, liquids, and solids. Gases 1.Take the shape of its container 2.Fill container 3.Very compressible 4.Lowest density Liquids 1.Take 2.As 3.Fs 4.sf Solids 1.Sf 2.Fs 3.Sf 4.Sf condensed phases The fundamental difference between states of matter is the distance between particles.

KMT of KMT of Gases: Negligible attractive forces between molecules

KMT of KMT of Liquids: Intermolecular forces of attraction hold molecules together.

KMT of KMT of Solids: Intermolecular forces of attraction lock molecules in place

The state a substance is in at a particular temperature and pressure depends on two opposing forces: –The kinetic energy of the particles. –The strength of the attractions between the particles.

Intermolecular Forces Forces of attraction between molecules. Not nearly as strong as the intramolecular attractions that hold atoms together in molecules. Also called van der Waals forces.

Dipole-Dipole Interactions Molecules that have permanent dipoles are attracted to each other. –The positive end of one is attracted to the negative end of the other. –These forces are only important when the molecules are close to each other. –The more polar the molecule, the greater the dipole dipole attractions. Dipole-dipole attraction ++ --

London Dispersion Forces

Are weak attractive forces between molecules resulting from small, instantaneous dipoles. Polarizability: is the tendency of an electron cloud to get distorted. These forces are present in all molecules, whether they are polar or nonpolar. London Dispersion Forces

Factors Affecting London Forces Surface area long, skinny molecules have stronger dispersion forces than short, fat ones because of greater contact. The strength of dispersion forces tends to increase with increased molecular weight. (Larger particles have larger electron clouds, which are easier to polarize.)

Which of the following has the highest boiling pt? Explain. a. HBr b. HCl

Which Has a Greater Effect: Dipole-Dipole Interactions or Dispersion Forces? If two molecules are of comparable size and shape, dipole-dipole interactions will likely be the dominating force. If one molecule is much larger than another, dispersion forces will likely determine its physical properties.

PRACTICE: 1)HBr boils at 206 K, while HCl boils at 19 K. a) Which molecule has the stronger dipole-dipole attraction? Explain. b) Which has stronger dispersion forces? Explain. c) Which force dominates in this case? Explain.

2) Explain the following data. n-pentane Boiling pt=36.1 o C dimethyl propane (Neopentane) Boiling pt= 9.5 o C

Hydrogen Bonding Unusually strong dipole-dipole attractions occurring among molecules where H is bonded to N, O, or F. Ex. HF, H 2 O, NH 3

Hydrogen Bonding 1.High electronegativity difference. 2.Charges are highly concentrated because of the small size. - H  + of one molecule can get very close to the  - end of another.

Hydrogen bonding in water.

Ion-Dipole Interactions Attraction between an ion and the end of a polar molecule. Ex. NaCl(aq)

SUMMARY 1)Compare 3 intermolecular attractive forces for the following molecules. Molecule Molar mass Dipole moment Bp CH 3 F 34 g/mol 1.81D -78 o C CH 3 OH 32 g/mol 1.70 D 65 o C Molecule dispersion force dipole H-bond CH 3 F slightly greater greater no H-bond CH 3 OH hydrogen bond Hydrogen bond dominates because CH 3 OH has a higher bp. HW # 37

2) Why is H 2 O a liquid and CO 2 a gas at room temperature? 3) Why is the mp of CCl 4 higher than CH 3 F? Molecule Melting pt CH 3 F -78 o C CCl o C Molecule dispersion force dipole H-bond CH 3 F less mass polar no H-bond CCl 4 higher mass non-polar no H-bond Dispersion forces dominate since CCl 4 has a higher m.p.