Lecture 27 © slg CHM 151

TOPICS:

1. Intermolecular Attractions2. Solubilities and Boiling Points3. Finals Week Overview

N

H

H

H

H

non polar

N 3.0 H 2.1

0.9polarbonds Br

F

F

F

F 4.0Br 2.8

1.2

polarbonds

dipole

F

C

HH

F

F 4.0C 2.5

1.5

polar bonds

C 2.5H 2.1

0.4slightly polar

dipole

Br

F

F

F

F

F 4.0Br 2.8

1.2

polarbonds

dipole

N

OO

O

O 3.5N 3.0

0.5bonds polar

non polar

N

H

H

H

N 3.5H 2.1

1.4

dipole

H

C

ClCl

Cl

Cl 3.0C 2.5

0.5bonds polar

C 2.5H 2.1

0.4

bonds slightly polar

N

OO

O 3.5N 3.0

0.5

bonds polar

Br

F

F

F 4.0Br 2.8

1.2

non polarBr

F

F F

F

F

Group Work

What volume of O2, collected at 22.0 oC and 728mm Hg would be produced by the decomposition of8.15 g KClO3, M = 122.5 g/mol?

2 KClO3 (s) 2 KCl (s) + 3 O2(g)

1. Calculate moles of O2 from equation

2. Calculate V of O2 from ideal gas law

8.15 g KClO3

122.5 g KClO3

1 mol KClO3

2 mol KClO3

3 molO2

= .0998 mol O2

V= ? LT= 22.0 oC = 295.1 K P= 728 mm Hg R= 62.4 L mm Hg K-1 mol-1

2 KClO3 (s) 2 KCl (s) + 3 O2(g)

8.15 g

122.5 g/mol

V, L =?

V = nRT

P=

.0998 mol x 62.4 L mm Hg K-1 mol-1 x 295.1 K

728 mm Hg

= 2.52 L

New Material, Not Included, Test 5 But included, ACS Test....

Attractive or Intermolecular Forces, an Overview:

•Ion-ion (greatest, full charges, solid metal salts)

•Ion- dipole (very strong: hydrated ions)

•dipole-dipole (polar molecules)

• “induced dipole” - “induced dipole” (close packed non polar molecules) (small but increases with surface area)

ION- ION

ION - DIPOLE

DIPOLE - DIPOLE (H-Bonding, greatest)

DIPOLE - INDUCED DIPOLE

INDUCED DIPOLE - INDUCED DIPOLE, (* tiniest of attractions but becomes very significant when molecules are very large!)

Strength of Attraction

O

HH

H2O MOLECULE: STRONG DIPOLE

X, O: 3.5 X, H: 2.1 X = 1.4

+O

H

H

O

H H

O

H H

O

H

H

- O

H

H

O

H H

O

H

H

O

H H

ION - DIPOLE ATTRACTIONS:Salts in water

C

H

H

Cl

H

~+

~+~+

~-

~+

~-

"DIPOLE"

2.1

2.5

3.0

Methyl Chloride: Dipole

DIPOLE-DIPOLE ATTRACTIONS

Random Liquid DIPOLE - DIPOLE ATTRACTIONS

Hydrogen Bonding:Especially Strong Dipole-Dipole Attraction

When H (X = 2.1) is bonded to F, O, N (X = 4.0, 3.5, 3.0), the bond is polarized strongly towards the moreelectronegative atom.

Recall that the H atom consists of one proton (p+) in the nucleus and one electron (e-) outside the nucleus.

Uneven sharing of that lone electron in a polarizedcovalent bond leave H as an exposed proton with no surrounding electronic cloud.

H F

X = 2.1 X = 4.0

H O

X = 2.1 X = 3.5

H N

X = 2.1 X = 3.0

HOp+

e-

H O

HFp+

e-

H F

H

Np+

e-

H N

H

p+

e-

F O N

atom

H’s lone electron

Exposed positive charge

When hydrogen exists in this polarized bonding situation (or as H+) it is often referred to as what itis: a “proton”.

More importantly, when this occurs, the H atom forms a secondary bond to a pair of unshared electrons onanother N, O or F as illustrated on the next slides. Thissecondary bond is described as “hydrogen bonding.”

Biologically, this “hydrogen bond” is of greatestconsequence: DNA and protein molecules are boundtogether by these fragile but essential attractions.

F H F H

HYDROGEN BOND

~-~+ ~+

~-

HYDROGEN BOND

NH

HH

NH

HH

NH

HH

NH

HH

Hydrogenbonds

O

H H

O

H H

O

H H

O

H H

Water, H2O, exhibits the highest degree of hydrogen bonding of all molecules, having two hydrogens and two unshared pairs per molecule.

When solid (ice), all the water molecules are lockedinto a rigid structure in which all the molecules are “tetrahedrally” joined to each other.

In liquid water the attachment is more mobile: the particles can flow past each other, exchanging one attraction for another (higher energy state!).

Escape to the gas (vapor) state requires especiallyhigh energy, and the boiling point for “hydrogenbonded” molecules is quite unusually high for the size of the molecule.

Special Reading Assignment

To really appreciate the impact of hydrogen bondingon the properties of water we experience, be sureto read the insert in Kotz, p. 594 entitled:

A CLOSER LOOK: The Unusual Properties of Water:A Consequence of Hydrogen Bonding

BP, Various Molecules

H2 -253 CH4 -164

N2 -196 NH3 -33

O2 -183 HF +19.5

F2 -188 H2O +100

H BONDINGNON POLAR

Finally, the weakest set of attractions, “induced dipole-induced dipole”:

When any molecule gets close enough to another molecule(liquid, solid state) so that the molecules are “touching”,induced polarity sets in.

The electron clouds surrounding any set of bonds in themolecule will be repulsed by similar clouds on another molecule.

The clouds will shift away from each other (“polarize”), andsome positive attraction from the nuclei of one atom canattract electron clouds on a neighboring atom.

+ ++

+++

Induced dipole-dipole: surface attractions

Exhibited by all molecules in liquid or solid state!

Effect of Intermolecular Attractions on BP’s

A liquid will boil at a temperature at which all its particles have sufficient energy to escape into thegas state.

Specifically, a liquid will boil when its “vapor pressure”is equal to the pressure on its surface, usually atmospheric.

“Vapor Pressure” is the pressure exerted by molecules escaping from the surface of any liquid.

“Normal Boiling Point” (H20, 100oC) is reached officiallywhen the vapor pressure of the liquid reaches 1 atm.

Volatile slightly volatile non-volatile

TALE OF THREE LIQUIDS AT ROOM TEMPERATURE

high vapor pressure

Evaporates quickly

low boiling point

low intermolecular attractions

low vapor pressure

Evaporates slowly

higher boiling point

medium intermolecular attractions

no RT vapor pressure

does not evaporate

high boiling point

high intermolecular attractions

a) mass of the particles involved (the bigger the more gravitational attraction)

b) surface area (the more surface area, the more“induced dipole-induced dipole effect”)

c) intermolecular attractions

Boiling Points are a composite property, resulting from:

By comparing particles of same general mass and size,one can deduce degree of intermolecular attraction.

The four following molecules have about same M and surface area, but note the range in BP’s.

Volatile

H3CC

O

CH 3

CH3COCH3 acetone

BP 56.2o

Dipole-Dipole attractions

H3C

H2C

CH 2

CH3CH2CH2CH3 BUTANE

BP -0.5o

induced dipole-dipole

CH 3

H3CCH

OH

CH 3

CH3CHOHCH3 isopropyl alcohol

BP 82.4o

Hydrogen Bonding

slightly volatile non-volatile

CH2OHCH2OH ethyleneglycol "antifreeze"

BP 198o

Hydrogen Bonding

H2C CH 2

OHOH

A word on attractions and solubilities: “Like dissolves like”:

Hydrocarbon, Organic Solvent (acetone, ethers) soluble:

• slightly polar substances• non polar substances

fats, oils, creams, many paints and dyes, etc

Water soluble:• Many ionic solids (especially Na+ NH4

+ salts)• hydrogen bonded substances (alcohols, sugars)• small polar substances (acetone)

Unit Five Test:

• will cover: a) shapes, bond and molecular polarity b) gas laws (just like homework)

provided: PT’s, R values, all needed electronegativity values • will not include chapter 13

“That’s All Folks!!!”

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