new unit 8

IMFAs

IMFA: intermolecular forces of attraction

“bricks”— individual atoms, ions, or molecules of a solid

“mortar”— holds the separate pieces together(the IMFA)

IMFA: intermolecular forces of attraction

types of IMFAstrongest

weakest

London forces

dipole-dipole attraction

hydrogen bond

metallic bond

ionic bond

covalent network

occurs between

non-polar molecules

polar molecules

ultra-polar molecules(those with H–F, H–O, or H–N bonds)

metal atoms

cations and anions (metals with non-metals in a salt)

atoms such as C, Si, & Ge (when in an extended grid or network)

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details about each IMFAstrongest

weakest

London forces

dipole-dipole attraction

hydrogen bond

metallic bond

ionic bond

covalent network

London (or dispersion) forcesnon-polar molecules (or single atoms)

normally have no distinct + or – poleshow can they attract each other enough

to condense or freeze?they form temporary dipoleselectron clouds are slightly distorted by

neighboring moleculessort of like water sloshing in a shallow

pan

London dispersion forces in action

non-polar molecules, initially with uniform charge distribution

1. temporary polarization due to any random little disturbance

δ+ δ-

2. induced polarization caused by neighboring molecule

3. induced polarization spreads

4. induced polarization reverses

dipole-dipole attractionspolar molecules have permanent dipolesthe molecules’ partial charges (δ+, δ-)

attract the oppositely-charged parts of neighboring molecules

this produces stronger attraction than the temporary polarization of London forcestherefore polar molecules are more likely to be

liquid at a temperature where similar non-polar molecules are gases

dipole-dipole attractionsδ+ δ-

hydrogen bonding (or ultra-dipole attractions)

H—F, H—O, and H—N bonds are more polar than other similar bondsthese atoms are very small, particularly HF, O, and N are the three most electronegative

elementsthese bonds therefore are particularly polar

molecules containing these bonds have much higher m.p. and b.p than otherwise expected for non-polar or polar molecules of similar mass

the geological and biological systems of earth would be completely different if water molecules did not H-bond to each other

hydrogen bonding (or ultra-dipole attractions)

non-polar molecules(lower boiling points)

ultra-polar molecule(much higher boiling point)

hydrogen bonds (between molecules, not within them)

hydrogen bonding (or ultra-dipole attractions)

H H

O H H

O

H H

OH H

O

Beware!!These are not hydrogen bonds. They are normal covalent bonds between hydrogen and oxygen.These are hydrogen bonds. They are between separate molecules (not within a molecule).

metallic bondingstructure

nuclei arranged in a regular grid or matrix

“sea of electrons”—delocalized valence electrons free to move throughout grid

metallic “bond” is stronger than van der Waals attractions but generally is weaker than covalent bond since there are not specific e– pairs forming bonds

resulting propertiesshiny surfaceconductive (electrically and

thermally)strong, malleable, and ductile

alloy = mixture of metals

ionic bonding (salts)structure: orderly 3-D array

(crystal) of alternating + and – charges

made ofcations (metals from left side of periodic

table)anions (non-metals from right side of

periodic table)

propertieshard but brittle (why?)non-conductive when solidconductive when melted or dissolved

why are salts hard but brittle?

1. apply some force

2. layer breaks off and shifts

3. + repels + – repels –

4. shifted layer shatters away from rest of crystal

covalent networksstrong covalent bonds hold together

millions of atoms (or more) in a single strong particle

propertiesvery hard, very strongvery high melting temperaturesusually non-conductive (except graphite)

examplescarbon (two allotropes: diamond, graphite)pure silicon or pure germaniumSiO2 (quartz or sand)other synthetic combinations averaging 4 e–

per atom: SiC (silicon carbide), BN (boron nitride)

m.p. = ~1600°C

m.p. = 3550°C

C60buckminsterfullerine

“bucky ball”

summary of propertiesstrongest

weakest

London

dipole

hydrogen

metallic

ionic

network

strength

soft and brittle

strong, malleable, ductile

hard but brittle

extremely hard

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m.p. & b.p.

low

medium to high

medium to high

very high

conductive?

no

very(delocalized e–)

if melted or dissolved(mobile ions)

usually not(except graphite)

consequences of IMFAsmelting points and boiling points rise with

strength of IMFAincreasing molar mass

substances generally mix best with other substances having the same or similar IMFAs”like dissolves like”non-polar mixes well with non-polarpolar mixes well with polar(polar also mixes well with ultra-polar and

ionic)other physical properties such as

strength, conductivity, etc. are related to the type of IMFA

predicting melting points, boiling pointsstronger IMFAs cause higher m.p. and

higher b.p.when atoms/ions/molecules are more strongly

attracted to each other, temperature must be raised higher to overcome the greater attraction

more polar molecules have higher m.p. and b.p.

atoms and molecules that are heavier and/or larger generally have higher m.p. and higher b.p. larger/heavier atoms (higher molar mass) have

more e–

larger e– clouds can be distorted (polarized) more by London or dipole forces, causing greater attraction

strategy to predict m.p. and b.p.first sort atoms/molecules into the six IMFA

categoriesthen sort those in each category from lightest

to heaviest

same IMFA: sort by molar mass

thus at room temperature: F2 (g)

Cℓ2 (g)

Br2 (ℓ)

I2 (s)

°C

–250

–200

–150

–100

–50

0

+50

+100

+150

ex: halogen familyall are non-polar (London

force) lowest to highest m.p. and

b.p. matches lightest to heaviest

–219.62F2

(38)

melt boil

–101.5Cℓ2

(71)

–7.2Br2

(160)

+113.7I2

(257)

–182.95F2

(38)

–34.04

+58.8

+184.4

Cℓ2

(71)

Br2

(160)

I2

(257)

same mass: sort by IMFA type

°C

–50

0

+50

+100

+150

ex: organic molecules

all are ~60 g/moldifferent types of

IMFA

–0.5 butane (non-polar)

+10.8 methyl ethyl ether (slightly polar)

+56.2 acetone (more polar)

+97.4 1-propanol (ultra-polar = H-bonds)

+198 ethylene glycol(can form twice as many H-bonds)

the stronger the IMFA, the higher the boiling point

isomers (and an isobar)

n- and neo pentane

glycerol and 1-propanol

1-propanol and methyl ethyl ketone

butane and 2-methylpropane

1-propanol and 2-propanol

soaps and emulsifiers

some molecules are not strictly polar or non-polar, but have both characteristics within the same molecule

non-polar

region

polar region

this kind of molecule can function as a bridge between molecules that otherwise would repel each other

oil

water

soap or

emulsifier

soaps and emulsifiers

with a soap or emulsifier present to surround it, a drop of non-polar oil can mix into polar water