Chapter 11 States of Matter Part 1 Intermolecular forces

the attractive forces between molecules

are known as intermolecular forces

if you consider the water molecule

the oxygen and the hydrogen atoms are

bonded to each other by chemical bonds

however two water molecules

are attracted to each other

by an intermolecular force

intermolecular forces are much weaker

than chemical bonds

the distance over which

the intermolecular force acts

is longer than the distance of chemical

bonds

so any bond that is in between molecules

is an intermolecular force

you are going to learn about five

intermolecular forces

from weakest to strongest

london forces

dipole-dipole attractions

hydrogen bonding

ion-dipole interactions and ionic

attractions

these intermolecular forces

determine the physical properties of

substances such as boiling point

they also affect how different

substances interact with each other

will two liquids mix

will these two substances be soluble

with each other

so the bulk properties that we observe

depend on

the intermolecular forces therefore it

is very important to understand and be

able to determine intermolecular forces

present in a

substance london forces also known as

dispersion forces

are due to temporary dipoles in an atom

or molecule

the electron cloud in an atom or

molecule

is not a rigid volume it can shift to

the right left from time to time

these temporary shifts create temporary

dipoles

for example a neon atom being a neutral

particle can still have a partial

negative and a partial positive side

because of the temporary shift of the

electrons to one side of the atom

making that side

more electron rich

if there's another neon atom with

similar charge separation

the opposite

charges

will attract each other

this attraction is called the london

force

these dipoles are temporary but when

they shift they shift together

maintaining the attractive force between

two atoms

all compounds exhibit london forces

because all compounds contain electron

clouds

however these forces are significant

only in nonpolar molecules

in polar molecules there are stronger

more significant forces

they still have london forces

but they are not significant

it is very important to know the factors

that affect the strength of london

forces

one of those factors is the size of the

molecule

the bigger the molecule the larger the

surface area and the stronger the london

forces

cling wrap for example

contains very large molecules called

polymers

which

interact with each other through london

forces

the layering of the

polymer chains

creates an attractive force

that's why the wrap clings

also the shape of the molecule affects

the london forces

the more linear the molecule the

stronger the london forces

compare these two isomers

with the same molecular formula but with

different structural arrangements

the linear arrangement

results in linear molecules

where there is

a greater area of interaction between

molecules

whereas the branched isomer

is

like a ball

and the interaction between two

molecules is over a very small area

so the molecule with the larger area for

interaction

will exhibit stronger intermolecular

forces

this is another example of how size

affects london forces

in the straight chain alkanes pentane

with five carbons and octane with eight

carbons

the interaction

surface area between molecules

is larger in octane

there is more surface contact among the

molecules

therefore octane has stronger

intermolecular forces

than pentane

london forces also explain why geckos

can walk upside down

on the ceiling

geckos have cete

hairs on their hands

if you take an electron microscopy image

of the hair area

you will see

micro hairs that extend from the hand

and these micro hairs split into nano

hairs

at the tip of which there is a flat area

called a spatula

this flat area

increases the surface area of contact

of the gecko's hand and the surface

there are about 200 000 sete at any time

applied on a surface

so this attractive force

allows the gecko to be able to walk

on a wall or upside down on the ceiling

the next stronger intermolecular force

is dipole-dipole attractions

this is observed in polar molecules

polar molecules have permanent dipoles

each molecule is a dipole die means two

pole means charge so there are permanent

charged

sides of the molecule acetone for

example has a permanent partial positive

and a permanent partial negative end

so two acetone molecules will attract

each other from partial negative end of

one

to the partial positive end of the other

these attractions are stronger than

london forces because permanent dipoles

are stronger than

temporary dipoles

of course molecules that are polar that

exhibit dipole-dipole attractions

also have london forces but these are

negligible

dipole-dipole interactions

decide the physical properties

the next stronger intermolecular force

is hydrogen bonding this is an extreme

case of dipole-dipole attractions

it is observed when there is a hydrogen

atom

directly bonded to a fluorine

oxygen or nitrogen

the resulting hydrogen bonding is much

stronger than dipole-dipole forces

the bonding requires the interaction of

a donor hydrogen

a hydrogen bonded to one of these atoms

and an acceptor pair of electrons

for example

in the

hn bond

the hydrogen is bonded to nitrogen and

the nitrogen has a lone pair

the lone pair

on a nitrogen on another molecule

will have an attractive force to the

hydrogen

on this molecule

examples of hydrogen nitrogen bonds that

show hydrogen bonding are ammonia

methylamine

dimethylamine

but not trimethylamine

this molecule trimethylamine

is not going to exhibit hydrogen bonding

because there is no hydrogen directly

bonded to nitrogen

likewise a hydrogen on an oxygen

in a molecule

will result in hydrogen bonding in

between molecules

water

and methanol are examples there are

oxygen atoms bearing

carrying hydrogen atoms but if you look

at the ether molecule

even though there is an oxygen and some

hydrogen atoms there will be no hydrogen

bonding because the oxygen itself does

not carry

a hydrogen atom

also the hydrogen fluoride molecule

exhibits hydrogen bonding but the only

example here is the hydrogen fluoride

itself because

fluorine makes only one bond

there is no other bond from fluorine

going to other atoms

in the diagram that is drawn for water

we can see the hydrogen bond donor and

the hydrogen bond acceptor

the donor

is

the hydrogen on the oxygen

and the acceptor is the lone pairs on

the oxygen

so water has both of them on the same

molecule it has the hydrogen

which is the donor

and the lone pair which is the acceptor

because water has two donor electron

pairs and two acceptor hydrogens

it can make

four hydrogen bonds per molecule

the two will be from hydrogen to the

lone pairs to another water

and two will be from the lone pairs on

itself to the hydrogens of two other

water molecules

hydrogen bonding is also

the attractive force that keeps the

double helix of dna together

the lone pairs on oxygen on one side of

the chain

attract the hydrogen

on the other side of the chain which is

bonded to nitrogen

the criteria for hydrogen bonding is

satisfied because the hydrogen is on a

nitrogen

and it's attracted to the lone pairs of

an oxygen

or in this case a nitrogen

water can make hydrogen bonding because

of the oh group on it

so any molecule with the oh group on it

can make hydrogen bonding

glycerol has three of them methanol has

only one but all these molecules are

capable of making hydrogen bonding

linear molecules if they are long enough

can exhibit intramolecular hydrogen

bonding

there could be an oh on one end of the

molecule and an oxygen on the other end

and it can close into a cyclic structure

it doesn't form an actual covalent bond

but just a hydrogen bond but keeps it in

cyclic form this is intermolecular

hydrogen bonding

between two different molecules hydrogen

bonding can exist for example

acetone

contains an oxygen with lone pairs those

lone pairs will be hydrogen bonding to

the hydrogen atoms of water molecules

same with ethanol

and water the lone pairs on oxygen can

hydrogen bond to water

but because it also contains a hydrogen

on an oxygen

they can hydrogen bond to

water molecules and also other ethanol

molecules as well

hydrogen bonding is the strongest

intermolecular force observed in neutral

molecules

therefore substances that exhibit

hydrogen bonding have abnormally high

boiling points

if you consider the hydrogen compounds

of group 6a elements

lower molecular weight compounds have

lower boiling points

from hydrogen tolerant to hydrogen

selenide to hydrogen sulfide

following this trend we would expect

water to have a boiling point of less

than negative degrees celsius

water is a very small and a very low

molecular weight compound however the

observed boiling point of water is

positive 100 degrees celsius

which is much much higher than negative

60 degrees celsius this extremely high

boiling point is due to hydrogen bonding

if water did not make hydrogen bonding

it would have a boiling point less than

the temperature of our planet

there will be no liquid water all water

will be found as gas

and because liquid water is needed for

life there would be no life on earth

the next intermolecular attraction is

ion attractions

as the name implies these are between an

ion

and a dipole a polar molecule

this is an important attractive force

often seen in biological systems

and because the ion carries a full

charge

it is stronger than hydrogen bonding

when sodium chloride dissolves in water

the attraction between the cation sodium

ion and water

its partial negative side is an ion

dipole attraction

similarly

the attraction between the chloride ion

and the partial positive side of the

water molecule

is also an ion dipole attraction

the strongest of all intermolecular

attractions is ionic attraction

it is the strongest because it involves

full charges

these are also called salt bridges

a carboxylate group of one molecule

and the protonated amine

with a positive charge

will attract each other by a very strong

attractive force

this is called the ionic bond or the

salt bridge but remember this is still

an intermolecular force

proteins and enzymes

they are made by amino acids linked to

each other

amino acids contain functional groups

and when placed along a long chain

can interact with each other with other

side groups

that are on different parts of the chain

this results in the folding of the amino

acid chain

in a specific way

which gives the enzyme its specific

shape the interactions between different

parts of the chain

are all intermolecular forces

for example a salt bridge keeps these

two parts together

a hydrogen bond keeps these two parts

together

again hydrogen bonds exist extensively

the hydrophobic interaction here

is

another word for london forces

there's another hydrogen bond that keeps

this two parts together

so as you see

intermolecular forces play a very

important role

in protein folding which gives the

enzymes their specific shape

in order to determine the intermolecular

forces in a compound

you have to first determine the polarity

of the compound

we can do that by a simple questionnaire

first asked whether the central atom

have any lone pairs or not

if yes

then the molecule is polar

as in the case of sulfur difluoride the

central atom has lone pairs it is a

polar molecule

if the answer is no there are no lone

pairs on the central atom ask the second

question

are the atoms bonded the same type

if yes the molecule is nonpolar this is

the case in carbon dioxide

the central atom carbon does not have

any lone pairs

and the atoms bonded are both oxygen

atoms

therefore

the molecule is nonpolar

if the answer is no

if the atoms bonded are not of the same

type then the molecule will be polar

once you determine whether the molecule

is polar or not you can determine the

intermolecular forces it will exhibit

if the molecule is not polar

it will exhibit only london forces

if the molecule is polar

it will exhibit dipole-dipole and london

forces dipole-dipole being the stronger

one

if the molecule is polar and

it contains a hydrogen on a nitrogen

oxygen or a fluorine

then hydrogen bonding will also be

present

it will be the strongest

intermolecular force

but if there's no hydrogen to nitrogen

oxygen fluorine connection

there will be no hydrogen bonding

looking at sulfur difluoride

it is a polar molecule

so it will have dipole-dipole and london

forces

but also it does not contain a hydrogen

on one of these atoms

therefore it does not exhibit hydrogen

bonding so it has

only dipole dipole and london forces

carbon dioxide we determined to be

a nonpolar molecule therefore it has

only london forces

let's identify the intermolecular forces

that are expected for these substances

oxygen is a nonpolar molecule

therefore it exhibits only

london forces

hydrogen peroxide is a polar molecule

and it also has hydrogen bonded to

oxygen

so it contains

london forces dipole-dipole forces and

hydrogen bonding

hydrogen bonding being the strongest of

all

tribroma methane

is a polar molecule

but it does not make hydrogen bonding

so therefore it has london forces

and dipole-dipole forces

now pause and determine the kinds of

intermolecular forces that exist in

these four substances

make sure you use the lewis structures

now take your time to answer this

question about the intermolecular forces

observed in the given molecules

look around you right now you will see

substances that are solids

liquids

such as water and gases such as the air

you are breathing

so what determines the physical state of

matter at a given temperature

why are certain things solids liquids or

gases

there are two major factors that

determine the state of the material at a

given temperature

the kinetic energy of the particles

try to break the particles free

separate them from each other

but the particles are held together by

intermolecular forces

these two factors

are in competition with each other

for a gas at room temperature

the intermolecular forces are so weak

that the kinetic energy has completely

dominated over them

therefore the gas molecules

are completely free from each other

they don't feel any attraction

towards each other

the kinetic energy has beaten

intermolecular forces therefore the

state is a gas

compare that to solids

the kinetic energy of the particles is

there

but the attractive forces intermolecular

forces are so strong

that the molecules

are frozen in place they cannot break

free from each other

in a liquid compared to a solid the

intermolecular forces are slightly

loosened but still

they are

not allowing the molecules to break free

from each other to form a gas

so intermolecular forces play an

important role in the physical state of

matter