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2.5 Intermolecular Forces

Recognise the different kinds of IMFs.

Describe how IMFs take place.

Relate physical properties of materials to IMFs.


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Non-polar Molecule Cl2


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in any liquid or solid there are bonds betweenmolecules. Called intermolecular bonds.

Intermolecular bonds need to be broken for asubstance to melt or boil.

The stronger the bonds, more energy is needed tobreak them.

(resulting in higher melting and boiling temps)

Smaller atoms exert more pull on bonding electrons as thenucleus is closer.

When different atoms are bonded the pull on electrons is

determined by the relative core charges of the atoms.


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Dipole Dipole bonds

Molecules with permanent dipoles have atoms withdifferent electronegativity values.

Slightly negative end of a molecule attracts to the

slightly positive end of another molecule, therefore

an intermolecular bond occurs:

dipole dipole bond

Used in holding polyester together


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Hydrogen Halides

Dipole Dipole bonds present + HCl -polar

+ HCl -+ HCl -

Resulting in high boiling points

The hydrogen halides are colourless gases at room

temperature, producing steamy fumes in moist air.


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polar molecules

Dipole - Dipole Interactions


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London Forces or Van der Waals Forces

Forces of attraction between two non polar

molecules (charge distribution)


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Temporary or instantaneous dipoles

If a molecule does not have a permanent dipole the electron

density in the molecule may be unevenly distributed at anyone time it has an instantaneous dipole.

If other molecules are close to a molecule with a dipole these may

cause an effect and produce an induced dipole.


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Induced dipole-dipole forces

Even in molecules with no polar bonds, there aretemporary dipoles due to uneven electrondistribution due to the constant movement ofelectrons.

This induces a temporary dipole in a neighbouringmolecule, producing a temporary induced dipole-dipole attraction.

The bigger the molecule (i.e. the more electrons),the greater the induced dipole-dipole.


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Weakest type of intermolecular bond. Can happen in all types of molecules.

instantaneous dipole induced dipole bonds

Krypton atoms:

electrons continually moving, creating instantaneousdipoles. When other krypton atoms approach an atomwith an instantaneous dipole they will produce aninduced dipole. Because the electrons are continuallymoving these bonds are continually forming andbreaking. The more electrons an atom has thegreater the attractions (therefore higherboilingand melting points).


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Instantaneousdipoleinduceddipole


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Poly(ethene) has instantaneous dipole induced dipole bonds

-These kind of forces occur between poly(ethene)

-Yet poly(ethene) is a solid at room temperature

Because:-Chains are long-Can pack closely together

-Therefore lots of bonds(although bonds initially very weak)


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Quick Questions Would you expect xenon or krypton to have the

higher boiling point?

How do instantaneous dipole-induced dipole

bonds arise between molecules of hydrogen?

What is the strongest type of intermolecular bond

between molecules of ethane?

Xenon more electrons, so instantaneous dipole induced dipole

larger, more energy is needed to pull xenon atoms apart.

At any one moment the electron cloud can be unevenly distributed

causing an instantaneous dipole, electrons in a nearby H can be

attracted or repelled causing an induced dipole. The +ve and ve

ends of these two molecules are temporarily attracted.

instantaneous dipole

induced dipole bonds


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In a hydrogen bond atoms held together because nuclei are bothattracted to the bonding pair.

Large dipole between a small hydrogen atom and a highlyelectronegative atom (such as O, F or N)

Hydrogen bonds

A strong intermolecular force it is NOT a bond!


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3 things needed for a hydrogen bond A large dipole between an H atom and a highly electronegative atom

(O, N or F)

The small H atom which can get very close to O, N, or F atoms in

nearby molecules.

A lone pair of electrons on the O, N or F atom, with which the

positively charged H atom can line up.

H

NH

H

H

NH

H

Liquids with hydrogen bonds have:

a higher than expected viscosity. (glycerol x3 OH groups)

are often soluble in water (H-bond between H20 and substance)

helps fibres absorb water (wool O and N atoms H-bond to water)


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Explain the trend in boiling points of the Hydrogen

Halides, HCl HI

Why does HF have a higher than expected value.

HF +600C

HCl -1200C

HBr -850C

HI -200C

HF 600C


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Explain the trend in boiling points of the Hydrogen

Halides, HCl HI

Why does HF have a higher than expected value.

HF +600C

HCl -1200C

HBr -850C

HI -200

C

Down the group HCl- HI: the atomic radius increases and the

surface area increases so there is more area for more IMF forces

to form. Also HI has the strongest induced dipole forces because

it has the greatest number of electrons, thus requiring more

energy to break and have a higher boiling point.

HF is a lot higher because it has the ability to hydrogen bond

which is a much stronger intermolecular force.


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Trends increasing surface area = increasing boiling point

Increasing Molecular/Atomic mass = increasing boilingpoint

Longer straight chain = higher boiling temp


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Volatility

Alcohols have higher boiling points (lessvolatile) than their corresponding alkane.

Why?

As well as the IMF that form between both alcohols

and alkanes, alcohols can form hydrogen bonds. The

electronegative oxygen can hydrogen bond to the

partially positive hydrogen of near by molecule.

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