intermolecular_forces1

8/11/2019 Intermolecular_Forces1

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Intermolecular Forces


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Intermolecular Forces

When molecules are attracted to other

molecules by a weaker force (when compared toan ionic bond or a covalent bond holding amolecule together)

Intermolecular forces refers to an attraction

between molecules, intramolecularforces arethe forces within the molecule themselves

when a substance evaporates, melts, orsublimates (change of state), the molecules gain

kinetic energy from an outside source, thisallows them to overcome the intermolecularforces holding them close together


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there are no intermolecular forces in ionic

compounds because ionic compounds do not

contain molecules! Ionic compounds are solids at room temperature

In an ionic crystal ionic bonds hold all the ions

together, there is no difference between the bonds

holding the compounds together and those holding

the crystal together

Ionic bonds are generally very strong and accounts

for the high melting pointIonic crystal for NaCl


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Many molecular substances are gases at roomtemperature, others are liquids with low boilingpoints or solids that melt easily

Forces between the molecules in molecular solids andliquids are relatively weak, addition of thermalenergy can easily overcome these intermolecularforces

The strength of intermolecular forces determines thephysical properties of molecular compounds: state,melting point, boiling point, surface tension,hardness, texture and solubility

As the intermolecular forces between moleculesincreases, the compounds m.p., b.p. and surfacetension also increase


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Types of Intermolecular Forces

Dipole-dipole Forces

Intermolecular force between oppositelycharged ends of a polar molecule

Ex: H-Cl

These are generally strong intermolecular forces

The more polar a substances is (refer toelectronegativity and shape), the stronger thedipole-dipole force of attraction


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London Dispersion Forces

All gases will condense to from liquids (even noble gases) when cooled

enough which implies that there is some type of attraction between the

entities when they are cooled These attractions are caused by the temporary shifts in the electron

cloud in an atom or molecule

This shift creates a temporary dipole for a fraction of a second that will

also cause a temporary dipole in an adjacent atom or molecule

Very short lived as the attraction between them continually forms andbreaks, as a result these London forces are very weak

The larger the molecule, the more electrons and protons there are

attracting one another, the stronger the forces, the higher the melting

point

London dispersion forces exist between all molecules even polar

molecules

In polar compounds the London forces are insignificant compared to the

much stronger dipole forces

dipole-dipole forces and London forces are called Van der Waals forces in

honour of the scientist who proposed their existence


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Hydrogen Bonds

special category of very strong dipole-dipoleintermolecular forces

a hydrogen bondis a particularly strong dipole-dipoleforce that occurs between two molecules

each consists of a hydrogen atom covalently bonded to ahighly electronegative atom of nitrogen, oxygen orfluorine

ex: H-F

two reasons for hydrogen bonding are the large differencein electronegativity and the small size of the hydrogenatom (positive pole is highly concentrated and attracts thenegative pole of a nearby molecule)


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...in biochemistry

hydrogen bonds play a significant role in

determining the shape and function of large,

biologically important molecules (proteins

such as enzymes, hormones, antibodies,

DNA)

DNA Protein


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Hydrogen Bonding and Water

one of most abundant compounds on Earths surface

approx. 22% of Earths fresh water is in Canada

in nature water contains many dissolved substances

necessary for life

the water cycle, plants and marshes recycle and purify water

but cannot take out some pollutants which are dangerous toour health and the health of the planet

water is becoming increasingly contaminated by caffeine,

cosmetics, hormones, food additive, antibiotics, despite our

desperate need for clean water when compared to other similar compounds, water is

differentit has some very strange properties


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a) unusually high melting and boiling points

water is the only substance that commonly

exists in three states

the mp and bp are much higher for water

than other similarly structured compounds

implying the forces holding water togetherare very strong

bent shape, polar bonds and two lone pairs

of electrons on the central oxygen atommake water one of the most polar molecules

on Earth


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b) low density of ice

solid ice floats on water, the solid is less dense

ice also acts as a thermal blanket holding in the heatof the water below it (as in a frozen lake)

at 4oC water is most dense and as it gets closer tofreezing it moves to the top of the lake and changesstate as it become less dense (leaving warmer waterbelow) because as water freezes the molecules losekinetic energy (slow down) and hydrogen bonds start toarrange the molecules into a crystalline structure (thinksnowflakes). This arrangement contains more spaces

between the molecules than just a randomarrangement of liquid water molecules so the solidexpands and is less dense.


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c) Unusually high surface tension

Attraction between molecules at the surface of

liquid water is very high

Surface tension due to hydrogen bonds causes

beads of water to form on smooth surfaces,

raindrops to form in free fall and explains why

insects can walk on water

Hydrogen bonds allow the water molecules to hold

on strongly to each other, pulling together and

resisting being broken apart so it shapes to thesmallest surface area (a droplet)


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d) Unusually high Specific Heat Capacity

Water holds thermal energy very well

without changing its temperature

Takes a lot of energy to increase the

temperature of water by 1 oC

This property allows large lakes and oceans

to influence climate and weather patterns

(think El Nino, El Nina)

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