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Inter-molecular Forces & Properties

K Warne Grade 10

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• Two different types of bonds occur in water.

• Intramolecular– Between hydrogen

atoms and oxygen atoms inside the molecules Covalent bonds

• Intermolecular– Between two

different water molecules Hydrogen bonds

Intermolecular forces

O+

-H

HO+ -

H

H

O+

-

H

H

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Bond Polarity in Water

The oxygen atom has greater electronegativity so it is surrounded by greater electron density than the hydrogen atoms. O

H

H

-

+

++ -

The water molecule is a DIPOLE - it has two oppositely charged “poles”.

OH

H+ -

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• The oppositely charged poles on the water molecules attract each other.

• This electrostatic attraction constitutes Hydrogen bonding.

• It is the strongest form of intermolecular attraction.

• Hydrogen bonding exists only between molecules in which hydrogen is bonded to a very electronegative atom (H-O-X, H-NX2 or H-F).

• Result in abnormally high boiling points.

Hydrogen Bonding

O+

-H

HO+ -

H

H

O+

-

H

H

Example:Water

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Boiling Points of Hydrides

Where will the next points be??

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Molecular solids

Iodine - I2

Strong covalent bonds

Weaker intermolecular bonds

Microscopic Structure

Covalently bonded molecules held together by weaker intermolecular bonding.

PROPERTIES (Macroscopic)

• Low melting points

• Sublimes if gently warmed

• Brittle

• Soluble in non-polar solvents

• Non - conducting

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Physical Properties

PHYSICAL PROPERIES• melting points

IMF vs Mp

The greater the inter

molecular forces the

higher the melting

point because more

energy would be

needed to overcome the

forces.

MeltingForces between particles must be overcome

IMF vs Mp & BpHalogenX2 (diatomic)

Molecular

Mass (Mr g.mol-1)

Mp / Bp (oC)

FlourineF2 pale yellow gas

19 x 2 =

38 -220 / -188

ChlorineCl2 pale green gas

35.5 x 2 =

71-101/ -35

BromineBr2 red volatile liquid

80 x 2 =

160-7 / 59

IodineI2 purple solid - sublimes

127 x 2 =

254114 / 184M

p &

Bp

incr

ease

s w

ith S

IZE

and

mol

ecul

e M

AS

S

. .: X .

. .

. .: X .

. .

Inter molecular forces increase in strength as molecules get bigger.

Density

Density = mass/volume (g.cm-3)

High density – solids

Many particles per cm3

Low density – gases

Few particles per cm3

1cm x 1cm x 1cm = 1cm3

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Effect of Temp

• Temperature is a measure of average kinetic energy.• As temperature rises – particles move faster – and further apart –

substances expand – used in a thermometer.

Maxwell-Boltzman Curve

Average Ek

Viscosity• Viscosity is a measure of

how thick (viscous) and sticky a liquid is.

• Viscosity reduces the ability of a liquid to flow.

• Liquids that flow readily (water) have a low viscosity.

• Viscosity is a function of (depends on) the attractive forces of the molecules of the liquid.

• Strong forces – high viscosity

• Temperature also greatly affects viscosity: as temperature increases, viscosity decreases.

Kinetic energy enables particles to overcome forces.

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Hi -

This is a SAMPLE presentation only.

My FULL presentations, which contain loads more slides and other resources, are freely available on my resource sharing website:

www.sciencecafe.org.za

(paste into your browser if link above does not work)Have a look and enjoy!

Keith Warne