Intermolecular Attractions and the properties of liquids and Solids

Chapter 12

12.3 Intermolecular Forces and properties of liquids and solids• Learning Objectives:

• To learn some general properties of liquids and solids and how these properties are related to the closeness of packing of molecules and to intermolecular attractive forces.

Physical properties of liquids and solids depends

1.On how Tightly Molecules Pack

2.On strength of intermolecular attractions.

Physical properties and intermolecular forces

Physical properties that depends how tightly molecules pack•Compressibility• Diffusion

Physical properties and intermolecular forces

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Compressibility

• Compressibility

Measure of ability of substance to be forced into smaller volume

Determined by strength of intermolecular forces

Gases highly compressible

Molecules far apart

Weak intermolecular forces

Solids and liquids nearly incompressible

Molecules very close together

Stronger intermolecular forces

The incompressibility of liquids is also the foundation of the engineering science of hydraulics, which uses fluids to transmit forces that lift or move heavy objects.

Application of incompressibility of liquids

Pascal’s law — states that when there is an increase in pressure at any point in a confined fluid, there is an equal increase at every other point in the container.

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Diffusion

• Movement that spreads one gas though another gas to occupy space uniformly

• Spontaneous intermingling of molecules of one gas with molecules of another gas

Occurs more rapidly in gases than in liquids

Hardly at all in solids

Diffusion

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Diffusion• In Gases

• Molecules travel long distances between collisions

• Diffusion rapid• In Liquids

• Molecules closer• Encounter more collisions• Takes a long time to

move from place to place• In Solids

• Diffusion close to zero at room temperature

• Will increase at high temperature

Properties that depend on strength of intermolecular attractions. •Retention of volume and shape•Surface Tension•Wetting of a surface by a liquid •Viscosity

Physical properties and intermolecular forces

Retention of volume and shape

• Retention of volume and shape• Solids retain both volume and shape• Strongest intermolecular attractions; Molecules closest• Liquids retain volume, but not shape• Attractions intermediate• Gases, expand to fill their containers• Weakest intermolecular attractions; Molecules farthest apart

Surface Tension

Surface tension is a physical property equal to the amount of force per unit area necessary to expand the surface of a liquid.

Surface tension forces are due to intermolecular forces between the liquid's molecules at the liquid's outer boundaries.

• Inside body of liquid• Intermolecular forces

are the same in all directions

• Molecules at surface • Potential energy

increases when removing neighbors

• Molecules move together to reduce surface area and potential energy

Surface Tension

• Causes a liquid to take the shape (a sphere) that minimizes its surface area• Molecules at surface

have higher potential energy than those in bulk of liquid and move to reduce the potential energy

Wax = nonpolar H2O = polar Water beads in order

to reduce potential energy by reducing surface area

Why does H2O bead up on a freshly waxed car instead of forming a layer?

Surface Tension

• Liquids containing molecules with strong intermolecular forces have high surface tension• Allows us to fill glass above rim• Gives surface rounded

appearance• Surface acts as “skin” that lets

water pile up• Surface resists expansion and

pushes back

Surface tension increases as intermolecular forces increase

Surface tension decreases as temperature increases

Walking on water

Wetting of a surface by a liquid

Wetting: ability of liquid to spread across surface to form thin film

•Greater similarity in attractive forces between liquid and surface, yields greater wetting effect

•Occurs only if intermolecular attractive force between surface and liquid about as strong as within liquid itself

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Wetting

Ex. H2O wets clean glass surface as it forms

H–bonds to SiO2 surface

• Does not wet greasy glass, because grease is nonpolar and water is very polar• Only London forces • Forms beads instead

Surfactants

• Added to detergents to lower surface tension of H2O

• Now water can spread out on greasy glass

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Surfactants (Detergents)

• Substances that have both polar and non-polar characteristics

• Long chain hydrocarbons with polar tail

OS

O

O Na+

O

O

O Na+

Nonpolar end dissolves in nonpolar grease Polar end dissolves in polar H2O Thus increasing solubility of grease in water

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Solubility

• “Like dissolves like”• To dissolve polar substance, use polar solvent• To dissolve nonpolar substance, use nonpolar solvent

• Compare relative polarity• Similar polarity means greater ability to dissolve in each

other • Differing polarity means that they don’t dissolve, they are

insoluble• Surfactants• Both polar and non-polar characteristics• Used to increase solubility

Surface Tension

Surfactant decrease surface tension

OS

O

O Na+

O

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Your Turn!

Which of the following are not expected to be soluble in water?

A. HF

B. CH4

C. CH3OH

D. All are soluble

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Viscosity

Viscosity is resistance to flow

•Measure of fluid’s resistance to flow or changing form

•Related to intermolecular attractive forces

Also called internal friction Depends on intermolecular attractions

Viscosity

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Viscosity

• Viscosity decreases when temperature increases

• Most people associate liquids with viscosity• Syrup more viscous than water

Gases have viscosity• Respond almost instantly to form-changing forces• Viscosity in gases arises from the molecular diffusion

that transports momentum between layers of flow. • The kinetic theory of gases allows accurate prediction of

the behavior of gaseous viscosity.

• Solids, such as rocks and glass have viscosity• Normally respond very slowly to forces acting to change their shape• Indeed, some scientists have claimed that amorphous solids, such as glass or many

polymers, are actually liquids with a very high viscosity

• Debatable by scientist?

Viscosity of solids

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Amorphous Solids (Glass)

• Have little order, thus referred to as “super cooled liquids”• Edges are not clean, but ragged due to the lack of order

Your Turn!

For each pair given, which is has more viscosity?CH3CH2CH2CH2OH, CH3CH2CH2CHO

C6H14, C12H26

NH3(l ), PH3(l )

A. CH3CH2CH2CH2OH C6H14NH3(l )

B. CH3CH2CH2CH2OH C12H26 NH3(l )

C. CH3CH2CH2CHO C6H14PH3(l )

D. CH3CH2CH2CHO C12H26 NH3(l )

E. CH3CH2CH2CH2OH C12H26 PH3(l )

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Effect of Intermolecular Forces on Viscosity

Acetone • Polar molecule• Dipole-dipole • London forces

Ethylene glycol• Polar molecule• Hydrogen-bonding• Dipole-dipole and • London forces

water• Polar molecule• Hydrogen-bonding• Dipole-dipole and • London forces

Lab: Molar mass of CO2

Experimentally determine the molar mass of CO2.

Molar mass= g/mole

We need grams and we need number of moles of a CO2 sample

Dry ice is CO2 grams by weighing

How do we get the number of moles

For moles of CO2. how about using PV=nRT

P= atm. Pressure in lab

V = can be measured Using water to determine the volume

R= gas constant

T= can be measured (room temperature we are working at)

Lab: Molar mass of CO2