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Tro Chpt. 11

Liquids, solids and intermolecular forces •  Solids, liquids and gases - A Molecular Comparison

•  Intermolecular forces

•  Intermolecular forces in action: surface tension, viscosity and capillary action

•  Vaporization and vapor pressure

•  Sublimation and fusion

•  Quantitative aspects of phase changes

•  Phase Diagrams

•  Skip sections 11.10-11.13

Tro 11.2

INTERMOLECULAR vs. INTRAMOLECULAR INTERACTIONS

+ 8.9 kJ

Tro 11.3

C + 4H 4 mol C-H bonds x 414 kJmol-1 or 1656 kJ per mol of methane!

1 joule: * the energy required to lift a small apple (102 g) one meter against Earth's gravity. * one hundredth of the energy a person can get by drinking a single 5 mm diameter droplet of beer. * The amount of energy released if a can of beer is dropped from wait height

Non-covalent, intermolecular interactions between covalent molecules (1) London (dispersion) forces (2) Dipole-dipole forces (3) Hydrogen bonding

TYPES OF INTERMOLECULAR INTERACTIONS

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ION–ION INTERACTION

Eatt ∝ Q1Q2

d

Opposite charges attract

EXAMPLE:

Q is charge

+ – d

Na+ Cl– 102 pm 181 pm

ION–DIPOLE INTERACTION

Eatt ∝ Q µ

d2

+ d δ– δ+

neutral polar molecule with dipole moment electrostatic interaction

Q charge µ dipole moment

ions in solution Na+ O H

:

:

H

DIPOLE–DIPOLE INTERACTION

Eatt ∝ µ4

d6

δ– δ+

δ– δ+ δ– δ+

δ+ δ-

attraction repulsion net effect, averaged over time

More polar molecules have larger attractive force

H Cl H Cl

EXAMPLE:

DISPERSION INTERACTION LONDON INTERACTION

• Nonpolar molecules will liquify, so there must be attractive intermolecular forces

• Electrons are always moving in molecules • At some times, there is an instantaneous dipole moment • When this occurs, a dipole is induced in the adjacent atom and an

attractive force is the result

Eatt ∝ 1 d6

• Highly polarizable molecules are more subject to dispersion forces • LDF increases with molecular size • Molecular shape is also involved • LDFs occur for all molecules

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DISPERSION INTERACTION

Ar Ar LDF only intermolecular force for noble gases

H Cl H Cl

LDF in addition to dipole-dipole

SIZE Ar BP = – 185.7 °C = 87.5 K

Xe BP = – 107.1 °C = 166.1 K

atomic size related to polarizability

Molar Mass and Boiling Point

Dipole Moment and Boiling Point Molecular Shape and Boiling Point

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Practice – Choose the Substance in Each Pair with the Highest Boiling Point

a)  CH4 CH3CH2CH2CH3

b)  CH3CH2CH=CHCH2CH3 cyclohexane

b.p. = 80.7 ˚C b.p. = 66.4 ˚C

b.p. = -164 ˚C b.p. = -0.5 ˚C

or

Practice – Choose the Substance in Each Pair with the Highest Boiling Point

a)  CH2FCH2F CH3CHF2

b)

b.p. trans: 47.5 °C cis: 60.3 °C

b.p. = -24.9 ˚C b.p. = 30.7 ˚C

• A ‘H’ bonded to a very electronegative atom (O, N, F) can interact with lone pair electrons on (O, N, F) on another molecule

• H-bond energies usually 4-25 kJ/mol • H bonds weak compared to covalent bonds • H bonds strong compared to intermolecular forces

• H-bonds are directional

• H-bond strength related to dipole moment of bond

• Properties of H2O are related to H-bonding in liquid and solid

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Practice – Choose the substance in each pair that is a liquid at room temperature (the other is a gas)

a)  CH3OH CH3CHF2

b)  CH3-O-CH2CH3 CH3CH2CH2NH2

Practice – Choose the substance in each pair that is more soluble in water

a)  CH3OH CH3CHF2

b)  CH3CH2CH2CH3 CH3NH2

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Tro 11.4

Surface tension: Molecules at the liquid surface have a higher potential energy than those in the interior. As a result, liquids tend to minimize their surface area and the surface behaves like a membrane or “skin”.

Surface tension allows a paper click to float on water!

Viscosity •  Viscosity: the resistance of a liquid to flow

–  1 poise = 1 P = 1 g/cm·s –  often given in centipoise, cP

•  larger intermolecular attractions = larger viscosity •  higher temperature = lower viscosity

Capillary Action •  the adhesive forces pull the surface liquid up the side of the tube,

while the cohesive forces pull the interior liquid with it •  the liquid rises up the tube until the force of gravity counteracts

the capillary action forces

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Water (dyed red) and mercury in a glass test tube

Equilibrium nature of phase changes Tro 11.5

Dynamic Equilibrium

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Vapor Pressure

Vapor Pressure as a function of temperature and intermolecular forces

Clausius-Clapeyron Equation

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Clausius-Clapeyron Equation

Clausius-Clapeyron plot for diethyl ether

Boiling Point

Determining vapor pressure under different conditions

The vapor pressure of ethanol is 115 torr at 34.9 ˚C. If ΔHvap 40.5 kJmol-1, calculate the temperature (in ˚C) when the vapor pressure is 760 torr.

T2 = 350 K or 77˚C

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Phase Changes - Sublimation and Deposition •  molecules in the solid have thermal energy that allows them to vibrate •  surface molecules with sufficient energy may break free from the surface and

become a gas – this process is called sublimation •  the capturing of vapor molecules into a solid is called deposition •  the solid and vapor phases exist in dynamic equilibrium in a closed container

–  at temperatures below the melting point –  therefore, molecular solids have a vapor pressure

solid gas sublimation deposition

Tro 11.6

Melting = Fusion

Tro 11.7 Quantitative aspects of phase changes:

How much energy is released when 2.5 mol of water vapor at 130˚C is cooled to ice at -40˚?

Stage 1: H2O (g) [130 ˚C] H2O (g) [100˚C]

q = n x Cwater(g) x ΔT

q = (2.5 moles) x (33.1 Jmol-1˚C-1) x (100-130˚C)

q = -2482 J

Stage 2: H2O (g) [100 ˚C] H2O (l) [100˚C]

q = n x (-ΔHvap)

q = 2.5 x (-Δ40.7 kJmol-1)

q = -102 kJ

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Stage 3: H2O (l) [100 ˚C] H2O (l) [0˚C]

q = n x Cwater(l) x ΔT q = (2.5 moles) x (75.4 Jmol-1˚C-1) x (0-100˚C)

q = -18850 J

Stage 4: H2O (l) [0 ˚C] H2O (s) [0˚C]

q = n x (-ΔHfus)

q = 2.5 x (-Δ6.02 kJmol-1)

q = -15.0 kJ

Stage 5: H2O (s) [0 ˚C] H2O (s) [-40˚C]

q = n x Cwater(s) x ΔT

q = (2.5 moles) x (37.6 Jmol-1˚C-1) x (-40 - 0˚C)

q = -3760 J

Using Hess’s law, sum of q for all 5 stages = -142 kJ

Wicked Weird Phases - Supercritical Fluid •  as a liquid is heated in a sealed container, more vapor collects

causing the pressure inside the container to rise –  and the density of the vapor to increase –  and the density of the liquid to decrease

•  at some temperature, the meniscus between the liquid and vapor disappears and the states commingle to form a supercritical fluid

•  supercritical fluid have properties of both gas and liquid states

Supercritical n-pentane

Typical phase diagram for most substances Tro 11.8 For most substances, increasing pressure will solidify a liquid (i.e. the

solid is more dense than the liquid). The opposite is true for water (note the negative slope for the solid-liquid line)

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Information extracted from phase diagrams

Starting at the triple point, what phase exists when the pressure is held constant and the temperature is increased to 0.5˚C

gas!

Starting at the triple point, what phase exists when the temperature is held constant and the pressure is increased to to 20 mm Hg? liquid!