Intermolecular forcesLiquids and Solids

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Chapter objectives

• Understand the three intermolecular forces in pure liquid in relation to molecular structure/polarity

• Understand the physical properties of liquids that is relevant to intermolecular force: vapor pressure and boiling

IntermolecularAttractive Forces

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• Stronger intermolecular force in the liquid prevent liquid molecules from escaping into gas state, causing lower vapor pressure

• Recall: substance with lower vapor pressure will have ________ (higher, lower) boiling point.

Intermolecular forces affect physical properties of solid and liquid

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Intermolecular forces affect physical properties of solid & liquid

• stronger intermolecular forces increases surface tension and viscosity

• stronger intermolecular forces reduces vapor pressure (retaining molecules in liquid state), thus increases boiling point

• likewise with melting point

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Intermolecular forces in Pure liquids

• Dispersion force (aka London force)• Dipole-dipole force• Hydrogen bonding

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Dispersion Forcesalso: London Forces or Induced Dipoles

Electrons on one molecule distorting the electron cloud on another

• ALL molecules have Dispersion Forces• Dispersion force is especially important

among nonpolar molecules

+----

- -- --

-

++

++

---

--

-------

--

-

++

++

---

--

-------

--

-

+----

- -- --

-

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Dispersion Forces: Instantaneous Dipoles

NonpolarSomewhat polar Polar

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Dispersion Force: Strength

• Electron mobility: how easily the electrons can move within a molecule, or be polarized. =O < =S -F < -Cl < -I

• more electrons + electron farther from the nuclei the larger the dipole that can be induced

• strength of the dispersion force gets Larger with larger molecules

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Permanent DipolesChapter 4:• Electronegativity difference

& Molecular Geometry some molecules have a Permanent Dipole: (+) (-)

• all polar molecules have a permanent dipole. H2O, NH3, HCl, etc.

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Dipole-to-Dipole Attraction

• Polar molecules have a permanent dipole

a + end and a – end

• the + end of one molecule will be attracted to the –end of another

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Attractive Forces

+ - + - + - + -

+++

+

____

Dispersion Forces – all molecules

Dipole-to-Dipole Forces – polar molecules

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Hydrogen Bonding• Molecules that have HF,

-OH or -NH groups have particularly strong intermolecular attractions

unusually high melting and boiling pointsunusually high solubility in water

Hydrogen Bond

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Intermolecular H-Bonding

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Hydrogen Bonding• A very electronegative atom X (X = F, O, N) is

bonded to hydrogen, the bonding electrons is pulled toward X.

Xδ–-Hδ+

• Since hydrogen has no other electrons, the nucleus becomes deshielded (“stripped”): -Hδ+

exposing the proton

• The exposed proton Hδ+ (center of positive charge) attracting all the electron clouds from neighboring molecules Xδ–-Hδ+ ••• Yδ–-

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H-Bonds vs. Chemical Bonds

• Hydrogen bonds are not chemical bonds• Hydrogen bonds are attractive forces

between molecules• Chemical bonds are attractive forces that

make molecules

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Hydrogen Bond in DNA double helix

•

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Types of Intermolecular ForcesType of Force

Relative Strength

Present in Example

DispersionForce

weak, but increases

with molar mass

all atoms and

moleculesH2

Dipole –Dipole Force

moderate only polar molecules HCl

Hydrogen Bond strong

molecules having H bonded to F, O or N

HF

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Attractive Forces and SolubilityLike dissolves Like

miscible = liquids that do not separate

• Polar molecules dissolve in Polar solventswater, alcohol, isopropanol, CH2Cl2H-bond: molecules with O or N higher solubility in H2O

• Nonpolar molecules dissolve in nonpolar solventsligroin (hexane), toluene, kerosene, CCl4

• if molecule has both polar & nonpolar parts, then hydrophilic - hydrophobic competition

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Solubility between two liquids: Immiscible Liquids

Pentane (C5H12) (C-H and C-C bond, nonpolar substance)is mixed with water (O-H bond, polar) the two liquids separate

they are moreattracted to their ownkind of molecule thanto the other.

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Physical Property: Interactions Between Molecules

• Many of the phenomena we observe are related to interactions between molecules that do not involve a chemical reaction

your taste and smell organs work because molecules interact with the receptor molecule sites in your tongue and nose

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Structure Determines Properties:Solids, Liquid and Gases

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Why is Sugar a Solid ButWater is a Liquid?

• The state a material exists in depends on the attraction between molecules and their ability to overcome the attraction

• The attractive forces between Ions or Molecules Their structurethe attractions are electrostaticdepend on shape, polarity, etc.

• The ability of the molecules to overcome the attraction Kinetic energy they possess

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Escaping from the Surface• Evaporation : molecules of a

liquid breaking free from the surface: Liquid Gas

also known as vaporization• Physical change • a substance is converted from its

liquid form to its gaseous formthe gaseous form is called a vapor

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Evaporation: Liquid Gas• Molecules of the liquid mix with and

dissolve in the air• happens at the surface• molecules on the Surface experience

a smaller net attractive force than molecules in the Interior

• but all the surface molecules do not escape at once, only the ones with sufficient kinetic energy to overcome the attractions will escape

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Condensation: Gas Liquid• in a closed container, after a liquid

evaporates, the vapor molecules are trapped and may eventually turn into liquid

• Condensation : the vapor molecules may eventually bump into and stick to the surface of the container or get recaptured by the liquid.

Physical change : Gas Liquid

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Dynamic Equilibrium

• Evaporation and Condensation are opposite processes

• eventually, the rate of evaporation and condensation in the container will be the same

• Dynamic equilibrium : opposite processes that occur at the same rate in the same system

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Evaporation ⇔ Condensation

Water is just added to the flask and it is capped, all the water molecules are in the liquid.

Shortly, the waterstarts to evaporate.Speed of evaporation >> Speed of condensation (Rateevap >> Ratecondsn)

Eventually, Rateevap = Ratecondsn

The air in the flaskis now saturatedwith water vapor.

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Vapor Pressure Pvap

• once equilibrium is reached, then the amount of vapor (mole nvap) in the container will remain the same

as long as you don’t change the conditions

• Vapor pressure: the partial pressure exerted by the vapor of the liquid.

• Depending on the temperature and strength of intermolecular attractions

VTRn

P vapvap

⋅⋅=:Law Gas Ideal

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Vapor Pressure increases as temperature increases

normal boiling point water

ethanolether

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Boiling and Boiling Point (b.p.)• Boiling: vapor pressure of the liquid is the same as

the atmospheric pressure. Liquid Gas. Pvap = Pair

• Boiling point: the temperature for boiling processnormal boiling point: temperature when Pair = 1 atm

b.p. of water is 100°C

• b.p. depends on Pairthe temperature of boiling water on the top of a mountain will be cooler than boiling water at sea levelOn top of Mount Whitney, b.p. of water is about 84°C

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Vapor pressure at given temperature vs. Normal Boiling point

• At the same temperature, different liquids have different vapor pressure (volatility)

• Liquids having higher vapor pressure are normally called “more volatile”

• Liquids having higher vapor pressure will have lower normal boiling points

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Energy flow: Evaporation vs. Condensation

Evaporation: Liquid absorbs heat from its surroundings to evaporate The surroundings cool off

• Endothermic: heat flows into a system from the surroundings

as alcohol evaporates off your skin, it causes your skin to cool

Condensation: Gas releases heat to its surroundings to reduce its temperature The surroundings warms up

• Exothermic: heat flows out of a system into the surroundings

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Temperature and Melting• For solid, temperature increases

until it reaches the melting point. Ice melts at 0°C.

• During melting: the temperature remains the same until it all turns to a liquid. solid liquid

• all the Energy from the heat source is for overcoming the attractive forces in the solid, not increase the temperature

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Sublimation vs. Deposition• Sublimation: the Solid form

changes directly to the Gaseous form. Solid Gas

without going through the liquid formDry ice (solid CO2) gas CO2

• like melting, sublimation is endothermic

• Deposition is the reverse of Sublimation, exothermic.

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Heating Curve:phase changes during heating solid ice at

1 atm

Temperature of water at Constant Heating

-200

20406080

100120140160

0 10 20 30 40 50 60 70 80 90 100

Time

Tem

pera

ture

(o C)

s s+l

l l+g

g

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Types of Crystalline Solids

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Molecular Crystalline Solids• Molecular solid: composite

units are molecules.

CO2 • CO2 •H2O •• H2O •• H2O

• Held together by intermolecular attractive forces

dispersion, dipole-dipole, or H-bonding

• generally low melting points and ΔHfusion

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Ionic Crystalline Solids• Ionic solids: composite units are

formula units. NaClNa+ ••• Cl–••• Na+ ••• Cl–

• Held together by Electrostaticforces between Cation+ and Anion–

arranged in a geometric pattern called a crystal lattice to maximize attractions

• generally higher melting points and ΔHfusion than molecular solids

because ionic bonds are stronger than intermolecular forces

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Atomic Crystalline Solids• Atomic solids: composite units

are individual atomsXe • Xe • Xe • Xe

• Held together by either covalent bonds, dispersion forces or metallic bonds

• melting points and ΔHfusionvary depending on the attractive forces between the atoms

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Types of Atomic Solids

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Types of Atomic SolidsCovalent

• Covalent Atomic Solids : atoms attached by covalent bonds. Diamond Carbon (tetrahedral, C-C bond).

effectively, the entire solid is one, giant molecule

Covalent bonds are strong very High melting points and

ΔHfusionHigh hardness

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Types of Atomic SolidsNonbonding

• Nonbonding Atomic Solid: held together by dispersion forces. Xenon solid (at low temperature)

Xe • Xe • Xe • Xe

Dispersion forces are relatively weak, very low melting points and

ΔHfusion

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Types of Atomic SolidsMetallic

• Metallic solids: held together by metallic bonds

• How: metal atoms release some of their electrons to be sharedby all the other atoms in the crystal

• Metallic bond: the attraction of the metal Cations M+ for the mobile electrons e-

often described as “islands of cations in a sea of electrons”

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Water: A Unique and Important Substance

• found in all 3 states on the Earth: Ice, Liquid, Vapor• the most common solvent (liquid) found in nature• without water, life as we know it could not exist

the search for extraterrestrial life starts with the search for water

• relatively high boiling point• expands as it freezes

most substances contract as they freezecauses ice to be less dense than liquid water