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You Ever Ask???• Why is water usually a liquid and

not a gas? • Why does liquid water boil at

such a high temperature for such a small molecule?

• Why does ice float on water?• Why do snowflakes have 6

sides?

• Why is I2 a solid whereas Cl2 is a gas?

• Why are NaCl crystals little cubes?

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Chapter 11: Liquids, Solids and Intermolecular Forces

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Chapter Objectives

• Kinetic-Molecular Description of Liquids & Solids

• Intermolecular (IMF) Attractions & Phase Changes

• Properties of Liquids– Viscosity, Surface Tension, Capillary Action– Evaporation, Vapor Pressure, – Boiling Points & Distillation, Heat transfer

• Phase ChangesThe following will NOT be covered in class

Types of crystals

Chapter Topics

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States of Matter

• List all the differences between– Solids– Liquids– Gases

• Kinetic Energy?

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States of Matter (KE of Matter)

The fundamental difference between states of matter is the distance between particles.

Intermolecular attractions in liquids & solids are strong: in liquids & solids are strong: KE of molecules << IMF

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States of Matter

Because in the solid and liquid states particles are closer together, we refer to them as condensed phases (depends on T and P).

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The States of Matter

• The state a substance is in at a particular temperature and pressure depends on two antagonistic entities:

– The kinetic energy of the particles

– The strength of the attractions between the particles

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Examples

1. What is the difference between intermolecular forces and intramolecular forces?

2. List all the intermolecular forces you are familiar with.

3. List all the intramolecular forces you are familiar with.

4. Which forces are stronger?

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Intermolecular Forces

The attractions between molecules are not nearly as strong as the intramolecular attractions that hold compounds together.

Intermolecular forces are weaker than intramolecular forces ( to break 2 O-H in bonds in water: 930 kJ/mol; to vaporize water: 43 kJ/mol)Responsible for the existence of condensed states (liquids, solids)

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Intermolecular Forces

They are, however, strong enough to control physical properties such as boiling and melting points, vapor

pressures, surface tension, and viscosities (reflect the strength of the bond).

INTRAINTRAmolecular forces—the forces that holds atoms molecular forces—the forces that holds atoms together o form moleculestogether o form moleculesINTERINTERmolecular forces: the forces between molecules, ions molecular forces: the forces between molecules, ions and molecules-ions.and molecules-ions.

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Intermolecular Forces

These intermolecular forces as a group are referred to as van der Waals forces (electrostatic forces).

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IMF Problem

For each of the molecules below, Determine the geometry of the moleculeDetermine the polarity of the moleculeList the types of intermolecular force which act between pairs of these molecules.

(a) CH4

(b) PF3 (c) CO2 (d) HCN,(e) HCOOH (methanoic acid)

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Types of IMF

• Ion – Ion • Van der Waals Forces

Dipole – dipole (for molecules with dipole moments)

Dipole – induced dipole

Dispersion forces (London)

Hydrogen Bond (special case of dipole-dipole (IMF)

London Dispersion Forces (induced dipole-dipole)

• Ion – induced dipole• Ion – dipole• Total attraction between molecules may depend on

more than one type of intermolecular force.

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Ion – Ion Forces

08M16VD1

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Ion-Ion Forces

• Ion-ion forces: electrostatic forces of attraction between __________________ of ionic compounds. Generally very strong → 250 kJ. (not a true intermolecular force)

• Ionic compounds: metal and nonmetal or polyatomic anions (NH4

+)

• Coulomb’s law & the attraction energy determine: – melting & boiling points of ionic compounds– the solubility of ionic compounds

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IMF: Ionic Solids

• Ion-ion interactions– force of attraction between two oppositely charged

ions is determined by Coulomb’s law

F

q q

d

+ -

2

• Energy of attraction between two ions is given by:

E = F d =

q q

dd

=q q

d

+ -

2

+ -

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Ion-Ion Forcesfor comparison of magnitude

Ion-Ion Forcesfor comparison of magnitude

NaNa++—Cl—Cl-- in salt in salt

These are the strongest These are the strongest forcesforces..

Lead to solids with high Lead to solids with high melting temperatures.melting temperatures.

NaCl (lattice energy = 788 NaCl (lattice energy = 788 kJ/mol), mp = 800 kJ/mol), mp = 800 ooCC

MgO (lattice energy = 3890 MgO (lattice energy = 3890 kJ/mol) , mp = 2800 kJ/mol) , mp = 2800 ooCC

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Covalent Bonding Forcesfor comparison of magnitude

Covalent Bonding Forcesfor comparison of magnitude

C–H, 413 kJ/molC–H, 413 kJ/mol

C=C, 610 kJ/molC=C, 610 kJ/mol

C–C, 346 kJ/molC–C, 346 kJ/mol

CN, 887 kJ/molCN, 887 kJ/mol

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Ion – Dipole Force

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Ion-Dipole Interactions

• Ion-dipole interactions are an important force in solutions of ions.

• The strength of these forces are what make it possible for ionic substances to dissolve in polar solvents.

NaNa++(g) + 6H(g) + 6H22O(l)→ [Na(HO(l)→ [Na(H22O)O)66]]++(aq) (aq) ΔΔHrxn = -405 kJHrxn = -405 kJ

Hydrated Ions?Hydrated Ions? Coordination Number? Coordination Number?

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Attraction Between Ions and Permanent Dipoles

Attraction Between Ions and Permanent Dipoles

HH

water dipole

••

••

O-

+

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Attraction between ions and dipole depends on Attraction between ions and dipole depends on ion ion chargecharge and and ion-dipole distance.ion-dipole distance.

Measured by ∆H for MMeasured by ∆H for Mn+n+ + H + H22O O →→ [M(H [M(H22O)O)xx]]n+n+

-1922 kJ/mol-1922 kJ/mol -405 kJ/mol-405 kJ/mol -263 kJ/mol-263 kJ/mol

Attraction Between Ions and Permanent Dipoles

Attraction Between Ions and Permanent Dipoles

OH

H+

-• • • O

H

H+

-• • • O

H

H+

-• • •

Na+Mg2+

Cs+

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Dipole – Dipole Force

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Dipole-Dipole Forces

Solid

Liquidliquid

Molecules that have permanent dipoles are attracted to each other.

The positive end of one is attracted to the negative end of the other and vice-versa.These forces are only important when the molecules are close to each other.Note the difference between solid and liquid.

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Effect of Dipole Moment on BP

Substance MM Dipole moment

BP, K

C3H8

propane

44 0.1 231

CH3OCH3

dimethyl ether

46 1.3 248

CH3CHOAcetyl aldehyde

44 2.7 294

CH3CNacetonitrile

41 3.9 355

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Dipole-Dipole Interactions

The more polar the molecule (higher μ), the higher is its boiling point.

• Basic attraction : electrostatic, Coulomb’s Law

Examples: HCl, CO, SO2, NF3, etc

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• Dipole-dipole are of the order of 5 to 20 kJ/mol. (KE due to temp at 25oC about 4 kJ/mol). Cmpds that have these forces (dipole-dipole) are frequently solids and liquids at room temp.

• The stronger the forces, the ______ the melting and boiling points of the compounds.

IMF: Dipole-Dipole

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Hydrogen Bond: type of dipole-dipole force

13M07AN2

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Boiling Points of Simple Hydrogen-Containing Compounds

The nonpolar series (SnH4 to CH4) follow the expected trend.

The polar series follows the trend from H2Te through H2S, but water is quite an anomaly.

EXPLAIN!

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Which of these are capable of forming hydrogen bonds among themselves?

a) CH3OH

b) C2H4

c) CH3NH2

d) HCN

e) NH4+

f) KF

g) CH3COOH

Intermolecular Forces: H-bond

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Hydrogen Bonding

• The dipole-dipole interactions experienced when H is bonded to N, O, or F (HIGH ELECTRONEGATIVITY) are unusually strong.

• Hydrogen nucleus is exposed.

• We call these interactions hydrogen bonds.

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Hydrogen Bonding

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H-Bonding Between Methanol and WaterH-Bonding Between Methanol and Water

H-bondH-bond--

++

--

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Hydrogen Bonding in H2OHydrogen Bonding in H2O

H-bonding is especially strong (40 kJ/mol) in water because

• the O—H bond is very polar

• there are 2 lone pairs on the O atom

Accounts for many of water’s (and other molecules such as DNA, proteins) unique properties such as anomalous high BP and high viscosity.

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Hydrogen Bonding in H2OHydrogen Bonding in H2O

Ice has open Ice has open lattice-like lattice-like structure.structure.

Ice density is Ice density is < liquid.< liquid.

And so solid And so solid floats on floats on water.water.

Snow flake: www.snowcrystals.com

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Hydrogen Bonding in H2OHydrogen Bonding in H2O

Ice has open lattice-like structure.Ice has open lattice-like structure.

Ice density is < liquid and so solid floats on water.Ice density is < liquid and so solid floats on water.

One of the VERY few substances where solid is LESS DENSE than the liquid.

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Hydrogen BondingHydrogen Bonding

H bonds leads to abnormally high boiling point of water.

See Screen 13.7See Screen 13.7

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Group VIA Group VIIA Group IVA Group VA

H2O 100 HF 20 NH3 -33 CH4 -161

H2S -65 HCl -85 PH3 -87 SiH4 -112

H2Se -45 HBr -69 AsH3 -60 GeH4 -90

H2Te -15 HI -35 SbH3 -25

Boiling Point of Hydrides in ºC

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Hydrogen Bonding in BiologyHydrogen Bonding in Biology

H-bonding is especially strong in biological H-bonding is especially strong in biological systems — such as proteins and DNA.systems — such as proteins and DNA.

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DNA — helical chains of phosphate groups and DNA — helical chains of phosphate groups and sugar molecules. Chains are helical because sugar molecules. Chains are helical because of tetrahedral geometry of P, C, and O.of tetrahedral geometry of P, C, and O.

Chains bind to one another by specific Chains bind to one another by specific

hydrogen bondinghydrogen bonding between pairs of between pairs of Lewis bases.Lewis bases.

——adenine with thymineadenine with thymine

——guanine with cytosineguanine with cytosine

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Portion of a Portion of a DNA chainDNA chain

Double helix Double helix of DNAof DNA

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Base-Pairing through H-Bonds

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Induced Dipole –Induced Dipole (London Dispersion Forces)

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London Dispersion Forces

While the electrons in the 1s orbital of helium would repel each other (and, therefore, tend to stay far away from each other), it does happen that they occasionally wind up on the same side of the atom.

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London Dispersion Forces

The helium atom becomes polar, with an excess of electrons on the left side and a shortage on the right side. Instantenous dipole forms (for an instant)

Instantaneous dipole

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London Dispersion Forces

Another helium nearby, then, would have a dipole induced in it, as the electrons on the left side of helium atom 2 repel the electrons in the cloud on helium atom 1.

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London Dispersion Forces

London dispersion forces, or dispersion forces, are attractions between an instantaneous dipole and an induced dipole.

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London Dispersion Forces

• These forces are present in all molecules, whether they are polar or nonpolar.

• The tendency of an electron cloud to distort in this way is called POLARIZABILITY.

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Forces Involving Dipole -Induced DipoleForces Involving Dipole -Induced Dipole

• Process of inducing a Process of inducing a dipole is dipole is polarizationpolarization

• Degree to which Degree to which electron cloud of an electron cloud of an atom or molecule can atom or molecule can be distorted in its be distorted in its polarizabilitypolarizability..

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IMF: London Dispersion Forces• Induced Dipoles: the temporary separation of positive

and negative charges in a neutral particle due to the proximity of an ion, dipole, or another induced dipole. On average μ = 0.

• London Dispersion Forces: attractive forces (electrostatic in origin) that arise as a result of temporary dipoles induced in atoms or molecules (instantaneous dipoles). Weak: 0.1- 5 kJ/mol

• Dispersion forces allow non-polar molecules to condense.

• Exist in all molecules!!!!! Importance depends on the type of intermolecular forces.

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IMF: London Dispersion Forces

• London Forcesvery weak

only attractive force in nonpolar molecules

Ar atom Cluster of Ar atoms

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Factors Affecting London Forces

• The shape of the molecule affects the strength of dispersion forces: long, skinny molecules (like n-pentane tend to have stronger dispersion forces than short, fat ones (like neopentane).

• This is due to the increased surface area in n-pentane.

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Effect of Geometry (shape) on BP of Molecules

Compound MM BP, ºC

N-butane 58 -0.45

Isobutane 58 -12.0

N-pentane 72 36.1

2-methyl butane 72 27.8

2,2 methyl propane 72 9.5

•Given the same molecular mass, GEOMETRY is important

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Factors Affecting London Forces

• The strength of dispersion forces tends to increase with increased molecular weight.

• Larger atoms have larger electron clouds, which are easier to polarize (larger polarizability.

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Boiling Points of HydrocarbonsBoiling Points of Hydrocarbons

CHCH44

CC22HH66

CC33HH88

CC44HH1010

MoleculeMolecule MM MM BP BP ((ooC)C) CHCH44 (methane) (methane) 1616 - 161.5- 161.5

CC22HH66 (ethane) (ethane) 3838 - 88.6 - 88.6

CC33HH88 (propane) (propane) 4444 - 42.1- 42.1

CC44HH1010 (butane) (butane) 58 58 - 0.5- 0.5

Note: linear relationship between BP and MM (Polarizability increases).

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Which Have a Greater Effect:Dipole-Dipole Interactions or Dispersion Forces?

• If two molecules are of comparable size and shape, dipole-dipole interactions will likely be the dominating force.

• If one molecule is much larger than another, dispersion forces will likely determine its physical properties.

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Dipole – Induced Dipole

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FORCES INVOLVING INDUCED DIPOLESFORCES INVOLVING INDUCED DIPOLES

How can non-polar molecules such as OHow can non-polar molecules such as O2 2 and Iand I22

dissolve in water?dissolve in water?

The water dipole The water dipole INDUCES a dipole a dipole in the Oin the O22 electric cloud. electric cloud.

Dipole-induced dipole

Dipole-induced dipole

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Forces Involving Dipole -Induced DIPOLEForces Involving Dipole -Induced DIPOLE

Solubility increases with mass of the gasSolubility increases with mass of the gas

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Forces Involving Dipole -induced DipoleForces Involving Dipole -induced Dipole

Consider IConsider I22

dissolving dissolving in ethanol, in ethanol,

CHCH33CHCH22OHOH..

OH

-

+

I-I

R-

+

OH

+

-

I-I

R

The alcohol The alcohol temporarily temporarily creates or creates or

INDUCES a a dipole in Idipole in I22..

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• Ion-dipole forces (very strong; solubility of ions in water)• Dipole-dipole forces (larger dipole moments)Dipole-dipole forces (larger dipole moments)

– Special dipole-dipole force: Special dipole-dipole force: hydrogen bondshydrogen bonds• Induced dipoles (occur in all substances; important for Induced dipoles (occur in all substances; important for

nonpolar molecules); increase with molar mass (glues); nonpolar molecules); increase with molar mass (glues); depend on geometry. For large molecules may exceed depend on geometry. For large molecules may exceed the force of dipole-dipole force (polymers, glues)the force of dipole-dipole force (polymers, glues)

Summary of Intermolecular Forces

In general: ionic forces the strongest; then hydrogen bonding; dipole-dipole; and lastly dispersion for species of similar molar mass. London Dispersion Forces exist in all molecules and ions.

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Summarizing Intermolecular Forces

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Summary of dipole forces

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Intermolecular Forces Summary

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Intermolecular Forces

Figure 13.13Figure 13.13

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Example

What type of intermolecular forces exist between the following pairs?

1. HBr and H2S

2. Cl2 and CBr4

3. I2 and NO3-

4. NH3 and C6H6

1. Dipole-dipole; dispersion; 2. dispersion; 3. ion-induced dipole; dispersion 4. dipole-induced dipole; dispersion

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Intermolecular Forces: Examples

5. Order the following compounds in order of increasing MP and BP:

a) N2, O2, H2

b) Cl2, F2, I2, Br2

c) SiH4, GeH4, SnH4, CH4

Order of MP and BP:

(a) H2< N2<O2

(b) F2<Cl2<Br2<I2

(c) CH4<SiH4<GeH4<SnH4

• Always compare like species.

6. But what about HF, HCl, HBr, HI

7. and H2O, H2S, H2Se, H2Te

8. and NH3, PH3, AsH3 and SbH3.

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Examples

1. List the IMF and arrange the substances BaCl2, H2, CO, and Ne in order of increasing boiling points.

2. In which of the following substances is hydrogen bonding possible?

a. Methaneb. Methyl alcohol

c. Hydrazine (H2NNH2)d. Methyl fluoridee. Hydrogen sulfidef. Carboxylic acid

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4. Arrange in order of increasing BP:

CO2 CH3OH CH3Br , RbF

5. Which one in each pair has the higher BP?

a) CH4 and C2H6 b) H2S and H2Te c) NH3 and PH3 d) HCl and HFe) I2 and ICl

Intermolecular Forces: Examples

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Example (London Forces)

6. Arrange the following molecules in order of increasing strength of intermolecular forces.

F2

Br2 Cl2

I2

7.Explain the trend in the normal boiling points of these liquids in terms of intermolecular forces.

CH4 normal boiling point: -161.5°CCF4 normal boiling point: -28°CCCl4 normal boiling point: +77°CCBr4 normal boiling point: +190°C

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8. (11.100) Which of the following substances has the highest polarizability?

CH4, H2, CCl4, SF6, H2S

Intermolecular Forces: Examples

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Identify “intermolecular” forces

9. H2O

10.CH2Cl211.KBr

12.F- + H2O

13. I2

14.CH3OH

15.PCl3

16.C6H6 17.Fe

18.CS2

19.BCl320.Na+ + NH3

21. Dimethyl ether (CH3OCH3) and ethanol (C2H5OH) have the same formula (C2H6O) but the BP of the ether is -25ºC and of the ethanol 78ºC. Explain.