Inter- molecular Forces

© 2006 Brooks/Cole - Thomson

Inter-Inter-moleculamolecular Forcesr Forces

Have studied Have studied INTRAINTRAmolecular molecular forces—the forces holding forces—the forces holding atoms together to form atoms together to form molecules.molecules.

Now turn to forces between Now turn to forces between molecules —molecules — INTERINTERmolecular forces. molecular forces.

Forces between molecules, Forces between molecules, between ions, or between between ions, or between molecules and ions.molecules and ions.


Ion-Ion Forcesfor comparison of

magnitudeNaNa++—Cl—Cl-- in salt in saltThese are the These are the

strongest forces.strongest forces. Lead to solids with Lead to solids with

high melting high melting temperatures.temperatures.

NaCl, mp = 800 NaCl, mp = 800 ooCCMgO, mp = 2800 MgO, mp = 2800 ooCC


Covalent Bonding Forcesfor comparison of

magnitude

C–H, 413 kJ/molC–H, 413 kJ/mol

C=C, 610 kJ/molC=C, 610 kJ/molC–C, 346 kJ/molC–C, 346 kJ/mol

CN, 887 kJ/molCN, 887 kJ/mol


Attraction Between Ions and Permanent

Dipoles

Water is highly polar Water is highly polar and can interact and can interact with positive ions to with positive ions to give give hydratedhydrated ions in water.ions in water.

HH

water dipole••

••

O-

+



Dipoles

Water is highly polar Water is highly polar and can interact and can interact with positive ions to with positive ions to give give hydratedhydrated ions in water.ions in water.

HH

water dipole••

••

O-

+



Dipoles

Many metal ions are hydrated. This is Many metal ions are hydrated. This is the reason metal salts dissolve in the reason metal salts dissolve in waterwater..


Attraction between ions and dipole depends on Attraction between ions and dipole depends on ion chargeion charge and and ion-dipole distanceion-dipole distance..

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

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


Dipoles

OH

H+

-• • • O

H

H+

-• • • O

H

H+

-• • •

Na+Mg2+

Cs+


Dipole-Dipole Forces

Such forces bind molecules having Such forces bind molecules having permanent dipoles to one another.permanent dipoles to one another.


Dipole-Dipole Forces

Influence of dipole-dipole forces is seen in Influence of dipole-dipole forces is seen in the boiling points of simple molecules.the boiling points of simple molecules.

CompdCompd Mol. Wt.Mol. Wt. Boil PointBoil PointNN22 2828 -196 -196 ooCCCOCO 2828 -192 -192 ooCCBrBr22 160160 59 59 ooCCIClICl 162162 97 97 ooCC


Hydrogen BondingA special form of dipole-dipole attraction, A special form of dipole-dipole attraction,

which enhances dipole-dipole attractions.which enhances dipole-dipole attractions.

H-bonding is strongest when X and Y are H-bonding is strongest when X and Y are N, O, N, O, or For F


Hydrogen Bonding in H2O

H-bonding is especially H-bonding is especially strong in water strong in water becausebecause

• the O—H bond is very the O—H bond is very polarpolar

• there are 2 lone pairs there are 2 lone pairs on the O atomon the O atom

Accounts for many of Accounts for many of water’s unique water’s unique properties.properties.


Hydrogen Bonding in H2OIce 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


Hydrogen Bonding in H2OIce has open lattice-like structure.Ice has open lattice-like structure.Ice density is < liquid and so solid floats on Ice density is < liquid and so solid floats on

water.water.

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


Hydrogen Bonding in H2OH bonds ---> abnormally high specific heat capacity of H bonds ---> abnormally high specific heat capacity of

water (4.184 J/g•K)water (4.184 J/g•K)This is the reason water is used to put out fires, it is the This is the reason water is used to put out fires, it is the

reason lakes/oceans control climate, and is the reason reason lakes/oceans control climate, and is the reason thunderstorms release huge energy.thunderstorms release huge energy.


Boiling Points of Boiling Points of Simple Hydrogen-Simple Hydrogen-

Containing Containing CompoundsCompounds

Active Figure 13.8Active Figure 13.8


Portion of Portion of a DNA a DNA chainchain

Double Double helix of helix of

DNADNA

H-bonding is especially strong in biological systems — such as H-bonding is especially strong in biological systems — such as DNA. DNA. DNA — helical chains of phosphate groups and sugar molecules. DNA — helical chains of phosphate groups and sugar molecules. Chains are helical because of tetrahedral geometry of P, C, and O.Chains are helical because of tetrahedral geometry of P, C, and O.Chains bind to one another by specific hydrogen bonding between Chains bind to one another by specific hydrogen bonding between pairs of Lewis bases.pairs of Lewis bases. ——adenine with thymineadenine with thymine ——guanine with cytosineguanine with cytosine


Base-Pairing through H-Bonds


Base-Pairing through H-Bonds


FORCES 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 INDUCESINDUCES a a dipole in the Odipole in the O22 electric electric cloud. cloud.

Dipole-Dipole-induced induced dipoledipole



Solubility increases with mass the gasSolubility increases with mass the gas



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

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


IM FORCES — INDUCED DIPOLES

Consider IConsider I22 dissolving dissolving in ethanol, in ethanol, CHCH33CHCH22OH.OH.

OH

-

+

I-I

R-

+

OH

+

-

I-I

R

The alcohol The alcohol temporarily temporarily creates or creates or

INDUCESINDUCES a a dipole in Idipole in I22..



Formation of a dipole in two nonpolar IFormation of a dipole in two nonpolar I22 molecules.molecules.

Induced dipole-induced dipole

The induced forces between IThe induced forces between I22 molecules are very weak, so molecules are very weak, so solid Isolid I22 sublimes (goes from a solid to gaseous molecules). sublimes (goes from a solid to gaseous molecules).



The magnitude of the induced dipole The magnitude of the induced dipole depends on the tendency to be distorted. depends on the tendency to be distorted.

Higher molec. weight ---> larger induced Higher molec. weight ---> larger induced dipoles.dipoles.

MoleculeMolecule Boiling Point Boiling Point ((ooC)C) CHCH44 (methane) (methane) - 161.5- 161.5 CC22HH66 (ethane) (ethane) - 88.6 - 88.6 CC33HH88 (propane) (propane) - 42.1- 42.1 CC44HH1010 (butane) (butane) - 0.5- 0.5


Boiling Points of Hydrocarbons

Note linear relation between bp and molar mass.

CHCH44

CC22HH66

CC33HH88

CC44HH1010


Intermolecular Forces Summary


Equilibrium Vapor PressureActive Figure 13.18


Liquids Equilibrium Vapor Pressure

FIGURE 13.18:FIGURE 13.18: VP as a function of T.VP as a function of T.1. The curves show all conditions of P and T where LIQ 1. The curves show all conditions of P and T where LIQ

and VAP are in and VAP are in EQUILIBRIUMEQUILIBRIUM2. The VP rises with T.2. The VP rises with T.3. When VP = external P, the liquid boils.3. When VP = external P, the liquid boils. This means that BP’s of liquids change with altitude.This means that BP’s of liquids change with altitude.4. If external P = 760 mm Hg, T of boiling is the 4. If external P = 760 mm Hg, T of boiling is the

NORMAL BOILING POINTNORMAL BOILING POINT5. VP of a given molecule at a given T depends on IM 5. VP of a given molecule at a given T depends on IM

forces. Here the VP’s are in the orderforces. Here the VP’s are in the order

CC 22HH 55HH 55CC22 HHHH 55CC 22 HHHH

waterwateralcoholalcoholetherether

increasing strength of IM interactionsincreasing strength of IM interactions

extensiveextensiveH-bondsH-bondsH-bondsH-bonds

dipole-dipole-dipoledipole

OOOOOO


Liquids

HEAT OF VAPORIZATIONHEAT OF VAPORIZATION is the heat is the heat req’d (at constant P) to vaporize the liquid.req’d (at constant P) to vaporize the liquid.

LIQ + heat ---> VAPLIQ + heat ---> VAPCompd.Compd. ∆H∆Hvapvap (kJ/mol) (kJ/mol) IM ForceIM ForceHH22OO 40.7 (100 40.7 (100 ooC)C) H-bondsH-bondsSOSO22 26.8 (-47 26.8 (-47 ooC)C) dipoledipoleXeXe 12.6 (-107 12.6 (-107 ooC)C) induced induced

dipole dipole


Phase Diagrams

Lines connect all conditions of T and P where EQUILIBRIUM exists Lines connect all conditions of T and P where EQUILIBRIUM exists between the phases on either side of the line. (At equilibrium between the phases on either side of the line. (At equilibrium particles move from liquid to gas as fast as they move from gas to particles move from liquid to gas as fast as they move from gas to liquid, for example.)liquid, for example.)


Phase Equilibria — WaterSolid-liquidSolid-liquid

Gas-LiquidGas-Liquid

Gas-SolidGas-Solid


Triple Point — Water

At the TRIPLE POINT all three phases are in equilibrium.


Phases Diagrams—Important Points for Water

T(˚C)T(˚C)P(mmHg)P(mmHg)Normal boil point Normal boil point 100100 760 760Normal freeze pointNormal freeze point 00 760 760Triple point Triple point 0.00980.0098 4.58 4.58


Critical T and P

.

LIQUID

GAS

Pcritical

Hig

h Pr

essu

re

High Temperature

Tcritical

Note that linegoes straight up

Above Above critical T no critical T no liquid exists liquid exists no matter no matter how high the how high the pressure.pressure.

As P and T increase, you finally reach the CRITICAL T and PAs P and T increase, you finally reach the CRITICAL T and P

At P < 4.58 mmHg and T < 0.0098 ˚C solid HAt P < 4.58 mmHg and T < 0.0098 ˚C solid H22O O can go directly to vapor. This process is called can go directly to vapor. This process is called SUBLIMATIONSUBLIMATIONThis is how a frost-free refrigerator works.This is how a frost-free refrigerator works.


Critical T and PCOMPDCOMPD TTcc((ooC)C) PPcc(atm)(atm)HH22OO 374374 218218COCO22 3131 7373CHCH44 -82-82 4646Freon-12Freon-12 112112 4141

(CCl(CCl22FF22))Notice that TNotice that Tcc and P and Pcc depend on depend on

intermolecular forces.intermolecular forces.


Solid-Liquid Equilibria

Raising the pressure at constant Raising the pressure at constant T causes water to melt.T causes water to melt.

The NEGATIVE SLOPE of the S/L The NEGATIVE SLOPE of the S/L line is unique to Hline is unique to H22O. Almost O. Almost everything else has positive everything else has positive slope.slope.

LIQUID H2OICEfavored atlow P

favored athigh P

SolidH2O

LiquidH2O

P

T

760mmHg

0 °C

Normalfreezingpoint

The behavior of water under pressure is an example of The behavior of water under pressure is an example of LE LE CHATELIER’S PRINCIPLE CHATELIER’S PRINCIPLE At Solid/Liquid equilibrium, At Solid/Liquid equilibrium, raising P squeezes the solid. raising P squeezes the solid. It responds by going to phase with greater density, i.e., the It responds by going to phase with greater density, i.e., the liquid phase.liquid phase.In any system, if you increase P the In any system, if you increase P the DENSITYDENSITY will go up. will go up.

Therefore — as P goes up, equilibrium favors phase with Therefore — as P goes up, equilibrium favors phase with the larger density (or the larger density (or SMALLERSMALLER volume/gram). volume/gram).

Documents

Inter- molecular Forces