Intermolecular Forces in Liquids Only

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Copyright 1999, PRENTICE HALL Chapter 11 1

A Molecular Comparison of Liquids andA Molecular Comparison of Liquids and

SolidsSolids


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SolidsSolids Physical properties of substances understood in terms

of kinetic molecular theory: Gases are highly compressible, assumes shape and volume

of container:

Gas molecules are far apart and do not interact much with each other.

Liquids are almost incompressible, assume the shape butnot the volume of container:

Liquids molecules are held closer together than gas molecules, but not so

rigidly that the molecules cannot slide past each other.

Solids are incompressible and have a definite shape andvolume:

Solid molecules are packed closely together. The molecules are so

rigidly packed that they cannot easily slide past each other.


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The 3 StatesThe 3 States


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SolidsSolids

Converting a gas into a liquid or solid requires themolecules to get closer to each other: cool or compress.

Converting a solid into a liquid or gas requires themolecules to move further apart: heat or reduce pressure.

The forces holding solids and liquids together are

called intermolecular forces.


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Intermolecular ForcesIntermolecular Forces

The covalent bond holding a molecule together is anintramolecular forces.

The attraction between molecules is an intermolecularforce.

Intermolecular forces are much weaker thanintramolecular forces (e.g. 16 kJ/mol vs. 431 kJ/molfor HCl).

When a substance melts or boils the intermolecularforces are broken (not the covalent bonds).

When a substance condenses intermolecular forcesare formed.


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1. Ion - Dipole

2. Dipole-Dipole Forces3. Dipole - Induced Dipole

4. Instantaneous Dipole-Induced Dipole

5. Hydrogen Bonding


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Table of Force EnergiesTable of Force Energies

Type of Force Energy (kJ/mol)

Ionic Bond 300-600Covalent 200-400

Hydrogen Bonding 20-40

Ion-Dipole 10-20

Dipole-Dipole 1-5

Instantaneous Dipole/

Induced Dipole 0.05-2


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IonIon--Dipole ForcesDipole Forces Interaction between an ion (e.g. Na+) and a dipole (e.g.

water).

Strongest of all intermolecular forces:

Since Q1 is a full charge and Q2 is a partial charge, Fis

comparatively large. Fincreases as Q increases and as ddecreases:

the larger the charge and smaller the ion, the larger the ion-dipole attraction.

221

d

QQkF!


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IonIon -- Dipole InteractionsDipole Interactions


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DipoleDipole--Dipole ForcesDipole Forces Interaction between an ion (e.g. Na+) and a dipole (e.g.

water).

Dipole-dipole forces exist between neutral polarmolecules.

Polar molecules need to be close together.

Weaker than ion-dipole forces:

Q1 and Q2 arepartialcharges.

2 21dQQkF!


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DipoleDipole--Dipole ForcesDipole Forces

dd


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DipoleDipole--Dipole ForcesDipole Forces There is a mix of attractive and

repulsive dipole-dipole forces as the

molecules tumble. If two molecules have about the

same mass and size, then dipole-dipole forces increase with

increasing polarity.


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London Dispersion ForcesLondon Dispersion Forces Weakest of all intermolecular forces.

It is possible for two adjacent neutral molecules toaffect each other.

The nucleus of one molecule (or atom) attracts theelectrons of the adjacent molecule (or atom).

For an instant, the electron clouds become distorted.

In that instant a dipole is formed (called aninstantaneous dipole).


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London Dispersion ForcesLondon Dispersion Forces


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London Dispersion ForcesLondon Dispersion Forces One instantaneous dipole can induce another

instantaneous dipole in an adjacent molecule (oratom).

The forces between instantaneous dipoles are calledLondon dispersion forces.

Polarizability is the ease with which an electron cloud

can be deformed. The larger the molecule (the greater the number of

electrons) the more polarizable.


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-Temporary Induced Dipole-Dipole interactions

Very Weak always present in the condensed

phase


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LondonLondon

DispersionDispersion

ForcesForces


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London Dispersion ForcesLondon Dispersion Forces London dispersion forces increase as molecular

weight increases.

London dispersion forces exist between all molecules.

London dispersion forces depend on the shape of themolecule.

The greater the surface area available for contact, the

greater the dispersion forces. London dispersion forces between spherical molecules

are lower than between sausage-like molecules.


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Intermolecular ForcesIntermolecular ForcesHydrogen BondingHydrogen Bonding

Special case of dipole-dipole forces. By experiments: boiling points of compounds with H-

F, H-O, and H-N bonds are abnormally high.

Intermolecular forces are abnormally strong.

H-bonding requires H bonded to an electronegativeelement (most important for compounds of F, O, andN). Electrons in the H-X (X = electronegative element) lie much

closer to X than H.

H has only one electron, so in the H-X bond, the H+ Hpresents an almost bare proton to the H- X.

Therefore, H-bonds are strong.


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HH--BondingBonding

Occurs when Hydrogen is attached to ahighly electronegative atom.

N-H N- O-H N- F-H N-

N-H O- O-H O- F-H O-

N-H

F- O-H

F- F-H

F-H+ H-

Requires Unshared Electron Pairs of Highly

Electronegative Elements


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Hydrogen

Bonding

in Water

Molecules


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Structure of Ice

Observe the orientation of theHydrogen Bonds

HB-ice


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Hydrogen bonds are responsible for: Ice Floating

Solids are usually more closely packed than liquids;

therefore, solids are more dense than liquids.

Ice is ordered with an open structure to optimize H-bonding. Therefore, ice is less dense than water.

In water the H-O bond length is 1.0 .

TheOH hydrogen bond length is 1.8 .

Ice has waters arranged in an open, regular hexagon.

Each H+ H points towards a lone pair on O. Ice floats, so it forms an insulating layer on top of lakes, rivers, etc.

Therefore, aquatic life can survive in winter.


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Why Does Ice Float?Why Does Ice Float?

D2O(s)H2O(s)


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The Boiling Points of the CovalentHydrides of theThe Boiling Points of the CovalentHydrides of the

Elements in Groups 4A, 5A, 6A, and 7AElements in Groups 4A, 5A, 6A, and 7A


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Hydrogen bonds are responsible for: Protein Structure

Protein folding is a consequence of H-bonding.

DNATransport of Genetic Information


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Intermolecular ForcesIntermolecular ForcesComparing Intermolecular ForcesComparing Intermolecular Forces


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Some Properties of LiquidsSome Properties of Liquids

ViscosityViscosity Viscosity is the resistance of a liquid to flow. A liquid flows by sliding molecules over each other.

The stronger the intermolecular forces, the higher the

viscosity.

Surface TensionSurface Tension

Bulk molecules (those in the liquid) are equallyattracted to their neighbors.


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Surface TensionSurface Tension Surface molecules are only attracted inwards towards

the bulk molecules. Therefore, surface molecules are packed more closely than

bulk molecules.

Surface tension is the amount of energy required toincrease the surface area of a liquid.

Cohesive forces bind molecules to each other.

Adhesive forces bind molecules to a surface.


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Surface TensionSurface Tension Meniscus is the shape of the liquid surface.

If adhesive forces are greater than cohesive forces, theliquid surface is attracted to its container more than thebulk molecules. Therefore, the meniscus is U-shaped (e.g.

water in glass). If cohesive forces are greater than adhesive forces, the

meniscus is curved downwards.

CapillaryAction: When a narrow glass tube is placed

in water, the meniscus pulls the water up the tube.

Intermolecular Forces in Liquids Only

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