Chapter 11

Intermolecular Forces

A phase is a homogeneous part of the system in contact with other parts of the system but separated from them by a well-defined boundary.

2 Phases

Solid phase - ice

Liquid phase - water

11.1

11.2

Intermolecular forces are attractive forces between molecules.

Intramolecular forces hold atoms together in a molecule.

Intermolecular vs Intramolecular

• 41 kJ to vaporize 1 mole of water (inter)

• 930 kJ to break all O-H bonds in 1 mole of water (intra)

Generally, intermolecular forces are much weaker than intramolecular forces.

“Measure” of intermolecular force

boiling point

melting point

Hvap

Hfus

Hsub

Intermolecular forces are feeble; but without them, life as we know it would be impossible. Water would not condense from vapor into solid or liquid forms if its molecules didn't attract each other. Intermolecular forces are responsible for many properties of molecular compounds, including crystal structures (e. g. the shapes of snowflakes), melting points, boiling points, heats of fusion and vaporization, surface tension, and densities. Intermolecular forces pin gigantic molecules like enzymes, proteins, and DNA into the shapes required for biological activity.

http://www.nationmaster.com/encyclopedia/Image:Myoglobin.png

Intermolecular Forces

1. London Forces (Dispersion Forces)

2. Dipole-Dipole Interactions

3. Ion-Dipole Interactions (Salt dissolving in solution)

4. Hydrogen Bonding

Dispersion ForcesOccur between every compound and arise from the net attractive forcesamount molecules which is produced from induced charge imbalances

The magnitude of the Dispersion Forces is dependent upon how easily itis to distort the electron cloud. The larger the molecule the greater it’s Dispersion Forces are.

Figure 10-8 Olmsted Williams

Figure 10-9 Olmsted Williams

Olmsted Williams Fig 10-10 Pg 437

The boiling point of alkanes increase with the length of the carbon chain. Long-chain alkanes have larger dispersion forces because of the increased polarizability of their larger electron cloud.

How molecular shape affects the strength of the dispersion forces

The shapes of the molecules also matter. Long thin molecules can develop bigger temporary dipoles due to electron movement than short fat ones containing the same numbers of electrons.Long thin molecules can also lie closer together - these attractions are at their most effective if the molecules are really close.For example, the hydrocarbon molecules butane and 2-methylpropane both have a molecular formula C4H10, but the atoms are arranged

differently. In butane the carbon atoms are arranged in a single chain, but 2-methylpropane is a shorter chain with a branch.

                                                                                                                                                            Butane has a higher boiling point because the dispersion forces are greater. The molecules are longer (and so set up bigger temporary dipoles) and can lie closer together than the shorter, fatter 2-methylpropane molecules.

http://www.chemguide.co.uk/atoms/bonding/vdw.html

Polarizability

11.2

the ease with which the electron distribution in the atom or molecule can be distorted.

Polarizability increases with:

• greater number of electrons

• more diffuse electron cloud

Dispersion forces usually increase with molar mass.

Is the Molecule Polar?

We have already talked about diatomic molecules. The moreElectronegative atom will pull the electron density of the bond Closer to itself giving it a partial negative charge leaving the otherAtom with a partially positive charge. Thus giving the molecule A dipole moment.

But what about molecules made up of more than two molecules?

Molecules with 3 Atoms

HCN

Even though the C-O bond is polar, the bonds cancel each other out because the molecule is linear the dipole moments are equal and in opposite directions.Therefore CO2 is non-polar.

The dipole moment between H-C points in the direction of C. The dipole moment points between C-N points in the direction of the N. Therefore the dipole vectors are additive and HCN is polar

CO2

SO2 is a polar molecule because the S-O dipole Moments don’t cancel each other out due to the angle

SO2

CCl4 is non-polar

CHCl3 is polar

Molecules with 4 Atoms

How to Determine if a MoleculeIs Polar

1. Draw Lewis Structure

2. If all of the regions of electron density arebound to the same thing (CCl4; CO2 ) than the molecule is non-polar

3. If the regions of electron density are not bound to the same thing than the molecule is polar (HCN; SO2)

10.2

Which of the following molecules are polar (have a dipole moment)?H2O, CO2, SO2, and CH4

O HH

dipole momentpolar molecule

SO

O

CO O

no dipole momentnonpolar molecule

dipole momentpolar molecule

C

H

H

HH

no dipole momentnonpolar molecule

Dipole-Dipole Forces

Attractive forces between polar molecules

Orientation of Polar Molecules in a Solid

11.2

Figure 10-11

Dipole Forces occur between molecules containing a dipole moment. The positive end of the dipole moment on one mole is attracted to theNegative end of the dipole moment on a nearby molecule.

Consider 2-methyl propane(left) and acetone (right) Both compounds are aboutEqual in size and shape therbyHaving similar dispersion forces,But Acetone contains anOxygen (red) and causes theMolecule to have a dipole Moment allowing it to haveDipole forces and thus a Higher boiling point

Olmsted Williams

Ion-Dipole Forces

Attractive forces between an ion and a polar molecule

11.2

Ion-Dipole Interaction

The larger the charge the stronger the force

Fig 10-34

A molecular picture showing the ion-dipole Interaction that helps a solid ionic crystal dissolve in water. The arrows indicate ion-dipole interactions.

Olmsted Williams

SO

O

What type(s) of intermolecular forces exist between each of the following molecules?

HBrHBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules.

CH4

CH4 is nonpolar: dispersion forces.

SO2

SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules.

11.2

Hydrogen Bond

11.2

The hydrogen bond is a special dipole-dipole interaction between they hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom.

A H…B A H…Aor

A & B are N, O, or F

Crystals of benzoic acid contain pairs of molecules held together head tohead by hydrogen bonds. These pairs then stack in planes which are Held together by dispersion forces.

Fig 10-16B Pg 444

Courtesy Stephen Frisch

Intermolecular Forces

1. London Forces (Dispersion Forces)

2. Dipole-Dipole Interactions

3. Ion-Dipole Interactions (Salt dissolving in solution)

4. Hydrogen Bonding

These forces affect how molecules will interact with each other andAs a general rule as the strength of the force increases the boiling Point of the compound increases

Surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area.

Strong intermolecular

forces

High surface tension

11.3

Liquids and Surface Tension

Properties of Liquids

Cohesion is the intermolecular attraction between like molecules

11.3

Adhesion is an attraction between unlike molecules

Adhesion

Cohesion

E

vapo

ratio

nGreatest

Order

LeastOrder

11.8C

onde

nsat

ion

T2 > T1

The equilibrium vapor pressure is the vapor pressure measured when a dynamic equilibrium exists between condensation and evaporation

H2O (l) H2O (g)

Rate ofcondensation

Rate ofevaporation=

Dynamic Equilibrium

11.8

A substance with a high Vapor pressure is consideredTo be volitile therefore, the lowerThe boiling point the higher the Vapor pressure and the weakerThe intermolecular forces

The boiling point is the temperature at which the (equilibrium) vapor pressure of a liquid is equal to the external pressure.

The normal boiling point is the temperature at which a liquid boils when the external pressure is 1 atm.

11.8

The critical temperature (Tc) is the temperature above which the gas cannot be made to liquefy, no matter how great the applied pressure.

The critical pressure (Pc) is the minimum pressure that must be applied to bring about liquefaction at the critical temperature.

11.8

Mel

ting

11.8F

reez

ing

H2O (s) H2O (l)

The melting point of a solid or the freezing point of a liquid is the temperature at which the solid and liquid phases coexist in equilibrium

11.8

Sub

limat

ion

11.8

Dep

ositi

on

H2O (s) H2O (g)

Molar heat of sublimation (Hsub) is the energy required to sublime 1 mole of a solid.

Hsub = Hfus + Hvap

( Hess’s Law)

A phase diagram summarizes the conditions at which a substance exists as a solid, liquid, or gas.

Phase Diagram of Water

11.9

11.9