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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.

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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


Generally, intermolecular forces are much weaker than intramolecular forces.

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)

“Measure” of intermolecular force

boiling point

melting point





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.

Intermolecular Forces we know it would

  • 1. London Forces (Dispersion Forces)

  • Dipole-Dipole Interactions

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

  • 4. Hydrogen Bonding

Dispersion Forces we know it would

Occur between every compound and arise from the net attractive forces

amount molecules which is produced from induced charge imbalances

Figure 10-8 Olmsted Williams

The magnitude of the Dispersion Forces

is dependent upon how easily it

is to distort the electron cloud.

The larger the molecule the greater

it’s Dispersion Forces are.

Figure 10-9 Olmsted Williams

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.

Olmsted Williams Fig 10-10 Pg 437

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.

Dispersion forces usually increase with molar mass. forces


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


Is the Molecule Polar? forces

We have already talked about diatomic molecules. The more

Electronegative atom will pull the electron density of the bond

Closer to itself giving it a partial negative charge leaving the other

Atom 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 forces

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



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

Molecules with 4 Atoms forces

CCl4 is non-polar

CHCl3 is polar

How to Determine if a Molecule forces

Is Polar

  • Draw Lewis Structure

  • If all of the regions of electron density are

  • bound to the same thing (CCl4; CO2 ) than the

  • molecule is non-polar

  • If the regions of electron density are not bound to

  • the same thing than the molecule is polar (HCN; SO2)

Which of the following molecules are polar (have a forces

dipole moment)?H2O, CO2, SO2, and CH4















dipole moment

polar molecule

dipole moment

polar molecule

no dipole moment

nonpolar molecule

no dipole moment

nonpolar molecule


Orientation of Polar Molecules in a Solid forces

Dipole-Dipole Forces

Attractive forces between polar molecules


Dipole Forces occur between molecules containing a dipole moment.

The positive end of the dipole moment on one mole is attracted to the

Negative end of the dipole moment on a nearby molecule.

Consider 2-methyl propane

(left) and acetone (right)

Both compounds are about

Equal in size and shape therby

Having similar dispersion forces,

But Acetone contains an

Oxygen (red) and causes the

Molecule to have a dipole

Moment allowing it to have

Dipole forces and thus a

Higher boiling point

Figure 10-11

Olmsted Williams

Ion-Dipole Interaction moment.

Ion-Dipole Forces

Attractive forces between an ion and a polar molecule

The larger the charge the stronger the force


Fig 10-34 moment.

Olmsted Williams

A molecular picture showing the ion-dipole

Interaction that helps a solid ionic crystal dissolve

in water. The arrows indicate ion-dipole interactions.

O moment.



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


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


CH4 is nonpolar: dispersion forces.


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


or moment.







Hydrogen Bond

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 & B are N, O, or F


Fig 10-16B moment.

Pg 444

Crystals of

benzoic acid

contain pairs of

molecules held

together head to

head by

hydrogen bonds.

These pairs

then stack in

planes which are

Held together by

dispersion forces.

Courtesy Stephen Frisch

Intermolecular Forces moment.

  • 1. London Forces (Dispersion Forces)

  • Dipole-Dipole Interactions

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

  • 4. Hydrogen Bonding

These forces affect how molecules will interact with each other and

As a general rule as the strength of the force increases the boiling

Point of the compound increases

Liquids and Surface Tension moment.

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


Adhesion moment.


Properties of Liquids

Cohesion is the intermolecular attraction between like molecules

Adhesion is an attraction between unlike molecules


Least moment.




T2 > T1




H moment. 2O (l) H2O (g)

Dynamic Equilibrium

Rate of


Rate of



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

A substance with a high

Vapor pressure is considered

To be volitile therefore, the lower

The boiling point the higher the

Vapor pressure and the weaker

The intermolecular forces


The moment. 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.


The moment. 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.


H moment. 2O (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 moment.

H moment. 2O (s) H2O (g)

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



DHsub = DHfus + DHvap

( Hess’s Law)


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

Phase Diagram of Water


11.9 moment.