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 - iceSolid phase - ice

Liquid phase - waterLiquid phase - water

A Molecular Comparison of Liquids and Solids

Recall that we go from a gas to a liquid to a solid, a substance’s molecules become closer to each other. We know this happens by decreasing the temperature, which also decreases the volume.

Intermolecular forces are the forces that hold solids and liquids together. These are the forces of attraction between molecules (inter-). (An intramolecular force is one that holds the atoms inside a molecule together via a covalent bond.)

Intermolecular forces are much weaker than intramolecular forces. When a substance melts or boils, the intermolecular forces are broken while the covalent bonds (intramolecular) stay in place. Intermolecular forces form when a substance condenses or freezes.

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

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

“Measure” of intermolecular forceboiling pointmelting point

Intermolecular Forces

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

Intermolecular Forces

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

INTRAMOLECUAR BONDINTRAMOLECUAR BOND

INTERMOLECULAR BONDINTERMOLECULAR BOND

Which is Which is a a stronger stronger bond?bond?

Generally, intermolecular forces are much Generally, intermolecular forces are much weaker than intramolecular forces.weaker than intramolecular forces.

1)1) Intermolecular forces are feeble; Intermolecular forces are feeble; a)a) Water would not condense from vapor into Water would not condense from vapor into

solid or liquid forms if its molecules didn't solid or liquid forms if its molecules didn't attract each other. attract each other.

2)2) Intermolecular forces are responsible for many Intermolecular forces are responsible for many properties of molecular compounds, properties of molecular compounds, a)a) including crystal structures (e. g. the shapes including crystal structures (e. g. the shapes

of snowflakes), melting points, boiling points, of snowflakes), melting points, boiling points, heats of fusion and vaporization, surface heats of fusion and vaporization, surface tension, and densities. tension, and densities.

3)3) Intermolecular forces pin gigantic molecules Intermolecular forces pin gigantic molecules like enzymes, proteins, and like enzymes, proteins, and a)a) DNA into the shapes required for biological DNA into the shapes required for biological

activity. activity.

Intermolecular ForcesIntermolecular Forces1)1) Ion-Dipole Interactions Ion-Dipole Interactions

(Salt dissolving in solution) (Salt dissolving in solution) 2)2) Dipole-Dipole InteractionsDipole-Dipole Interactions3)3) Dispersion London ForcesDispersion London Forces4)4) Hydrogen BondingHydrogen Bonding

1) An ion-dipole force is the interaction of an ion and a polar solvent, such as water. This is the strongest of all intermolecular forces. A larger charge and smaller ion means a larger ion-dipole attraction.

Example: Example: When NaCl is dissolved in water, the When NaCl is dissolved in water, the sodium and chloride ions are pulled apart and sodium and chloride ions are pulled apart and kept separated by the water molecules. The kept separated by the water molecules. The sodium cation is attracted to the partially sodium cation is attracted to the partially negative end of the water molecule (oxygen). negative end of the water molecule (oxygen). The chloride anion is attracted to the partially The chloride anion is attracted to the partially positive end of the water molecule (hydrogen).positive end of the water molecule (hydrogen).

1) Ion-Dipole Interactions1) Ion-Dipole Interactions• A ion-dipole interactions are an important force A ion-dipole interactions are an important force

in solutions of ions.in solutions of ions.• The strength of these forces are what make it The strength of these forces are what make it

possible for ionic substances to dissolve in polar possible for ionic substances to dissolve in polar solvents.solvents.

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

Olmsted Williams

2) Dipole-dipole forces2) Dipole-dipole forces are the third strongest are the third strongest intermolecular force. Dipole-dipole forces intermolecular force. Dipole-dipole forces exist between neutral polar molecules. exist between neutral polar molecules.

For dipole-dipole forces to be effective, the For dipole-dipole forces to be effective, the polar molecules must be close together. There polar molecules must be close together. There is a mix of repulsion and attraction between is a mix of repulsion and attraction between the ends of the dipoles.the ends of the dipoles.

A stronger bond means that there is a A stronger bond means that there is a higher melting point and boiling point higher melting point and boiling point because more energy is needed to break because more energy is needed to break the bond.the bond.

2) Dipole-Dipole Interactions2) Dipole-Dipole Interactions• Molecules that have Molecules that have

permanent dipoles are permanent dipoles are attracted to each other.attracted to each other.– The positive end of The positive end of

one is attracted to the one is attracted to the negative end of the negative end of the other and vice-versa.other and vice-versa.

– These forces are only These forces are only important when the important when the molecules are close to molecules are close to each other.each other.

Dipole-Dipole Forces

Attractive forces between polar molecules

Orientation of Polar Molecules in a Solid

11.2

From experimental data, it was found that the boiling points of compounds with H-F, H-O, and H-N bonds were abnormally high. This means the intermolecular bonds in these molecules had to be stronger than normal; however, the only forces present were dipole-dipole forces.

This led to the discovery of a special case of dipole-dipole forces: hydrogen bonding. Hydrogen bonding is the second strongest of all intermolecular forces.

Hydrogen bonding requires a hydrogen bonded to a strongly electronegative element (F,O or N usually). In these bonds, the electrons lie closer to the electronegative element. Since hydrogen has only one electron, the slightly positive hydrogen presents an almost bare proton.

In many cases, this almost bare proton will attract other (partially) negatively charged atoms besides the one it is bonded to (usually F, O, or N, on another molecule). This will create a “bond” between the hydrogen and the other atom. While this is not a true bond, there is a much stronger electrostatic attraction than normal which affects the properties of the substances involved.

CCH2

C

OO

HH

H δ+Hydrogen bondingδ-Oδ-

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

3) Hydrogen Bonding3) Hydrogen Bonding• The dipole-dipole The dipole-dipole

interactions interactions experienced when experienced when H is bonded to N, H is bonded to N, O, or F are O, or F are unusually strong.unusually strong.

• We call these We call these interactions interactions hydrogen bondshydrogen bonds..

It is possible for two adjacent neutral molecules to affect each other. The nucleus of one molecule (or atom) attracts the electrons of the adjacent molecule (or atom), and vise versa. For an instant, the electron clouds become distorted. In that instant, a dipole is formed. One instantaneous dipole can induce another instantaneous dipole in an adjacent molecule or atom.

4) Instantaneous dipoles are called London dispersion forces. These are the weakest of all intermolecular forces, but they exist between all molecules.

Polarizability is the ease with which an electron cloud can be deformed. A larger molecule is more polarizable because it has more electrons. Thus, London dispersion forces increase with increasing molar mass.

4) London Dispersion Forces4) London Dispersion Forces

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

4) London Dispersion Forces4) London Dispersion Forces

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

PolarizabilityPolarizability

11.2

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

Polarizability increases with:Polarizability increases with:

• greater number of electronsgreater number of electrons

• more diffuse electron cloudmore diffuse electron cloud

Dispersion forces Dispersion forces usually increase usually increase with molar mass.with molar mass.

Hydrogen bonding

Interacting molecules or ionsInteracting molecules or ions

Are ions involved?Are polar molecules involved?

Are polar molecules involved?

Are hydrogen atoms bonded to F, O or N?

London dispersion forces only

Dipole-dipole forces

Ionic bonding

yes

yes

yes

yesno

no

no

no

Increasing strengthIncreasing strength

Ion-dipole forces