INTERMOLECULAR FORCES

MRS. LOZANO

CHEMISTRY

ATTRACTIVE FORCES electrostatic in nature

Intramolecular forces

bonding forces

These forces exist withinwithin each molecule.They influence the chemicalchemical properties of the substance.

Intermolecular forcesnonbonding forces

These forces exist betweenbetween molecules.They influence the physicalphysical properties of the substance.

BONDING IS AN INTRAMOLECULAR FORCE

Which is stronger? Intermolecular or Intramolecular?

adhesive forcesstronger

cohesive forces

H2O

capillarity

Hg

3 INTERMOLECULAR FORCES

1. DIPOLE-DIPOLE forces

Cl2 molecules

3. DISPERSION/ VANDERWAALS

forces

2. ION-DIPOLE forces

3 INTERMOLECULAR FORCES

1. DIPOLE-DIPOLE forces

When elements in a molecule have very different electronegativities each side has an opposite charge. We call this polar. This results in two poles, and we call it a di-pole!

INTERMOLECULAR FORCE: DIPOLE-DIPOLE

This occurs when two polar substances come together. They are attracted because of the partial charges on each molecule!

As suggested through (a) Lewis structures and (b) ball-and-stick models, each water molecule is linked to four others through hydrogen bonds. Each H atom lies along a line that joins two O atoms. The shorter distances (100 pm) correspond to covalent bonds, and the longer distances (180 pm) to the hydrogen bonds.

The H-bonding ability of the water molecule.

WATER: dipole-dipoleHydrogen bonding: a special type ofdipole-dipole bond.

THE HYDROGEN BONDa dipole-dipole intermolecular force

The elements which are so electronegative are F, O, and N.

A hydrogen bond may occur when an H atom in a molecule, bound to small highly electronegative atom.

..H O..

FH

..

..

..

..NH

HYDROGEN BONDING IN ACETIC ACID: VINEGAR

SOLUTION:

C2H6(a) CH3OH(b) CH3C NH2

O

(c)

(a) C2H6 has no H bonding sites.

(c)(b)C O H

H

H

H

COH

H

H

H

CH3C N

O

H

H

CH3CN

O

H

H

CH3CN

O

H

H

CH3CN

O

H

H

Which one will H bond?

Figure 12.19 The molecular basis of surface tension.

hydrogen bondingoccurs in three

dimensions

hydrogen bondingoccurs across the surface

and below the surfacethe net vectorfor attractive

forces is downward

HYDROGEN BONDING IN DNAHydrogen bonding connects A to T and G to C, holding the two strands together!

Each NH group is hydrogen bonded to a CO group one helical turn (3.6 amino acid units) away in the same chain, giving a fairly rigid cylindrical structure with side chains on the outside.

INTERMOLECULAR FORCES IN PROTEIN STRUCTURE

The 4 types of interactions that determine tertiary structure of proteins are shown. Though varying in strength, each set of interactions is significant in the formation of intrachain bonds that fixes the structure of the overal polymer.

Figure 13.7 The forces that maintain protein structure.

Figure 13.10 Intermolecular forces and membrane structure.

Figure 12.14Hydrogen bonding and boiling point: Stronger intermolecular forces result in a higher boiling point

3 INTERMOLECULAR FORCES

2. ION-DIPOLE forces

INTERMOLECULAR FORCE: ION-DIPOLE

When ionic substances dissolve in water you have an ion interacting with the partial charges on water. These are Ion-Dipole forces

3 INTERMOLECULAR FORCES

Cl2 molecules

3. DISPERSION/ VANDERWAALS

forces

VANDERWAALS FORCE OR DISPERSION FORCE: This occurs in a non polar substance which can have an instantaneous dipole causing weak attractions

(a) In an unpolarized molecule the electron charge distribution is symmetrical. (b) A displacement of electron charge density (to the left) produces an instantaneous dipole. (c) The instantaneous dipole on the left induces charge separation in the molecule on the right, making it also a dipole, called an induced dipole. The attraction between the two dipoles constitutes an intermolecular force.

Liquid Helium boils at 4.22 K or -269 °C. WHY SO LOW?

Gecko toes: setae and van der Waals forces(seta is a stiff hair or bristle)

Recent studies of the spatula tipped setae on gecko footpads demonstrate that the attractive forces that hold geckos to surfaces are van der Waals interactions between the finely divided setae and the surfaces themselves. Every square millimeter of a gecko's footpad contains about 14,000 hair-like setae.

Figure 12.15 Dispersion forces among nonpolar molecules.

separated Cl2

molecules

instantaneous dipoles

When molecules move VERY SLOWLY they can cause the electrons to temporarily move to one side. This causes a temporary change in charge on each side and can induce (cause) a dipole in the neighboring molecule

Non-polar substances are only able to do this at a very low temperature.

Types of bondWhat force(s) would a diatomic molecule such as

N2 have?

London Dispersion because they have the same electronegativity

What force(s) would a molecule such as NH3 (ammonia) have?

All forces… they have hydrogen bonding which makes them a dipole, but at low temps london dispersion can also occur