Intermolecular Forces

Intramolecular Forces vs. Intermolecular Forces

Intramolecular Forces Chemical bonds

Intermolecular Forces Attractive forces between molecules

What are intermolecular forces?

NOT chemical bonds, less strength

Attractive forces between molecules involved in covalent bonding

Molecular level, not individual atoms

Develop solid and liquid physical properties

Types London Dispersion Dipole-Dipole Ion-Dipole Hydrogen Bonding

(London) Dispersion Forces

Attractive force between dipoles

TEMPORARY charge separation in an atom (instantaneous dipole) Sudden dipole in one atom causes “domino”

effect—influences electron distribution within adjacent atoms (induced dipoles

At SOME point in time, electron density is greater around one atom than the other Induces temporary dipoles in adjacent molecules

Found in ALL atoms/molecules

Intermolecular Force Strength

Influenced by:

1) Polarizability

2) Molecular Shape

1. Polarizability

How easily can a dipole be induced in a molecule or atom How easily can the electron density of an

atom/molecule be altered

polarizability, strength of intermolecular forces (WHY?)

**Atomic Size and Mass have an influence on polarizability

Example 1:

Which molecule is more likely to a gas at room temperature: F2 or Br2?

2. Molecular Shape

More contact with adjacent molecules creates more dispersion forces Bigger/elongated molecules have this advantage Compact molecules do not have as much surface area

Example 2:

Which compound has the strongest intermolecular forces?

Dipole-Dipole Forces

PERMANENT charge separation (dipole)

All polar molecules More intermolecular forces (dipole-dipole +

dispersion)

Molecules arranged so the POSITIVE end of one molecule is oriented toward the NEGATIVE end of another molecule Molecular ends with like charges repel each other

Example 3: Propane vs. acetaldehyde

Which has stronger intermolecular forces?

Example 4

Based on intermolecular forces, arrange the following in order from decreasing to increasing boiling point: CBr4

CH3CH2CH2CH3

F2

CH3CHO

Example 5

Examine the following 2 chemical compounds: BrCl and IBr. Which compound is a gas at room temperature? A solid?

Ion-Dipole

Attractive forces between ions and polar molecules Results from soluble ionic compounds in polar solvent (ex. Water) Ions split up in solvent and are surrounded by water molecules

Negative portion of water molecule surrounds POSITIVE ion

Positive portion of water molecule surrounds NEGATIVE ion

Hydrogen Bonding

Type of intermolecular force, strong dipole-dipole force

H atom is bonded to an electronegative (nonmetal) atom and is attracted to the electronegative (nonmetal) atom in a neighboring molecule

Majority of H-bonding occurs among small, very electronegative, nonmetal atoms (N, O, F)

Dotted lines represent hydrogen bonding

Hydrogen Bonding (cont.)

Considered a FORCE, not a type of bond

Force holding water molecules together

Results from a negative charge on an atom and a positive charge on hydrogen

H-X format

Example: H2O, HF

Unique Properties of Water due to Hydrogen Bonding

Freezes and expands as solid

Solid is less dense than liquid (ex. Ice floats)

High melting point

High boiling point H-bonding strength greater than other

intermolecular forces

Highest surface tension (Hg only exception)

Other compounds can easily dissolve in it

High specific heat

Biological Applications to H-Bonding

Protein Structure

Enzyme Activity

DNA structure

ALL DUE TO HYDROGEN BONDING !

Intermolecular Force Strength

All chemical compounds have dispersion forces

Other intermolecular forces are dependent on molecular structure

Generally, increasing molecular weight indicates an increase in intermolecular forces Analyze molecular structure and types of

intermolecular forces for compounds of similar molecule weight.

Similar forces, look at molecular weight

Strength of Intermolecular Forces

Example 6:

The following compounds have similar molecular weights. Arrange in order from DECREASING to INCREASING boiling point. Formaldehyde (H2C=O)

Methanol (CH3—OH)

Ethan (CH3CH3)

Example 7:

Arrange in order from DECREASING to INCREASING boiling point. Carbon tetrachloride (CCl4)

Acetone (C3H6O)

Tetrabromobutane (C4H6Br4)

Example 8:

Arrange in order from INCREASING to DECREASING boiling point. Octanoic acid—CH3(CH2)6COOH

Decane—CH3(CH2)8CH3

Nonanal—CH3(CH2)7CHO