INTERMOLECULAR FORCES
1. Van der Waals Forces or London Dispersion Force

Weak force.

Brought about by the fluctuating charge of the molecule due to electron
distribution.

This force results to temporary dipole, causing induced polarization.

An increase in the number of carbon causes an increase Van der Waals
Force.

Branching causes a decrease of Van der Waals Force.
2. Dipole-Dipole

Observed among partly ionic or polar molecules.

Brought about by the interaction of two charges at both ends – one is
slightly positive and the other is slightly negative.

It induces polarization causing a positive charge to be negative.
o Partially negative elements: Halogens, oxygen
o Partially positive element: Carbon
3. Hydrogen Bonding

Strongest type of intermolecular force

Can either be intramolecular or intermolecular
o Intermolecular H-bonding raises the boiling point of organic
compounds.
o Intramolecular H-bonding lowers the boiling point because of the
decreased tendency for interaction with other molecules.

Involves an interaction between a polarized OH bond and NH bond.

Happens when two molecules that contain a hydrogen atom bonded to a
very electronegative atom.
Type of Force
Relative Strength
Exhibited by
Van der Waals
Weak
All molecules
Dipole-Dipole
Moderate
Molecules with net dipole
Molecules with an O-H, NH, or H-F bond
Hydrogen Bonding
Strong
Example
CH4
H2O
H2O
H2O
Experiment 8: Boiling Point
 It is the temperature at which the vapor pressure of the substance is equals the pressure
of the atmosphere above it.
 It is a physical constant that can be used in identification and characterization, as well as
a criterion of purity of a substance.
 Pure compounds have constant boiling point.
 Mixtures have a boiling point range except for azeotropes.
 Compounds involving ionic bonds have higher boiling point because the amount of heat
required to separate the ions is higher than the amount required to separate molecules
in covalent compounds.
 Presence of impurities may affect the boiling point of a liquid.

Non-volatile impurities usually increase the boiling point of the liquid due to
a decrease in the vapor pressure.

Volatile impurities usually decrease the boiling point of the liquid.
Test Compound:
n-Butyl Alcohol
Boiling Point: 118°C
Test Compound:
n-amyl Alcohol
Boiling Point: 136-138°C
Test Compound:
tert-Butyl Alcohol
Boiling Point: 82°C
N-butyl alcohol vs n-amyl alcohol

n-amyl alcohol has more carbon, thus, has stronger Van der Waals force.
N-butyl alcohol vs tert-butyl alcohol

tert-butyl alcohol has lower boiling point because of the branching.
Experiment 9: Melting Point
 It is the temperature at which the solid becomes a liquid.

Solids with greater cohesive forces exhibit higher melting points.

Pure crystalline solids normally have sharp melting points.
Test Compound:
Stearic acid
Melting Point: 68-70°C
- long straight chain; Van der Waals
Test Compound:
Benzoic acid
Melting Point: 120-122°C
Benzoic acid - (carboxyl group) OH makes the benzoic acid more
open for h-bonding.
- More h-bonds means more difficult to melt.
Test Compound:
Salicylic acid
Melting Point: 155-160°C
Salicylic has carboxylic acid group thus it has more intermolecular
forces making it more difficult to melt.
Experiment 10: Solubility
Test Compound:
OH
n-Propyl Alcohol
n-Propyl alcohol + Water
n-Propyl alcohol + Ether
 soluble
 soluble
Solubility Class: S1 (water soluble)
Structural Effect:
- Hydroxyl group easily forms H-bonds with water.
- The length of the carbon chain also implies weak Van der Waals
force.
Test Compound:
Naphthalene
Naphthalene + Water  insoluble
Naphthalene + NaOH  insoluble
Naphthalene + HCl  insoluble
Naphthalene + H2SO4  insoluble
Solubility Class: INERT
Structural Effect:
- The fused benzene ring makes the compound resonance stabilize.
Test Compound:
Urea
Urea + Water  soluble
Urea + Ether  insoluble
O
C
H2N
NH2
Solubility Class: S2 (water soluble)
Structural Effect:
- Soluble in water because of the NH group.
- The double bond could also form H-bonds.
- This is an example of an exemption to “like dissolve like.”
Test Compound:
Aniline
NH2
Aniline + Water  insoluble
Aniline + NaOH  insoluble
Aniline + HCl  soluble
C6H5NH2 + HCl  C6H5N+H3ClSolubility Class: BASIC
Structural Effect:
- NH site reacts with Cl.
Test Compound:
Benzoic Acid
O
OH
Benzoic acid + Water  insoluble
Benzoic acid + NaOH  soluble
C6H5COOH + NaOH  C6H5COO-Na+ + H2O
Benzoic acid + NaHCO3 partially soluble
C6H5COOH + NaHCO3  C6H5COO-Na+ + H2O + CO2
Solubility Class: A1 (acidic)
Structural Effect:
- COOH group is drawn to the benzene ring.
- The hydroxyl group in H2O cannot interact with any of the groups
in benzoic acid.

The more the intramolecular H-bonding the more insoluble the sample is.

Polar solvents dissolve the solute because of dipole-dipole interaction.

Non-polar solvents dissolve the solute because of dipole-dipole interaction.
Experiment 11: Acidity and Basicity
 Arrhenius acid – a substance that produces hydrogen ions H+ or hydronium ions in
water.

Arrhenius acids are frequently referred to as proton donors, hydrogen ion
donors, or hydronium ion donors

Chemical formula is written with the acidic hydrogen first.
o
Example: HCl
 Arrhenius base – a substance that produces hydroxide ions OH- in water.

NaOH is an example of an Arrhenius base.
 Bronsted-Lowry acid – a substance that donates a proton (H+).
 Bronsted-Lowry base – a substance that accepts a proton (H+).
Arrhennius
Bronsted-Lowry
Lewis
Acid
Ionize to give H+
A proton donor
An electron pair acceptor
Base
Ionize to give OHA proton acceptor
An electron pair donor