Intermolecular Forces

Explore Van der Waals' forces and Hydrogen bonding

Compare Substances

Boiling Point (°C)

100°C50°C0°C-50°C-100°C-150°C

C H_{4}

C_{2} H_{6}

C_{3} H_{8}

C_{4} H_{10}

C H_{3} O H

C_{2} H_{5} O H

C_{3} H_{7} O H

Substance (Molar Mass)

Van der Waals' Forces

As molar mass increases, Van der Waals' forces become stronger, increasing boiling point.

Two CH₄ molecules

Lower molar mass (or smaller surface area for isomers)

\to

fewer electrons (or smaller area of contact)

\to

weaker attraction.

Two C₄H₁₀ molecules

Higher molar mass (or larger surface area for isomers)

\to

more electrons (or greater area of contact)

\to

stronger attraction.

Strength: Weak, but increases with molar mass and surface area.

Hydrogen Bonding

Alcohols have significantly higher boiling points than alkanes of similar molar mass due to Hydrogen bonding.

Three H₂O molecules

δ+ on H (bonded to O) is attracted to the lone pair on a neighbouring O atom.

Electronegativity (Pauling scale)

Period 1

H2.1

He—

Period 2

Li0.9

Be1.5

B2.0

C2.5

N3.0

O3.5

F4.0

Ne—

Period 3

Na0.9

Mg1.2

Al1.5

Si1.8

P2.1

S2.5

Cl3.0

Ar—

Low ENHigh ENF, O, N (H-bond partners)

F, O and N are the most electronegative elements — when H is bonded to them, the bond is highly polar and hydrogen bonding can occur.

Across a period, electronegativity increases as the increasing nuclear charge attracts bonding electrons more strongly, whereas down a group, it decreases because additional electron shells increase shielding and distance from the nucleus.

Requirements:

H

atom covalently bonded to a highly electronegative atom (

F

O

, or

N

) which attracts the lone pair on a neighboring molecule's

F

O

, or

N

atom.

Strength: Stronger than other Van der Waals' forces, leading to abnormally high boiling points.