A STUDY OF INTERMOLECULAR FORCES OF ATTRACTION | EXPERIMENT 3

London dispersion, dipole–dipole, dipole–induced dipole

Types of van der Waals forces

Ion–dipole

Non–van der Waals force

London dispersion forces (LDF)

Caused by temporary instantaneous dipoles; present in all molecules

Polarizability

Ease of distortion of electron cloud; increases with molar mass → stronger LDF

London dispersion forces

Main IMF in O₂(l)

H bonded to N/O/F and a lone pair on N/O/F

Hydrogen bonding occurs between

Ion–dipole

Strongest IMF among the four

Ion–dipole interactions

Force responsible for dissolving ionic salts in polar solvents

Polar molecules with permanent dipoles

Dipole–dipole interactions occur between

London dispersion

IMF present in all substances

Hydrogen bonding

Strongest IMF in HF(l)

Deflection of water stream

Due to alignment of permanent dipoles with charged rod

No deflection — nonpolar

Vegetable oil and deflection

Ethanol deflects less → less polar than water

Ethanol vs water deflection

High surface tension from H-bonding

Water’s bead shape on wax

Due to strong surface tension from hydrogen bonds

Paper clip floating on water

Breaks hydrogen bonds → lowers surface tension → clip sinks

Detergent effect on surface tension

Water has stronger IMFA overall

Stronger IMF: water vs ethanol

Lower surface tension and weaker IMF

Vegetable oil spreading on wax

Strength of polarity/dipole interactions

Deflection magnitude relates to

LDF < Dipole–dipole < H-bonding < Ion–dipole

Ranking IMF (weak → strong)

Oil < Ethanol < Water

Deflection rank: oil, ethanol, water

↑ molecular weight and surface area

Factors increasing LDF

HF and H₂O (hydrogen bonding)

Strongest IMF pair