Topic 4 - Intermolecular Forces and the States of Matter: Intermolecular Forces

Intermolecular Forces

• Additional forces (non-bonding attractions/repulsions between particles) that act when two substances are mixed; weaker than intramolecular bonds.

• Set physical properties (solid/liquid/gas at room temperature, miscibility/solubility).

• Electrostatic in nature — formed through interactions between positive (+) and negative (–) regions.

• Strength determined by molecule/ion's ability to form/maintain dipoles (permanent or temporary).

• Strength from weakest to strongest:

  • Dispersion forces (aka London dispersion forces);

  • Dipole-dipole forces;

  • Hydrogen bonding (extreme form of dipole-dipole forces).

• All are much weaker than covalent/ionic bonds (e.g., melting/vaporising water needs far less energy than breaking O–H bonds), but cumulative effects can be large.

Dispersion Forces - Origin and Mechanism

Dispersion forces are present between ALL molecules and other atoms, ions or molecules.

• Electron clouds fluctuate → momentary uneven electron distribution → instantaneous dipole (areas of positive and negative partial charge).

• This can induce an opposite dipole in a neighbour → attraction.

• Attractive alignments last longer than repulsive ones → net attraction.

Dispersion Forces - Strength

Dispersion forces are present between ALL molecules and other atoms, ions or molecules.

What controls their strength

1. Number of electrons (size of e⁻ cloud):

   • More electrons → bigger, more distortable cloud → larger instantaneous dipoles → stronger dispersion.

2. Polarisability (ease of e⁻ cloud distortion): Increases with larger electron clouds and with shape. 

   • Linear/extended molecules → larger, extended surface area → electron farther away from the nucleus → weak electron-nucleus attraction → fluid cloud → more easily polarised → stronger dispersion.

   • Compact/branched molecules → smaller contact area → electron closer to the nucleus → strong electron-nucleus attraction → stiff cloud → less polarizable → weaker dispersion.

Dispersion Forces - Property Trends

Dispersion forces are present between ALL molecules and other atoms, ions or molecules.

• Boiling/melting points rise as IMFs strengthen because more heat is needed to overcome attractions.

• Alkanes: Longer chains (larger clouds) and more linear shapes → higher boiling points than shorter/branched isomers.

Dipole–dipole Forces

• Who has them: Interaction between polar molecules (permanent dipoles with δ+ / δ–).

• Charge separation by strength: temporary (dispersion) < permanent dipole separation < ionic.

• Mechanism: Close approach between polar molecules allows alignment (δ+ near δ–) → attraction; net effect is attractive overall.

• Relationship to dispersion: Occur in addition to dispersion and are typically stronger than dispersion for small molecules.

Hydrogen Bonding

A type of chemical bond that occurs when a hydrogen atom, which is covalently bonded to an electronegative atom (like oxygen, nitrogen, or fluorine), is attracted to another nearby electronegative atom. An extreme form of dipole-dipole interaction.

 

Hydrogen Bonding - Conditions for its Formation

1. Hydrogen Bond Donor

   • H atom must be covalently bonded directly to a highly electronegative atom (N, O, or F).

   • This creates a strong permanent dipole with the H atom becoming partially positive (δ+).

   • The δ+ hydrogen is available for attraction.

2. Hydrogen Bond Acceptor

   • Must be a strongly electronegative atom (N, O, or F) with one or more lone pairs of electrons.

   • If there are multiple electronegative atoms in a molecule to be chosen as the hydrogen bond acceptor, the most electronegative atom is likely to be the one

   • Does not require a hydrogen attached to it.

   • The lone pairs interact with the δ+ hydrogen of the donor.

3. Nature of H-Bonds

   • Strength: ~1/10 of a covalent bond.

   • Easily broken/reformed

   • Represented by dashed lines between donor and acceptor.

Hydrogen Bonding - Strength

Why are H-bnds so strong?

• Hydrogen has only one electron, leaving its nucleus exposed when bound to N, O, or F.

• Allows very close approach of electronegative atoms → unusually strong dipole-dipole interaction.

Intermolecular Forces & Solubility - General Rule

“Like dissolves like.”

• Polar solvents dissolve polar solutes.

• Non-polar solvents dissolve non-polar solutes.

• Polar & non-polar do not mix.

 

Intermolecular Forces & Solubility - Mixing Process

For solute to dissolve:

• Solute–solute and solvent–solvent interactions must be disrupted.

• Replaced by solute–solvent interactions (intermolecular forces) that are equal to or stronger.

Solute

Substance being dissolved (minor component).

Solvent

Substance doing the dissolving (major component).

Soluble

Capable of dissolving.

Miscible

• When two liquids dissolve in all proportions to form one phase.

• In the case of a liquid being soluble in another liquid, the term miscible is often used in place of soluble.