Intermolecular Forces Notes

Intermolecular Forces

Learning Outcomes

• Identify the types of intermolecular forces: dipole-dipole, dipole-induced-dipole, and London (dispersion) forces, which are all van der Waals forces.

• Deduce the type(s) of intermolecular force present based on electronegativity differences and structural features of molecular compounds.

• Describe the main features of each type of intermolecular force.

Intermolecular vs. Intramolecular Forces

• Intramolecular forces: Bonds within a molecule (e.g., ionic and covalent bonds).

• Intermolecular forces: Forces between molecules.

• Melting or boiling a substance requires overcoming intermolecular forces, not intramolecular forces.

• Intramolecular forces are much stronger than intermolecular forces.

Types of Intermolecular Forces

• London (dispersion) forces

• Dipole-dipole attraction

• Hydrogen bonding

London (Dispersion) Forces

• Electrons in atoms are in constant motion, leading to temporary, asymmetrical distributions.

Temporary Dipoles

• At any given time, electron distribution may not be perfectly symmetrical, creating a slight surplus of electrons on one side of the atom.

• This creates a temporary dipole that lasts for a very short time due to constant electron movement.

• Temporary dipoles are constantly appearing and disappearing.

Temporary Induced Dipoles

• A temporary dipole in one atom can induce a dipole in an adjacent atom.

• Electrons in the adjacent atom are repelled by the negative part of the dipole and attracted to the positive part, causing them to move accordingly.

General

• London (dispersion) forces are present between all atoms and molecules.

• They are the reason all compounds can be liquefied and solidified.

• Strength ranges from $1 kJmol^{-1}$ to $50 kJmol^{-1}$.

Factors Affecting Strength

• Number of electrons: More electrons lead to a greater likelihood of distortion, increasing the frequency and magnitude of temporary dipoles. Stronger dispersion forces result in larger melting and boiling points.

• Surface area: Larger surface area allows for more contact with adjacent molecules, increasing the ability to induce a dipole and resulting in greater London (dispersion) forces and higher melting/boiling points.

Number of Electrons

• The greater the number of electrons in a molecule, the greater the likelihood of a distortion and thus the greater the frequency and magnitude of the temporary dipoles.

• The dispersion forces between the molecules are stronger, and the melting and boiling points are larger.

Surface Area

• The larger the surface area of a molecule, the more contact it will have with adjacent molecules.

• The greater its ability to induce a dipole in an adjacent molecule, the greater the London (dispersion) forces and the higher the melting and boiling points.

Permanent Dipole-Dipole Attractions

• In addition to London (dispersion) forces from temporary dipoles, some molecules also have permanent dipoles.

• Permanent dipole-dipole bonding is an attraction between a permanent dipole on one molecule and a permanent dipole on another.

• It usually results in slightly higher boiling points than expected from temporary dipoles alone by slightly increasing the strength of intermolecular attractions.

• The effect of dipole-dipole bonding can be seen by comparing the melting and boiling points of different substances, which should have London (dispersion) forces of similar strength.

Comparing Butane and Propanone

• For small molecules with the same number of electrons, dipole-dipole attractions are stronger than dispersion forces.

• Butane and propanone have the same number of electrons.

• Butane is nonpolar and has only dispersion forces.

• Propanone is polar and has dipole-dipole attractions and dispersion forces.

• More energy is required to break the intermolecular forces between propanone molecules than between butane molecules.

• Result: propanone has a higher boiling point than butane.

Dipole-Induced Dipole Attraction

• Some mixtures contain both polar and nonpolar molecules (e.g., $HCl$ and $Cl_2$).

• The permanent dipole of a polar molecule can cause a temporary separation of charge on a non-polar molecule.

• This force acts in addition to London dispersion forces between nonpolar molecules and dipole-dipole forces between polar molecules.

Hydrogen Bonding

• Hydrogen bonding is the strongest type of intermolecular force and is a special type of permanent dipole-permanent dipole bonding.

Requirements:

• A species with an $O$, $N$, or $F$ atom (very electronegative) with an available lone pair of electrons.

• A hydrogen atom attached to an $O$, $N$, or $F$ atom.

• When hydrogen is covalently bonded to an electronegative atom ($O$, $N$, or $F$), the bond becomes highly polarized.

• The $H$ becomes so $\delta +$ charged that it can form a bond with the lone pair of an $O$, $N$, or $F$ atom in another molecule.

• Hydrogen bonds are represented by dots or dashes between $H$ and the $N/O/F$ element.

Number of Hydrogen Bonds

• The number of hydrogen bonds depends on:

  • The number of hydrogen atoms attached to $O$, $N$, or $F$ in the molecule.

  • The number of lone pairs on the $O$, $N$, or $F$.

Hydrogen Halides - Boiling Point

• The hydrogen halides do not show perfect periodicity.

• The boiling point of hydrogen fluoride, $HF$, is much higher than periodic trends would indicate because:

  1. $HF$ has hydrogen bonds between molecules, which is the strongest intermolecular force. More energy (hence higher boiling point) is required to overcome this strong force.

  2. $HCl$, $HBr$, and $HI$ do not experience hydrogen bonds or dipole-dipole forces. All of these molecules will experience London dispersion forces, and as you progress down the period from $HCl$ to $HI$, the molecular size (i.e., number of electrons) on the halogen will increase. This causes the strength of the London dispersion force to increase.

Comparing Boiling Points