Intermolecular Forces and Hydrogen Bonding

Hydrogen Bonding

• Hydrogen bonding occurs when hydrogen is bonded to N, O, or F.

• Example: Would $NO<em>2$, $H</em>2S$, $NH<em>3$, or $CBr</em>4$ form hydrogen bonding with water molecules?

• Answer: $NH_3$ would form hydrogen bonds with water molecules.

Methane (\CH4) and Hydrogen Bonding

• Question: Can $CH_4$ form hydrogen bonding? Why?

• Answer: No, because the electronegativity difference between carbon and hydrogen is almost zero, making the molecule nonpolar.

Boiling Point and Hydrogen Bonding

• A substance capable of hydrogen bonding has a higher boiling point than a similar substance that doesn't hydrogen bond.

Water's Ability to Dissolve Ionic Compounds

• The force that explains the ability for water molecules to dissolve ionic compounds is ion-dipole attraction.

Physical Properties and Intermolecular Forces

• Physical properties that typically increase when intermolecular forces increase include:

  • Melting Point

  • Viscosity

  • Boiling Point

  • All of these

Strength of London Dispersion Forces

• The strength of London dispersion forces increases with the number of electrons and surface area of the molecule.

• Example: Which of these molecules has the strongest London dispersion force? $\text{I}<em>2$, $Br</em>2$, $Cl<em>2$, $F</em>2$

• Answer: $I_2$ has the strongest London dispersion force because it has the largest number of electrons.

Intermolecular Forces in All Molecules

• The type of intermolecular force found in any molecule regardless of polarity is London dispersion forces.

Pure Substances Forming Hydrogen Bonds

• Which of the following pure substances forms hydrogen bonds?

  • HBr

  • HF

  • HCl

• Answer: HF.

High Melting and Boiling Point of Water

• The very high melting and boiling point of water is explained by hydrogen bonds between water molecules.

Predominant Intermolecular Force in Pure Water

• The predominant intermolecular force present in a sample of pure water is the hydrogen bond.

Predominant Intermolecular Force in Pure Oxygen

• The predominant intermolecular force present in a sample of pure oxygen ($O_2$) is London dispersion.

Intermolecular Force and Molecular Mass

• London dispersion force increases with increasing molecular mass.

Temporary Dipoles and Intermolecular Forces

• The intermolecular force caused by a temporary dipole is London dispersion.

Molecules Interacting via Hydrogen Bonds

• Which of the following molecules is most likely to interact using hydrogen bonds? $\text{CH}<em>3\text{NH}</em>2$, $\text{CH}<em>4$, $\text{CH}</em>3\text{CH}<em>3$, $N</em>2$

• Answer: $\text{CH}<em>3\text{NH}</em>2$

Weakest Intermolecular Force

• The weakest intermolecular force is London dispersion.