Hydrogen Bonding Tutorial

Introduction

• Hydrogen Bond: A strong, special type of dipole-dipole interaction.
• Occurs when hydrogen is attached to nitrogen, oxygen, or fluorine.

Comparison of Bonds

• HF vs CO Bond:
  • Both are highly polar.
  • Electronegativity Values:
    • Hydrogen: 2.1
    • Fluorine: 4.0
    • Carbon: 2.5
    • Oxygen: 3.5
  • Electronegativity Difference:
    • HF: 1.9
    • CO: 1.0

Why Hydrogen Bonds are Stronger

• Higher Polarization: Greater electronegativity difference in HF.
• Smaller Atomic Size: Hydrogen is smaller than carbon.
  • Atomic Radius: Hydrogen - 37 pm, Carbon - 77 pm.

Electrostatic Force of Attraction

• Force is inversely related to the square of the distance between charges.
• Reducing distance increases force exponentially, e.g., by a factor of 4 when distance is halved.
• Smaller size of hydrogen reduces distance, increasing attraction force.

Intramolecular vs Intermolecular Forces

• Intramolecular Force: Covalent bond within a molecule, e.g., between hydrogen and fluorine in HF.
• Intermolecular Force: Hydrogen bond between molecules, e.g., between HF molecules.

Hydrogen Bond in Water

• Oxygen has a partial negative charge; hydrogen has a partial positive charge.
• Attraction between oxygen of one water molecule and hydrogen of another constitutes a hydrogen bond.

Energy Considerations

• Breaking Hydrogen Bonds: Endothermic process, requires heat input.
  • Enthalpy of Vaporization: 40.7 kJ/mol to vaporize water.
• Breaking Covalent Bonds: Much more energy-intensive.
  • OH Bond Energy: 467 kJ/mol.
  • Total energy to break OH bonds in water: 934 kJ (467 kJ each for two bonds).
• Covalent bond is about 23 times stronger than hydrogen bond between water molecules.

Conclusion

• Hydrogen bonds are significant but weaker compared to covalent bonds.
• Important to understand the distinction between different types of bonds and forces to grasp molecular interactions effectively.