Lecture Notes: Chemical Bonds and Properties of Water

Chemical Bonds

• Strongest to Weakest Bonds:
  • Nonpolar Covalent Bonds: Strongest in biological systems due to equal sharing of electrons and similar electronegativities (e.g., Carbon and Hydrogen).
  • Polar Covalent Bonds: Weaker than nonpolar due to unequal sharing of electrons (e.g., Oxygen and Hydrogen).
  • Ionic Bonds: Electrons are completely transferred, creating attraction between opposite charges (e.g., Sodium and Chloride).
  • Hydrogen Bonds: Weak bonds that form between molecules with polar covalent bonds.
  • Van der Waals Interactions: Weakest bonds formed due to temporary dipoles.

Covalent Bonds

• Nonpolar Covalent Bonds:
  • Electrons are shared equally.
  • Atoms have similar electronegativities.
  • Example: Carbon (2.5) and Hydrogen (2.1).
• Polar Covalent Bonds:
  • Electrons are shared unequally.
  • One atom is significantly more electronegative.
  • Example: Oxygen (3.5) vs. Hydrogen (2.1).

Ionic Bonds

• Formation: More electronegative atom steals an electron, forming charged ions (e.g., Na+ and Cl-).

Hydrogen Bonds

• Formation: Between partially charged regions of water molecules.
• Strength: Collectively strong but individually weak.

Van Der Waals Interactions

• Formation: Due to temporary dipoles in atoms/molecules coming close.
• Found in many large molecules and lipids.

Properties of Water

• Essential for Life:
  • Necessary for most biochemical reactions.
  • Makes up most cells and organisms.

Unique Characteristics

• Polarity and Hydrogen Bonding:
  • Water is polar and forms hydrogen bonds.
  • Essential for its role in life.

Hydrophilic vs. Hydrophobic

• Hydrophilic:
  • Water-loving; dissolve in water.
  • Made of polar covalent, hydrogen, or ionic bonds.
• Hydrophobic:
  • Water-fearing; do not dissolve in water.
  • Made of nonpolar covalent bonds.

States and Density

• Solid vs Liquid Water:
  • Ice is less dense than water, allowing it to float.

Impact of Freezing

• Expansion upon Freezing:
  • Can destroy cells by bursting.
  • Used beneficially in flash freezing.

Heat Capacity and Vaporization

• High Heat Capacity:
  • Absorbs a lot of energy without temperature change.
• High Heat of Vaporization:
  • Requires significant energy to transition from liquid to gas.

Solvent Abilities

• Dissolving Power:
  • Polar and ionic substances dissolve, forming hydration spheres.

Cohesion and Adhesion

• Cohesion:
  • Water molecules stick to each other, forming surface tension.
• Adhesion:
  • Water molecules stick to other materials (e.g., glass), causing capillary action.

Conclusion

• The lecture concludes with a note on upcoming topics: pH, buffers, carbon, and functional groups.