Covalent Bonds, Polarity, Hydrogen Bonds, and Water: Quick Review

Covalent Bond Types: Polar vs Nonpolar

• Covalent bonds involve sharing electrons.

• Two types:

  • Nonpolar covalent: electrons shared equally.

  • Polar covalent: electrons shared unequally.

• Quick check: polar = unequal sharing, nonpolar = equal sharing.

• Electronegativity difference determines polarity: let $\Delta EN = |EN<em>A - EN</em>B|$.

  • Small $\Delta EN$ → mostly nonpolar (equal sharing).

  • Large $\Delta EN$ → polar (unequal sharing).

Partial Charges and Bond Polarity

• Polar covalent bonds create partial charges:

  • Partial negative on the more electronegative atom ($\delta^-$).

  • Partial positive on the less electronegative atom ($\delta^+$).

• Nonpolar covalent bonds have no partial charges (charges are effectively balanced).

Hydrogen Bonds

• Hydrogen bonds are electrostatic attractions between partial charges, not full charges.

• They are weaker than ionic bonds because they involve partial charges.

• They can form between different molecules or within regions of the same molecule that have polar covalent bonds.

• They underpin water’s network: many H-bonds between water molecules.

Properties of Water

• Cohesion: water molecules stick to each other via hydrogen bonds.

• Adhesion: water molecules stick to other surfaces via hydrogen bonds.

• Surface tension: result of cohesion at air-water interface (e.g., water striders).

• Capillary action: adhesion to surfaces plus cohesion pull water up narrow tubes.

• Meniscus: curved surface from adhesion to tube walls.

• High specific heat capacity: $C = \dfrac{q}{m \Delta T}$; resists temperature changes.

• High heat of vaporization: large energy to convert liquid water to gas (evaporative cooling).

• Water as solvent: polar covalent bonds make water a good solvent for ionic compounds and many polar molecules.

• Dissolution mechanisms:

  • Ions (e.g., NaCl): water’s partial charges stabilize ions (hydration) and separate them.

  • Polar molecules (e.g., sugars): water forms hydrogen bonds with polar regions to dissolve them.

  • Nonpolar molecules (e.g., oils): lack of partial charges -> cannot form hydrogen bonds with water; water network excludes them; they separate (hydrophobic effect).

Hydrophilic vs Hydrophobic

• Hydrophilic: molecules with polar covalent or ionic bonds that can form hydrogen bonds with water; dissolves readily in water.

• Hydrophobic: molecules with nonpolar covalent bonds; do not form hydrogen bonds with water; tend to separate from water.

Five-Step Framework: From Bond Type to Solubility

1) Identify the main bonds in the molecule: polar or nonpolar.
2) Define polarity: polar → unequal sharing; nonpolar → equal sharing.
3) Consequences of charges: polar → partial charges ((\delta^+ / \delta^-)); nonpolar → no charges.
4) Can the molecule form hydrogen bonds with water?

• If polar, likely yes; if nonpolar, typically no.
  5) Solubility outcome:

• Polar → hydrophilic (water-loving).

• Nonpolar → hydrophobic (water-fearing).

Examples to Tie Concepts Together

• Water–water interactions: hydrogen bonding leads to cohesion.

• Water–ammonia interactions: both have polar covalent bonds; can form hydrogen bonds with each other.

• Salt in water: hydration shells form around Na+ and Cl− due to polar water molecules, enabling dissolution.

• Sugar in water: dissolves as molecules via hydrogen bonding; covalent bonds within sugar remain intact.

• Oil in water: nonpolar, hydrophobic; water forms a separate hydrogen-bond network, excluding oil.