Water & the Hydrogen Bond
Cohesion: This refers to the tendency of water molecules to stick to one another due to hydrogen bonding. This collective strength is responsible for surface tension, allowing the surface of water to resist external force and support small objects.
Adhesion: This is the attraction between water molecules and different polar substances. When water adheres to the walls of plant vessels or glass tubes, it can counteract gravity through capillary action.
Surface Tension Mechanics: Molecules at the surface experience a net inward pull because they lack neighbors above them to hydrogen bond with, creating a 'skin-like' effect on the liquid surface.
High Specific Heat: Water can absorb or release large amounts of heat with only a slight change in its own temperature. This occurs because much of the absorbed energy is used to break hydrogen bonds before the molecules can begin moving faster (increasing kinetic energy).
Evaporative Cooling: As water evaporates, the 'hottest' molecules (those with the highest kinetic energy) leave the liquid phase first. The remaining liquid is cooled, a process vital for organisms to maintain stable internal temperatures.
Heat of Vaporization: The high energy required to transform liquid water into gas is a direct result of the need to completely overcome the hydrogen bonds holding the molecules together.
Expansion upon Freezing: Unlike most substances that contract and become denser when solidifying, water expands. As temperature drops, molecules move too slowly to break hydrogen bonds, and they lock into a rigid, crystalline hexagonal lattice.
Buoyancy of Ice: In the solid lattice, water molecules are spaced further apart than in liquid water, making ice less dense. This allows ice to float, providing an insulating layer that protects aquatic life in sub-zero environments.
Liquid State Dynamics: In liquid water, hydrogen bonds are constantly breaking and reforming (lasting only picoseconds), allowing molecules to pack more closely together than in the fixed solid structure.
| Feature | Covalent Bond (Intramolecular) | Hydrogen Bond (Intermolecular) |
|---|---|---|
| Location | Within a single molecule | Between separate molecules |
| Strength | Very Strong (approx. 400 kJ/mol) | Weak (approx. 20 kJ/mol) |
| Nature | Sharing of electron pairs | Electrostatic attraction of partial charges |
| Function | Determines molecular identity | Determines physical properties (BP, MP) |
Identify the Donor/Acceptor: In exams, always check if the Hydrogen is attached to N, O, or F. If it is attached to Carbon (C-H), it cannot participate in hydrogen bonding because the C-H bond is not polar enough.
Explain Properties via Bonds: When asked why water has a high boiling point compared to similar-sized molecules (like ), always mention that extra energy is required to break the extensive network of intermolecular hydrogen bonds.
Avoid the 'Strong Bond' Trap: Never describe a hydrogen bond as 'strong' in an absolute sense; it is only strong relative to other Van der Waals forces, but much weaker than covalent or ionic bonds.
Check the State: Remember that the number of stable hydrogen bonds is highest in ice (4 per molecule) and lower/more transient in liquid water and steam.