Hydrogen Bonding | Chemical Bonding Class 11

Hydrogen Bonding

Intermolecular Forces & Physical Properties | Chemical Bonding

1. What is Hydrogen Bonding?

                    Definition: It is the electrostatic force of attraction between a covalently bonded Hydrogen atom of one molecule and a strongly Electronegative atom (Fluorine, Oxygen, or Nitrogen) of another (or same) molecule.
                

Representation: $H^{\delta+} - X^{\delta-} \dots\dots H^{\delta+} - X^{\delta-}$ (Dotted line represents H-bond).

Conditions:

The molecule must contain a highly electronegative atom (F, O, N).
The size of the electronegative atom should be small.

Note: Chlorine has high electronegativity (same as N) but large size, so it rarely forms effective H-bonds.

2. Types of Hydrogen Bonding

A. Intermolecular H-Bonding

Formed between two different molecules (same or different compounds).

Examples: Water ($H_2O$), Ammonia ($NH_3$), Hydrogen Fluoride ($HF$), Alcohol + Water.
Consequences:
- Higher Boiling Point: Molecules associate, requiring more energy to separate (e.g., $H_2O$ is liquid, $H_2S$ is gas).
- Solubility: Compounds like alcohol are soluble in water due to H-bonding.
- Viscosity: Glycerol is viscous due to extensive H-bonding.

B. Intramolecular H-Bonding

Formed between hydrogen and an electronegative atom within the same molecule.

Examples: o-Nitrophenol, Salicylaldehyde.
Consequences:
- Chelation: Forms a ring structure.
- Lower Boiling Point: Prevents association with other molecules. (e.g., o-Nitrophenol is more volatile than p-Nitrophenol).

3. Comparison Table

Property	Intermolecular	Intramolecular
Association	Molecules join together	No association (Discrete)
Boiling Point	Increases	Comparatively Low
Solubility in Water	Usually Increases	Decreases
Example	Para-Nitrophenol	Ortho-Nitrophenol

4. Structure of Ice & Density Anomaly

In ice, each oxygen atom is tetrahedrally surrounded by four hydrogen atoms (2 covalent bonds + 2 hydrogen bonds).

                    This creates an open cage-like structure with vacant spaces. Hence, the volume of ice is greater than water, and Density of Ice < Density of Water. This is why ice floats.
                

Maximum Density of Water: At $4^\circ C$. Upon heating from $0^\circ C$ to $4^\circ C$, H-bonds break, cage collapses, and molecules come closer (Density increases).

5. Strength & Importance

Strength: H-bond (8-42 kJ/mol) is weaker than Covalent (>200 kJ/mol) but stronger than Van der Waals forces.
Order of Strength: $H-F \dots H > H-O \dots H > H-N \dots H$.
Biology: H-bonds hold the two strands of DNA together (A-T and G-C pairing) and determine protein structure.

Practice Quiz

Test your knowledge on Hydrogen Bonding.

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