Hydrogen Bonding — Chemistry Mnemonic
Target: mnemonic for hydrogen bonding types intramolecular intermolecular
Why is this hard to memorize?
Hydrogen bonding is a special type of intermolecular force that occurs when H is bonded to highly electronegative atoms — F, O, or N. This "FON" rule is the starting point. H-bonds explain why water has an unusually high boiling point, why ice floats, why DNA has a double helix, and why proteins fold into specific shapes. NEET and JEE test: conditions for H-bonding, intermolecular vs intramolecular H-bonds, effects on boiling point and solubility, and comparing H-bond strength (F–H···F > O–H···O > N–H···N).
Classic mnemonics you should know
"H-bond requires H attached to F, O, or N — "FON friends of H""
Only F, O, and N are electronegative enough to create the large δ+ on H needed for hydrogen bonding. H bonded to Cl, S, or C does NOT form traditional H-bonds (though weak C–H···O interactions exist). "FON" = the three friends of hydrogen bonding.
"F–H···F > O–H···O > N–H···N — "FON in order of EN""
The more electronegative the atom, the stronger the H-bond. Fluorine (EN 4.0) creates the strongest H-bonds. Oxygen (3.5) is next. Nitrogen (3.0) creates the weakest of the three. This explains why HF has a higher BP than expected.
"Inter = between molecules (raises BP). Intra = within one molecule (lowers BP!)"
Intermolecular H-bonds link different molecules → harder to separate → higher boiling point. Intramolecular H-bonds are within the same molecule → molecule is more compact, less able to form intermolecular bonds → lower boiling point than expected. Classic example: ortho-nitrophenol (intra) has lower BP than para-nitrophenol (inter).
The complete list
- H-bond requires H bonded to F, O, or N
- Intermolecular (between molecules)
- Intramolecular (within molecule)
- F–H···F strongest, N–H···N weakest
- Raises boiling point (intermolecular)
- Explains water's high BP and ice density
- DNA base pairing uses H-bonds
- Protein secondary structure (α-helix, β-sheet)
Frequently asked questions
Why does water have an unusually high boiling point?
Water (H₂O, MW=18) boils at 100°C, while H₂S (MW=34) boils at −60°C. The difference is hydrogen bonding: each water molecule can form up to 4 H-bonds (2 as donor through H, 2 as acceptor through O lone pairs), creating a strong network. Breaking this network requires a lot of energy → high boiling point.
Why does ice float on water?
In ice, H-bonds form a rigid, open hexagonal lattice with empty spaces. This crystal structure is less dense than liquid water (where molecules are more randomly packed and closer together). Ice density = 0.917 g/cm³ vs liquid water = 1.000 g/cm³. This is crucial for aquatic life — ice insulates the water below.
What is the difference between intermolecular and intramolecular H-bonds?
Intermolecular: H-bonds between DIFFERENT molecules. Effect: increases boiling point, increases viscosity. Example: water, HF, ethanol. Intramolecular: H-bond WITHIN the same molecule (requires donor and acceptor groups close together). Effect: decreases boiling point (molecule is self-satisfied, less attraction to neighbours). Example: ortho-nitrophenol.
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