What is a hydrogen bond?
When H is bonded to a highly electronegative atom like F, O or N, the shared pair moves towards that atom. H is left with a partial positive charge (δ+). This small, exposed δ+ H is attracted to a lone pair on an electronegative atom of another molecule (or another part of the same molecule). This attraction is the hydrogen bond.
It is drawn with dots: X–H···Y (X, Y = F, O, N).
Conditions
- H must be bonded to a small, highly electronegative atom (F, O, N).
- The other atom must have a lone pair.
- Cl is electronegative but too large, so its H-bonds are very weak.
Strength
A hydrogen bond (≈ 10–40 kJ mol⁻¹) is much weaker than a covalent bond (≈ 400 kJ mol⁻¹) but stronger than ordinary van der Waals forces. Strength order: F–H···F > O–H···O > N–H···N.
Intermolecular hydrogen bonding
The H-bond forms between two different molecules, of the same or different kinds.
- HF: zig-zag chains (H–F···H–F···H–F). This is why HF is a liquid at room temperature (bp 19.5 °C) while HCl is a gas (bp −85 °C).
- Water: each molecule forms up to 4 H-bonds. Water boils at 100 °C, while H₂S (no strong H-bond) boils at −60 °C.
- Alcohol + water: ethanol dissolves in water because it forms H-bonds with water.
- Ammonia: NH₃ (bp −33 °C) boils higher than PH₃ (−88 °C).
Effects: higher melting and boiling points, higher solubility in water, higher viscosity (glycerol) and molecules that pair up (acetic acid dimers).
Intramolecular hydrogen bonding
The H-bond forms between two groups of the same molecule. It needs the groups to be close together so that a ring (usually 5 or 6 atoms, including the H) can form.
o-Nitrophenol: the –OH and –NO₂ groups are next to each other on the ring. The H of –OH bonds to an O of –NO₂ inside the same molecule. This "uses up" the H, so the molecules cannot link to each other much.
p-Nitrophenol: the groups are on opposite ends. They cannot reach each other, so H-bonds form between molecules instead.
Result: o-nitrophenol has a lower boiling point, is steam volatile and less soluble in water; p-nitrophenol has a higher boiling point. Salicylaldehyde and o-hydroxybenzoic acid also show intramolecular H-bonds.
Why hydrogen bonds matter
- Ice floats: the open, H-bonded cage of ice has more empty space, so ice is less dense than liquid water.
- Life: H-bonds hold the two strands of DNA together and fold proteins into their shapes.
- Sweat cools us: breaking H-bonds while water evaporates takes a lot of heat from the skin.
- KHF₂ exists (HF₂⁻ ion, F–H···F⁻), but KHCl₂ does not.
Try it: water vs spirit drops
Put one drop of water and one drop of spirit (or sanitiser) on a steel plate. Predict first: which drop keeps a rounder bead, and which dries first? Water has more H-bonds, so its drop stays rounder (higher surface tension) and dries later. Then, in the free-play step, count the H-bonds shown for water and compare with CH₄.
Key formulas and definitions
- Hydrogen bond: X–H···Y, where X, Y = F, O or N (with a lone pair on Y)
- Strength: F–H···F > O–H···O > N–H···N
- Covalent bond ≈ 400 kJ mol⁻¹ ≫ H-bond ≈ 10–40 kJ mol⁻¹ > van der Waals
- Intermolecular H-bond ⇒ higher bp, more soluble; intramolecular ⇒ lower bp, steam volatile
- Water: up to 4 H-bonds per molecule
Worked examples
1. Which of these can form hydrogen bonds between their own molecules: CH₄, NH₃, HCl, H₂O?
Step 1: Check for H bonded to F, O or N. Step 2: NH₃ (N–H) and H₂O (O–H) have it. CH₄ has C–H; HCl has H–Cl (Cl is too large). Answer: NH₃ and H₂O.
2. Why is HF a liquid at room temperature while HCl is a gas?
Step 1: F is small and the most electronegative atom → strong F–H···F bonds. Step 2: HF molecules join into chains, so more energy is needed to separate them. Step 3: Cl is large; HCl has only weak attractions. Answer: H-bonding raises the boiling point of HF (19.5 °C) far above HCl (−85 °C).
3. Explain why H₂O boils at 100 °C but H₂S boils at −60 °C.
Step 1: O is much more electronegative than S and small. Step 2: Water molecules form strong intermolecular H-bonds (up to 4 each). Step 3: These must be broken before boiling, which needs extra energy. Answer: H-bonding makes water's boiling point about 160 °C higher.
4. Which has the higher boiling point: o-nitrophenol or p-nitrophenol? Why?
Step 1: In o-nitrophenol the –OH and –NO₂ are next to each other → intramolecular H-bond. Step 2: In p-nitrophenol they are far apart → intermolecular H-bonds link many molecules. Answer: p-nitrophenol has the higher boiling point; o-nitrophenol is steam volatile.
5. Explain why ice floats on water.
Step 1: In ice, each water molecule is H-bonded to 4 others at tetrahedral angles. Step 2: This makes an open cage with lots of empty space. Step 3: On melting, some H-bonds break and molecules pack closer. Answer: ice is less dense than water, so it floats.
6. Ethanol (C₂H₅OH) mixes with water in all amounts but dimethyl ether (CH₃OCH₃, same formula) is much less soluble. Explain.
Step 1: Ethanol has an O–H group, so it can both give an H and accept an H-bond with water. Step 2: Dimethyl ether has no O–H; it can only accept an H-bond. Answer: ethanol forms more and stronger H-bonds with water, so it dissolves fully.
7. Arrange by boiling point: CH₄, NH₃, H₂O, HF (use H-bond strength and number).
Step 1: CH₄ has no H-bonds → lowest (−161 °C). Step 2: NH₃ has weaker N–H···N → −33 °C. Step 3: HF has the strongest H-bond but only 1 H and forms chains → 19.5 °C. Step 4: H₂O makes up to 4 H-bonds per molecule → 100 °C. Answer: CH₄ < NH₃ < HF < H₂O.
Common mistakes
- Calling the O–H bond inside a water molecule a hydrogen bond. That is a covalent bond; the H-bond is the attraction between molecules.
- Thinking any H atom can hydrogen-bond. The H must be bonded to F, O or N.
- Mixing up intra and inter: intramolecular H-bonds LOWER the boiling point compared with the isomer that has intermolecular H-bonds.
- Believing H-bonds are as strong as covalent bonds. They are about 10–20 times weaker.