The hydrogen bond

What is a hydrogen bond?

In a molecule of hydrogen fluoride, fluorine pulls the shared pair strongly towards itself. The hydrogen end is left with a partial positive charge, $\delta+$, and the fluorine end with $\delta-$.

The bare hydrogen nucleus is tiny. So its $\delta+$ charge is very concentrated, and it is attracted to a lone pair on a fluorine atom of a neighbouring molecule. This attraction is a hydrogen bond. It is drawn as a dotted line: $\text{H-F} \cdots \text{H-F}$.

A strong hydrogen bond needs hydrogen joined to a small, highly electronegative atom: fluorine, oxygen or nitrogen. A hydrogen bond is much weaker than a covalent bond, but much stronger than the ordinary attractions between molecules.

Two kinds

• Intermolecular hydrogen bonds join different molecules. Hydrogen fluoride forms zigzag chains, and water forms a network in which each molecule can take part in up to four hydrogen bonds.
• Intramolecular hydrogen bonds form within one molecule, when the hydrogen and the lone pair are close together in the same molecule. In ortho-nitrophenol, the $\text{O-H}$ group and the nitro group sit side by side, and a hydrogen bond joins them inside the molecule.

In para-nitrophenol the two groups are on opposite sides of the ring, too far apart. So its molecules hydrogen bond to each other. That is why ortho-nitrophenol is the more volatile of the two: its molecules are not held to their neighbours.

What hydrogen bonds do

Boiling points. Down group 16, boiling points usually rise with molar mass: $\text{H}_2\text{S}$ boils at $213\ \text{K}$, $\text{H}_2\text{Se}$ at $232\ \text{K}$ and $\text{H}_2\text{Te}$ at $271\ \text{K}$. Water should boil lowest of all. Yet it boils at $373\ \text{K}$, because every molecule must break free of its hydrogen bonds. Hydrogen fluoride and ammonia stand out in their groups for the same reason.

Ice floats. In ice every water molecule is held by four hydrogen bonds in an open cage, with much empty space. When ice melts, the cage partly collapses and the molecules pack closer. So ice is less dense than liquid water.

Solubility. Ammonia, ethanol and sugar dissolve well in water, because they form hydrogen bonds with water molecules.

Practise with new numbers

Try it in Python

The program steps down group 16 and estimates how high water would boil without hydrogen bonds, by carrying the trend of the heavier hydrides back.

Common mistakes

• Calling any bond to hydrogen a hydrogen bond. The covalent $\text{O-H}$ bond inside water is not one. The hydrogen bond is the weaker pull between an $\text{H}$ and a lone pair elsewhere.
• Expecting hydrogen bonds with carbon, chlorine or sulphur. Strong hydrogen bonds need $\text{F}$, $\text{O}$ or $\text{N}$.
• Thinking intramolecular hydrogen bonds raise the boiling point. They tie a molecule to itself, not to its neighbours, so they lower it compared with an isomer that bonds between molecules.
• Thinking ice is denser than water. Its open cage of hydrogen bonds makes it less dense.

Remember these

• A hydrogen bond: $\text{H}$ joined to $\text{F}$, $\text{O}$ or $\text{N}$, attracted to a lone pair on another such atom.
• Intermolecular: between molecules (HF, $\text{H}_2\text{O}$). Intramolecular: within one molecule (ortho-nitrophenol).
• It raises the boiling points of $\text{H}_2\text{O}$, HF and $\text{NH}_3$, makes ice less dense than water, and helps solubility in water.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why does methane, $\text{CH}_4$, not form hydrogen bonds?

2. Which boils higher, ethanol or dimethyl ether? Both have the formula $\text{C}_2\text{H}_6\text{O}$.

3. Why is ortho-nitrophenol more volatile than para-nitrophenol?

4. Why do pipes sometimes burst when water freezes in them?