Question 1

What exactly is a hydrogen bond?

Accepted Answer

It is an attraction between a partially positive hydrogen covalently bonded to F, O or N and a lone pair on another F, O or N atom. It is represented as X-H···Y.

Question 2

Why is the H-O-H angle in water 104.5°?

Accepted Answer

Oxygen has two bonding pairs and two lone pairs. Stronger lone-pair repulsion compresses the O-H bonds, so water is bent with an H-O-H angle of 104.5°.

Question 3

Why does water have the highest boiling point in this comparison?

Accepted Answer

Each water molecule can donate two hydrogen bonds and accept two more. This extensive network needs more energy to break than the networks in NH3 and HF, while CH4 has no comparable hydrogen bonding.

Question 4

Why is o-nitrophenol steam volatile?

Accepted Answer

Its phenolic O-H forms an intramolecular hydrogen bond with a nearby nitro oxygen. This reduces association between separate molecules, so the compound vaporizes more easily.

Question 5

Why does p-nitrophenol have a higher boiling point?

Accepted Answer

The donor and acceptor groups are too far apart for an intramolecular hydrogen bond. Neighbouring p-nitrophenol molecules instead form intermolecular H-bond chains that need more energy to separate.

Question 6

Why does ice float on liquid water?

Accepted Answer

Hydrogen bonds organize ice into an open tetrahedral cage containing empty space. This makes ice less dense than liquid water.

Question 7

What happens to hydrogen bonds when ice becomes liquid?

Accepted Answer

Some hydrogen bonds break and reform. Molecules can move closer, the open cage collapses partly, and the density of liquid water becomes higher than that of ice.

Hydrogen Bonding & Water — Molecular Network in 3D

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