2.2.2Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Hydrogen Bonding — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 18 of 53 Unit quiz
Section 1 of 4Conditions and Examples
A hydrogen bond needs a very polar X-H bond and a nearby lone pair, so only a few molecules qualify.
Donor
- Donor Hydrogen must be covalently bonded to nitrogen, oxygen or fluorine, which pull the bonding pair strongly and leave hydrogen strongly δ⁺.
Acceptor
- Acceptor The δ⁺ hydrogen attracts a lone pair on an N, O or F atom in a neighbouring molecule.
Common examples
- Common examples Water, alcohols, carboxylic acids, ammonia, amines and hydrogen fluoride all form hydrogen bonds between their own molecules.
Water
- Water Each water molecule can form up to four hydrogen bonds: two through its hydrogens and two to the lone pairs on its oxygen.
Ammonia
- Ammonia Nitrogen in ammonia has only one lone pair, so ammonia forms fewer hydrogen bonds per molecule than water does.
Section 2 of 4Anomalous Properties of Water
Hydrogen bonding makes water behave very differently from the other Group 16 hydrides.
Anomalous Properties of Water
High melting and boiling points
- High melting and boiling points A lot of energy is needed to separate hydrogen-bonded molecules, so water is a liquid at room temperature unlike H₂S.
Boiling point trend
- Boiling point trend H₂S, H₂Se and H₂Te rise with electron number because London forces grow, but H₂O breaks the trend by being far higher.
Surface tension
- Surface tension Surface molecules are pulled inwards by hydrogen bonds, giving a skin that resists forces and lets small insects walk on water.
Ice is less dense
- Ice is less dense In ice each molecule is held in an open lattice by hydrogen bonds, so the molecules are slightly further apart than in liquid water.
Section 3 of 4Why Ice Floats
1
As liquid water cools, hydrogen bonds form and break constantly and the molecules remain fairly close together.
2
At freezing each oxygen becomes bonded through hydrogen bonds to four neighbours in a regular tetrahedral arrangement.
3
This arrangement is an open lattice with empty space inside, so the molecules are held further apart than in the liquid.
4
The same mass occupies a larger volume, so ice is about 9% less dense than liquid water.
5
Ice therefore floats, insulating the water below and allowing aquatic life to survive when lakes freeze at the surface.
Section 4 of 4Exam Checklist & Tips
2.2.2 – Key Points for the Exam
- Electronegativity The ability of an atom to attract the bonding pair of electrons in a covalent bond.
- Lone pair A pair of outer electrons not used in bonding that can accept a hydrogen bond.
- Surface tension The tendency of a liquid surface to resist external forces because surface molecules are pulled inwards.
- Open lattice A structure with gaps between molecules, as seen in ice, which lowers the density.
Common Mistakes
- Saying hydrogen bonds are the bonds between hydrogen and oxygen inside water;
- they are between separate molecules.
- Forgetting that the lone pair on the acceptor atom is essential, not just a highly electronegative atom.
- Explaining the high boiling point of water by stronger covalent O-H bonds rather than stronger intermolecular forces.
- Stating that ice is denser because the molecules are packed more tightly;
- the open hydrogen-bonded lattice makes ice less dense.
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