AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The importance of hydrogen bonding in the low density of ice and the…
Full specification wording
Forces between molecules: The importance of hydrogen bonding in the low density of ice and the anomalous boiling points of compounds.
Water is exceptional because each molecule can form up to four hydrogen bonds, two through its hydrogen atoms and two through the lone pairs on its oxygen.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Water is exceptional because each molecule can form up to four hydrogen bonds, two through its hydrogen atoms and two through the lone pairs on its oxygen. When water freezes, these hydrogen bonds hold the molecules in a rigid, open tetrahedral cage, keeping them further apart on average than in the liquid, so ice is less dense than water and floats. On melting, some hydrogen bonds break and the structure collapses inwards. The same bonding explains the anomalous boiling points of water, ammonia and hydrogen fluoride, each far above the trend set by the other hydrides of its group. Water tops even hydrogen fluoride because it forms more hydrogen bonds per molecule.
What earns the marks
- Each water molecule forming up to four hydrogen bonds using its hydrogen atoms and the lone pairs on oxygen.
- The rigid open or tetrahedral lattice holding the molecules further apart than in the liquid.
- The resulting lower density of ice so that it floats on water.
- Hydrogen bonding being stronger than the dipole-dipole and induced dipole forces in the other hydrides of the group.
Common mistakes and how to put them right
Saying the water molecules themselves expand on freezing.
Claiming the hydrogen bonds lengthen rather than the lattice holding molecules apart.
Explaining the high boiling point of water by strong covalent bonds.
Stating that hydrogen fluoride boils higher than water, ignoring the number of hydrogen bonds per molecule.
Exam tips
- Draw the open lattice with the hydrogen bonds marked as dashed lines and the lone pairs shown.
- Say more hydrogen bonds per molecule when comparing water with hydrogen fluoride.
- For an anomalous boiling point question, always compare with the trend in the rest of the group.
Try the lesson
The start of the Forces between molecules: The importance of hydrogen bonding in the low density of ice and the anomalous boiling points of compounds. lesson, made from checked teaching notes for this specification point.
Have a go first
Why does ice float on water?
Listen
Water is exceptional because each molecule can form up to four hydrogen bonds, two through its hydrogen atoms and two through the lone pairs on its oxygen. When water freezes, these hydrogen bonds hold the molecules in a rigid, open tetrahedral cage, keeping them further apart on average than in the liquid, so ice is less dense than water and floats. On melting, some hydrogen bonds break and the structure collapses inwards. The same bonding explains the anomalous boiling points of water, ammonia and hydrogen fluoride, each far above the trend set by the other hydrides of its group. Water tops even hydrogen fluoride because it forms more hydrogen bonds per molecule. A strong answer earns marks for these points. each water molecule forming up to four hydrogen bonds using its hydrogen atoms and the lone pairs on oxygen. the rigid open or tetrahedral lattice holding the molecules further apart than in the liquid. the resulting lower density of ice so that it floats on water. hydrogen bonding being stronger than the dipole-dipole and induced dipole forces in the other hydrides of the group. Exam tip: Draw the open lattice with the hydrogen bonds marked as dashed lines and the lone pairs shown.
Key words
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four hydrogen bonds per water molecule
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open lattice in ice
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density anomaly
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anomalous boiling points
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See every definition
- four hydrogen bonds per water molecule
- Each water molecule can form up to four hydrogen bonds, two through its hydrogen atoms and two through the lone pairs on its oxygen.
- open lattice in ice
- When water freezes, hydrogen bonds hold the molecules in a rigid, open tetrahedral cage, keeping them further apart on average than in the liquid.
- density anomaly
- Ice is less dense than water and floats, and on melting some hydrogen bonds break and the structure collapses inwards.
- anomalous boiling points
- Hydrogen bonding explains why water, ammonia and hydrogen fluoride boil far above the trend set by the other hydrides of its group.
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