Quick Notes

Quick Notes

Chemistry Chapter 5: Chemical Bond
5.4 Hydrogen Bond
5.4 Hydrogen Bond
CHEMISTRY • Form 4 • Chapter 5: Chemical Bond

5.4Hydrogen Bond

Hydrogen bonds are a special type of intermolecular attraction that significantly influence the physical properties of substances. They explain fascinating real-world phenomena, such as why ice floats on water, why hair sticks together when wet, why licking a finger helps turn book pages, and why certain liquids like ethanol have high boiling points and dissolve well in water.

Learning Objectives

  • Explain with examples the formation of a hydrogen bond.
  • Explain the effect of the hydrogen bond on physical properties of substances, such as boiling point and solubility in water.

Formation and Effects of Hydrogen Bonds

See how hydrogen bonds form between molecules and how they affect daily phenomena and boiling points.

Brief Explanation

Understand how hydrogen bonds form and why they matter.

Hydrogen bonds are attraction forces between a hydrogen atom, H, that has bonded with an atom of high electronegativity, such as nitrogen, N, oxygen, O, or fluorine, F, with a nitrogen, N, oxygen, O, or fluorine, F, atom in another molecule.

Formation in Water

A water molecule consists of two hydrogen atoms and one oxygen atom bonded covalently by sharing electrons. The oxygen atom has high electronegativity. An attraction force forms between a hydrogen atom, H, in one water molecule and an oxygen atom, O, from another water molecule. This attraction is a hydrogen bond.

Other Examples

Hydrogen fluoride, HF, and ammonia, NH3, also form hydrogen bonds. Hydrogen chloride, HCl, does not form hydrogen bonds because chlorine is not one of the highly electronegative atoms, N, O, or F, specified for hydrogen bonding.

Substance Forms hydrogen bonds? Reason
Water, H2O Yes Hydrogen is bonded to oxygen, O.
Hydrogen fluoride, HF Yes Hydrogen is bonded to fluorine, F.
Ammonia, NH3 Yes Hydrogen is bonded to nitrogen, N.
Hydrogen chloride, HCl No Chlorine is not N, O, or F.
Ice floats on water: Hydrogen bonding causes ice to have a lower density than water, so ice floats on the surface of water.

Try to Answer First

Answer mentally, then check the explanation.

1

What is the definition of a hydrogen bond?

Answer: Hydrogen bonds are attraction forces between a hydrogen atom that has bonded with an atom of high electronegativity, such as nitrogen, oxygen, or fluorine, with a nitrogen, oxygen, or fluorine atom in another molecule.
2

Why does ethanol have a higher boiling point than chlorine?

Answer: In ethanol, strong hydrogen bonds are present between molecules in addition to weak Van der Waals forces, requiring more heat energy to overcome, whereas chlorine molecules do not form hydrogen bonds and only have weak Van der Waals forces.
3

Why does a wet finger help to turn the pages of a book?

Answer: Water molecules on the wet finger form hydrogen bonds with the oxygen and hydrogen atoms of cellulose in the paper, causing the paper to stick to the finger.

Common Mistakes

Avoid these errors when explaining hydrogen bonds.

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Confusing the covalent bond inside a molecule, intramolecular, with the hydrogen bond between molecules, intermolecular.
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Assuming any compound containing hydrogen forms hydrogen bonds, for example mistakenly thinking HCl forms hydrogen bonds, even though hydrogen bonding strictly requires N, O, or F.
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Forgetting that Van der Waals forces still exist in molecules that have hydrogen bonds.

Concept Misunderstandings

Compare each misconception with the correct fact.

Misconception

Hydrogen bonds are actual covalent or ionic chemical bonds within a molecule.

Correct Fact

A hydrogen bond is an intermolecular attraction force between separate molecules, or distant parts of a large molecule, not a covalent sharing of electrons.

Misconception

Wet hair sticks together because the hair proteins bond directly to each other more strongly when wet.

Correct Fact

Protein molecules stop bonding to each other when wet; instead, water molecules act as bridges by forming hydrogen bonds between different hair protein molecules.

Summary

Key facts to remember.

  • Hydrogen bonds form between a H atom bonded to a highly electronegative atom, N, O, or F, and an electronegative atom of another molecule.
  • Key examples: H2O, HF, NH3, and ethanol, C2H5OH.
  • Explains daily phenomena: wet hair sticking together, wet finger turning book pages, and low density of ice.
  • Physical property effects: higher boiling points, requiring more heat energy to break, and increased solubility in water through hydrogen bonds with water molecules.

Short Activity

Complete all parts before checking your score.

A. Objective Quiz

1

Which of the following pairs of elements must be bonded to hydrogen in a molecule for it to form hydrogen bonds with neighboring molecules?

2

Why does ethanol, C2H5OH, dissolve readily in water?

B. Fill in the Blanks

3

Hydrogen bonds are attraction forces between a hydrogen atom bonded to a highly electronegative atom and an atom of nitrogen, oxygen, or ______ in another molecule.

4

More ______ energy is required to overcome weak Van der Waals forces and break strong hydrogen bonds, resulting in higher boiling points.

C. Match the Phenomenon with Its Explanation

Drag each phenomenon on desktop. On mobile, tap a phenomenon and then tap the matching box.

Phenomenon / Substance

Wet finger turns book page Wet hair sticks together High boiling point of ethanol Hydrogen chloride, HCl

Explanation

1Water forms hydrogen bonds with cellulose in the paper.
2Water molecules form hydrogen bonds with different hair protein molecules.
3Strong hydrogen bonds between molecules require extra heat energy to break.
4Cannot form hydrogen bonds because chlorine is not N, O, or F.
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Hydrogen bond Electronegativity Van der Waals attraction force Cellulose Boiling point Solubility