Hydrogen Bond

HYDROGEN BOND

• LEARNING STANDARD:
• At the end of the lesson, pupils should be able to:

• Explain with examples the formation of a

hydrogen bond

• Explain the effect of the hydrogen bond on

physical properties

WHAT IS A Hydrogen bonds are attraction forces
HYDROGEN between hydrogen atom that has
BOND? bonded with an atom of high
electronegativity, such as nitrogen, N,
oxygen, O or fluorine, F in another
molecule.
For example water molecule, H2O can
form hydrogen bonds among water
molecules, H2O

The electronegative atom that is bonded
to hydrogen atom has the following
properties:

❑The atom should be electronegative in

nature

❑The atom should posses at least one lone

pair of unshared electron

❑The atom should be considerably small.
❑These conditions are fulfilled by 3 atoms

namely nitrogen, oxygen and fluorine

Electronegativity is the strength of
an atom in a molecule to attract
electron towards its nucleus.
Examples of electronegative atoms
are chlorine, oxygen and nitrogen
Notes: electronegativity generally
increases from left to right across a
period and decreases down a
periodic table.



• A polar molecule is a molecule

containing polar bonds. Polar bonds
form when there is a difference
between the electronegativity of the
atoms participating in a bond.

• Example: Hydrogen chloride is a

polar molecule. This is because
chlorine atom in the hydrogen
molecule is more electronegative
than hydrogen. The pair of electrons
are more closely with chlorine atom
than with the hydrogen atom





EXAMPLE OF
HYDROGEN
BONDING

HYDROGEN BOND AMONG WATER MOLECULES

HYDROGEN
BOND IN WATER



HOW When hydrogen is covalently bonded with
HYDROGEN the electronegative oxygen atom, the pair
BOND electrons are more closely with the oxygen
FORMS IN atom than with the hydrogen atom.
WATER?
This leads to the formation of partially
positive charge (δ+) on hydrogen atom, H
and partially negative charge (δ-) on
oxygen atom, O

The partially positive charged hydrogen
atom is then attracted by the other partially
negative charged oxygen atom is known as
hydrogen bond.

Hydrogen bond exists between the H atom in an
HF (hydrogen fluoride) molecule and the F atom
of an adjacent HF molecule.

δ+ δ- δ+ δ-

H-F---H-F
Hydrogen bond

HOW Fluorine is the more electronegative atom, so the
HYDROGEN pair of electrons shared in the bonds are more
BOND FORMS closely with the fluorine atom than with the hydrogen
IN atom.
HYDROGEN This leads to the formation of partially positive
FLUORIDE charge (δ+) on hydrogen atom, H and partially
MOLECULE negative charge (δ-) on fluorine atom, F

The partially positive charged hydrogen atom is
then attracted by the other partially negative
charged fluorine atom is known as hydrogen bond.

TASK 1 :
EXPLAIN
FORMATION
OF HYDROGEN
BOND IN
AMMONIA

1. Nitrogen is the more electronegative than
hydrogen atom, so the pair of electrons shared in
the bonds are more closely with the nitrogen atom
than with the hydrogen atom.

2. This leads to the formation of partially positive
charge (δ+) on hydrogen atom, H, and partially
negative charge (δ-) on nitrogen atom, N.

3. The partially positive charged hydrogen atom is
then attracted by the other partially negative
charged nitrogen atom to form hydrogen bond.

TASK 2: EXPLAIN FORMATION OF HYDROGEN
BOND IN ETHANOL, C2H5OH

1. Oxygen is more electronegative than hydrogen, so
the pair of electrons shared in the bonds are more
closely with the oxygen than with the hydrogen.

2. This leads to the formation of partially positive
charge (δ+) on hydrogen atom, H and partially
negative charge (δ-) on oxygen atom, O.

3. The partially positive charged hydrogen atom is then
attracted by the other partially negative charged
oxygen atom to form hydrogen bond.

EFFECT OF
HYDROGEN
BONDS ON THE
PHYSICAL
PROPERTIES OF
SUBSTANCES

EFFECT OF HYDROGEN BOND ON PHYSICAL
PROPERTIES OF THE SUBSTANCE

1. Boiling point
✓ Hydrogen bonding is stronger than Van der
Waals force.
✓ Its accounts for the abnormally high boiling
points of molecules attracted by hydrogen bond.

Effect on • Compounds in the form of liquids reach boiling
boiling
points point when the attraction forces between
molecules are overcome.

• In the covalent compound of ethanol, C2H5OH,

there are hydrogen bonds formed between
molecules, other than weak Van der Walls
attraction forces.

• Hydrogen bonds are harder to break.
• More heat energy is required to overcome the

weak Van der Waals attraction forces, besides
breaking the hydrogen bonds. As a result, the
boiling point of ethanol, C2H5OH is high.

• On the other hand, molecules like chlorine, Cl2

which do not form hydrogen bonds have lower
boiling point compared to ethanol.

EXAMPLE 1 Relative Molecular Boiling Point /˚C
Mass
Substance 78
46 -42
Ethanol (C2H5OH)
Propane (C3H8) 44

1. Ethanol and propane have almost the same relative molecular

mass and size.

2. The boiling point of ethanol is higher than propane

3. Ethanol has a hydrogen atom attach to an oxygen atom as in

water molecule.

4. There are hydrogen bonds between ethanol molecules, more

energy is needed to overcome the bond before it boils.

5. Propane is a non-polar molecules. Propane molecules are

attracted by weak intermolecular forces (Van der Waals
forces) There are no hydrogen bonds between propane
molecules

TASK 3: EXPLAIN WHY BOILING POINT OF WATER IS
HIGHER THAN BOILING POINT OF HYDROGEN SULPHIDE

Substance Relative Boiling Point
Molecular /˚C
Water
(H2O Mass 100
Hydrogen 18
sulphide -60
(H2S) 34

1. The relative molecular mass of hydrogen
sulphide is higher than water, but the boiling
point of water is higher than hydrogen
sulphide.

2. There are hydrogen bonds between water
molecules, more energy is needed to break
the bonds before it boils.

3. There is no hydrogen bond between
hydrogen sulphide molecules due to the low
electronegativity of Sulphur atom.

EFFECT OF HYDROGEN BOND ON PHYSICAL
PROPERTIES OF THE SUBSTANCE

2. Solubility in water
✓ Increases solubility of the compound
(especially organic compound) in water due to
the formation of hydrogen bond between the
molecules of the compound and water
molecules.

Effect on • Molecules that can form hydrogen bond with
solubility
in water water have a higher solubility in water.

• Covalent compounds that are dissolved in water

are due to the formation of hydrogen bond
between the molecules and water.

• Example: Ethanol is a polar molecule that

dissolves in water. Ethanol form hydrogen bond
with water molecules.

• Other examples of molecules which form

hydrogen bond with water molecules are
ammonia, NH3, hydrogen chloride, HCl and
sugar, C6H12O6. The polarity of these molecules
indicates that they will dissolves in water.

• Non-polar molecules is insoluble in water

SOLUBILITY OF AMMONIA IN WATER

SOLUBILITY OF ETHANOL IN WATER



SOLUBILITY
OF GLUCOSE
IN WATER

EFFECT OF HYDROGEN BOND ON PHYSICAL
PROPERTIES OF THE SUBSTANCE

3. Density of water
✓ The hydrogen bond in H2O also accounts for
the lower density of ice compared to liquid
water.

✓ In ice , the water molecules occupy larger
volume compared to liquid water because the
stronger hydrogen bond in ice hold the water
molecules apart in a rigid structure















• The molecules are further away from each other at this
point than when the water is warmer. Therefore, ice is
less dense that water and floats.

• The water molecules in ice take up about 9% more
space than liquid water, which means ice is about 9%
less dense than water

ROLES OF HYDROGEN BOND IN
DAILY LIFE

EXAMPLE OF ROLE OF HYDROGEN BOND IN
DAILY LIVES

1. Wet hair will stick together because of the hydrogen
bond between water molecules// wet hair sticky
compared to dry hair.

2. Flipping the paper is easier when the fingertips are
wet with water compared to dry finger tips.

WET HAIR IS STICKY COMPARED TO DRY
HAIR

• The keratin molecules(protein molecules) that

form the outer layer of the hair is a polar
molecule.

• Hydrogen bond is formed when partially

positive charged of hydrogen atom in water
attracted to the partially negative charged of
oxygen atom in keratin.

• When hair is wet, protein molecules (keratin

molecules) no longer form hydrogen bonds
among themselves. Instead, protein molecules
will form hydrogen bonds with water molecules.

• This causes hair to stick together.

• Have you ever come across the problem of turning the pages

of a book where the pages stick together?. To overcome the
problem, you lick your finger before turning the pages. Why?

HYDROGEN
BOND IN
CELLULOSE
MOLECULE

• The cellulose molecule that form the paper is a polar molecule
• Hydrogen bond is formed when partially positive charged of

hydrogen atom in water is attracted to the partially negative
charged of oxygen atom in cellulose.

• When fingertips are wet, there are hydrogen atom between

water molecules and cellulose in paper, thus it is easier to flip
paper.

• When fingertips are dry, no hydrogen bond formed between

water and cellulose in paper, thus it is difficult to flip the
paper.