Transcript Hydrogen bonding

2 types of Intermolecular forces
1. Van der Waal’s forces
(attraction between partial + charge on one
molecule & partial - charge on another molecule)
>
2. Hydrogen bonding
hydrogen bond
Formation of hydrogen bonds between HF molecules.
Electrostatic attraction exists between partial positive
charge of H atom and the lone pair electrons of F
atom of another HF.
hydrogen bond
Formation of hydrogen bonds between H2O
molecules.
Electrostatic attraction exists between partial positive
charge of H atom and the lone pair electrons of O atom
of another H2O.
hydrogen bond
Formation of hydrogen bonds between NH3 molecules.
Electrostatic attraction exists between partial positive
charge of H atom and the lone pair electrons of N
atom of another NH3.
Class practice 27.3
Identify the hydrogen atoms of the following species
that are capable of forming hydrogen bonding with
water molecules.
(a) CH3OH
(b)
(c)
Hydrogen bond
-- between H atom (bonded to F,O,N) and lone pair of electron (on F,O,N)
(a)
(b)
(c)
A27.3
(a) CH3OH
(b)
(c)
(b)
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Boiling point (°C)
H2O
HF
AsH3
NH3
H2S
HCl
PH3
H2Te
GeH4
SiH4
CH4
Period
SbH3
HI
SnH4
H2Se
HBr
• Molecular size of hydride molecules increases down
a group
•  the van der Waal’s forces between molecules
increases down a group
Boiling point (°C)
H2O
HF
AsH3
NH3
H2S
HCl
PH3
H2Te
GeH4
SiH4
CH4
Period
SbH3
HI
SnH4
H2Se
HBr
• High electronegativities of F, O and N.
• Besides van der Waal’s forces, there are hydrogen
bonds between molecules of NH3 , H2O and HF.
• However, there is weak van der Waal’s forces
between other molecules only.
• Hydrogen bond is stronger than van der Waal’s
forces
• A lot more energy is needed to break hydrogen
bonds between molecules
•  The melting and boiling pt of NH3 , H2O and
HF are much higher than expected.
Molecular size of hydride
molecules increases down a
group
 the VDW forces between
molecules increases down a
group
V
• Besides van der Waal’s forces, there are hydrogen
bonds between H2O molecules.
• However, In H2S, H2Se, H2Te , there are weak van
der Waal’s forces between molecules only.
• Hydrogen bond is stronger than van der Waal’s
forces
• A lot more energy is needed to break hydrogen
bonds between molecules
•  The melting and boiling pt of water are much
higher than expected.
2. Surface tension
Fig. 27.11 Droplets of water are caused by high
surface tension that pulls water molecules into a
sphere.
2. Surface tension
There are extensive hydrogen bonds
between water molecules.
The surface tension of water is much higher
than that of most other common liquids.
hydrogen bond
Liquid
Relative surface tension
C6H12
18.4 ( no of H bonding per molecule= 0)
CH3OH(methanol)
22.6
( no of H bonding per molecule= 1)
CH3CH2OH
22.8 ( no of H bonding per molecule= 1)
H2O
72.3 ( no of H bonding per molecule= 2)
hydrogen bond
3. Viscosity
Viscosity
The resistance of a
liquid to flow.
The higher the viscosity of a liquid, the more
slowly it flows.
Strong intermolecular forces hold molecules
together and do not allow them to move
past one another easily.
Liquid
water
molecules
held by
Hydrogen
bonds
held by
Benzene
molecules
Weak intermolecular
forces
Liquid
Relative viscosity
Benzene
1
Water
15
Table 27.4 Relative viscosities of some liquids at 25°C.
Water has high melting and boiling points,
high surface tension and is more viscous
than benzene.
Experiment 27.1
Experiment 27.1
Structure and bonding of ice
The oxygen atom of
each water molecule
forms hydrogen bonds
with two hydrogen
atoms of nearby water
molecules.
a water
molecule
hydrogen
bond
hydrogen
bond
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hydrogen atom
oxygen atom
The two hydrogen atoms of
each water molecule also
form hydrogen bonds with
oxygen atoms of nearby
water molecules.
hydrogen
bond
hydrogen
bond
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hydrogen atom
oxygen atom
The central oxygen atom of each water
molecule has a tetrahedral arrangement of
two lone pairs (forming hydrogen bonds)
and two bond pairs.
1
4
2
3
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Fig. 28.3
A water molecule can
form hydrogen bonds
with four other water
molecules.
In solid ice, the tetrahedral arrangement
repeats over and over again, resulting in a
regular open network structure of water
molecules.
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Fig. 28.4
The structure of ice.
hydrogen bond
empty space
a water molecule
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Fig. 28.5
The oxygen atoms in
the structure of ice
are arranged in a
hexagonal shape.
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Fig. 28.6
The hexagonal symmetry
of a snowflake reflects the
structure of ice.
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Explanation
In ice, water molecules are arranged in an
orderly manner in an open network structure
because of extensive hydrogen bonding.
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In this open structure, water molecules are
further apart than they are in liquid water.
liquid water
ice
melts
open structure
collapses
Think about
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water molecules tend to
pack more closely together
presence of extensive
hydrogen bonding
between water
molecules
regular open
network structure
High viscosity
relatively high
melting point
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Ice
low density
Ethanol CH3CH2OH
hydrogen bond
The hydrogen atom of an ethanol
molecule can form a hydrogen bond with
the oxygen atom of another water molecule.
The hydrogen atom of an ethanol
molecule can form a hydrogen bond with
the oxygen atom of another ethanol
molecule.
Ethanol CH3CH2OH
hydrogen bond
The hydrogen atom of an ethanol
molecule can form a hydrogen bond with
the oxygen atom of another water molecule.
The hydrogen atom of an ethanol
molecule can form a hydrogen bond with
the oxygen atom of another ethanol
molecule.
High Solubility in water
Ethanol CH3CH2OH
hydrogen bond
The hydrogen atom of an ethanol
molecule can form a hydrogen bond with
the oxygen atom of another water molecule.
The hydrogen atom of an ethanol
molecule can form a hydrogen bond with
the oxygen atom of another ethanol
molecule.
High boiling point
Ethanol CH3CH2OH
hydrogen bond
The hydrogen atom of an ethanol
molecule can form a hydrogen bond with
the oxygen atom of another water molecule.
The hydrogen atom of an ethanol
molecule can form a hydrogen bond with
the oxygen atom of another ethanol
molecule.
High viscosity
Ethanol is completely miscible with water,
and has high boiling point. It is as viscous
as water.
Ethanoic acid CH3COOH
hydrogen bonds
Fig. 27.16 There are hydrogen bonds between the
base pairs on the nucleic acid chains.
Effect of hydrogen bonding on
DNA
Hydrogen bonds between specific base
pairs hold two nucleic acid chains of a DNA
molecule together.
The presence of hydrogen bonds helps
maintain the double helical shape of the
molecules.
b.p / density/ viscosity of molecules
Affected by
Strength of van der Waal’s forces
Affected by
Molecular
size
Shape
Presence and no. of hydrogen bonds
(1. Presence of lone pair
electrons on F,O,N on one
molecule
Polarity of molecules
2. Presence of H attached
to F,O,N on another
molecule)
No. of hydrogen bonds per molecule
= minimum no. of lone pair electrons on
F,O,N
/ no, of H attached to F,O,N
Molecular crystals
Crystals having an ordered arrangement of
molecules are called molecular crystals.
Examples: ice, table sugar and iodine
ice
table sugar
Iodine molecules are
arranged orderly in
iodine crystal.
These molecules are
closely packed together,
but they are still
separate molecules.
Fig. 28.1 The crystal
structure of iodine.
These molecules are
held together by
relatively weak
intermolecular forces.
Structure and bonding of ice
The oxygen atom of
each water molecule
forms hydrogen bonds
with two hydrogen
atoms of nearby water
molecules.
a water
molecule
hydrogen
bond
hydrogen
bond
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hydrogen atom
oxygen atom
The central oxygen atom of each water
molecule has a tetrahedral arrangement of
two lone pairs (forming hydrogen bonds)
and two bond pairs.
1
4
2
3
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Fig. 28.3
A water molecule can
form hydrogen bonds
with four other water
molecules.
In solid ice, the tetrahedral arrangement
repeats over and over again, resulting in a
regular open network structure of water
molecules.
Learning tip
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Fig. 28.4
The structure of ice.
ice
(ball)
hydrogen bond
empty space
a water molecule
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Explanation
In ice, water molecules are arranged in an
orderly manner in an open network structure
because of extensive hydrogen bonding.
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In this open structure, water molecules are
further apart than they are in liquid water.
liquid water
ice
melts
open structure
collapses
Think about
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water molecules tend to
pack more closely together
2. Melting point
Water has a high melting temperature
compared with substances of similar
molecular masses.
Substance
Relative molecular mass Melting point (°C)
Nitrogen
18
−210
Ammonia
17
−78
Water
18
0
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presence of extensive
hydrogen bonding
between water
molecules
relatively high
melting point
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Ice
regular open
network structure
low density
Structure and bonding of
fullerenes
Fullerenes are molecules composed entirely
of carbon atoms, in the form of hollow spheres
or hollow tubes.
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Buckminsterfullerene (or buckyball)
The first fullerene discovered was
buckminsterfullerene.
Fig. 28.10 (a) The structure of buckminsterfullerene.
(b) A soccer ball.
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Each carbon atom is connected to three
other carbon atoms by one double covalent
bond and two single covalent bonds.
The atoms are arranged in a pattern of 20
hexagons and 12 pentagons on the surface
of the sphere.
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Other related molecules composed of only
carbon atoms were also discovered.
C28
C32
C50
C70
Fig. 28.11 Some of the more stable members of the
fullerene family. (a) C28 (b) C32 (c) C50 (d) C70
Do you know?
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Class practice 28.2
1. Solubility
Graphite
Diamond
C60
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insoluble
in all liquid
solvents
dissolves
in organic
solvent
2. Electrical conductivity
buckminsterfullerene (C60) is an electrical insulator.
Substance
Graphite
√ (with delocalized e-)
Diamond
X
C60
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Electrical conductivity
X (simple molecular structure,
No ions, no delocalized electrons
http://en.wikipedia.org/wiki/Diamond_cubic
1. Melting point
C60 molecules are held together by weak
van der Waals’ forces.
Substance
Melting point (°C)
Graphite
3730 (Giant covalent structure)
Diamond
3550(Giant covalent structure)
C60
1070(Simple molecular
structure)
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3. Strength and hardness
Buckminsterfullerenes are relatively strong
and hard compared with most other
molecular solids.
Fig. 28.13
The C60 molecules are packed
closely together in solid state.
buckminsterfullerene
molecule (C60)
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