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.
Formation of hydrogen bonds between H2O molecules.
hydrogen bond
Electrostatic attraction exists between partial positive charge of H atom and the lone pair electrons of O atom of another H2O.
Formation of hydrogen bonds between NH3 molecules.
hydrogen bond
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)
• Molecular size of hydride molecules increases down a group
• the van der Waal’s forces between molecules increases down a group
Boili
ng p
oint
(°C)
Period
H2O
H2SH2Se
H2TeHF
HClHBr
HINH3
PH3
AsH3 SbH3
CH4
SiH4
GeH4
SnH4
• 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
• 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.
Fig. 27.11 Droplets of water are caused by high surface tension that pulls water molecules into a sphere.
2. Surface tension
2. Surface tensionThere 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
The resistance of aliquid to flow.
The higher the viscosity of a liquid, the more slowly it flows.
Viscosity
Strong intermolecular forces hold molecules together and do not allow them to move past one another easily.
Liquid water
molecules
Benzene molecules
Hydrogen bonds
Weak intermolecular forces
held by
held by
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 27.1Experiment 27.1 Experiment 27.1Experiment 27.1
P. 33 / 15
The oxygen atom of each water molecule forms hydrogen bonds with two hydrogen atoms of nearby water molecules.
Structure and bonding of ice
a water molecule
hydrogenbond
hydrogenbond
hydrogen atom
oxygen atom
P. 34 / 15
The two hydrogen atoms of each water molecule also form hydrogen bonds with oxygen atoms of nearby water molecules.hydrogen
bond
hydrogenbond
hydrogen atom
oxygen atom
P. 35 / 15
2
1
3
4
The central oxygen atom of each water molecule has a tetrahedral arrangement of two lone pairs (forming hydrogen bonds) and two bond pairs.
Fig. 28.3A water molecule canform hydrogen bonds with four other water molecules.
P. 36 / 15
In solid ice, the tetrahedral arrangement repeats over and over again, resulting in a regular open network structure of water molecules.
P. 40 / 15
ExplanationIn ice, water molecules are arranged in an orderly manner in an open network structure because of extensive hydrogen bonding.
Open network structure!
P. 41 / 15
liquid water
In this open structure, water molecules are further apart than they are in liquid water.
melts
open structure collapses
water molecules tend to pack more closely together
Think aboutThink about
ice
P. 42 / 15
Ice
presence of extensive hydrogen bonding between water molecules
regular open network structure
low densityrelatively highmelting point
High viscosity
Ethanol CH3CH2OH
hydrogen bond
The hydrogen atom of an ethanol molecule can form a hydrogen bond with the oxygen atom of another ethanol molecule.
The hydrogen atom of an ethanol molecule can form a hydrogen bond with the oxygen atom of another water molecule.
Ethanol CH3CH2OH
hydrogen bond
The hydrogen atom of an ethanol molecule can form a hydrogen bond with the oxygen atom of another ethanol molecule.
The hydrogen atom of an ethanol molecule can form a hydrogen bond with the oxygen atom of another water 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 ethanol molecule.
The hydrogen atom of an ethanol molecule can form a hydrogen bond with the oxygen atom of another water 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 ethanol molecule.
The hydrogen atom of an ethanol molecule can form a hydrogen bond with the oxygen atom of another water molecule.
High viscosity
Fig. 27.16 There are hydrogen bonds between the base pairs on the nucleic acid chains.
hydrogen bonds
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 Presence and no. of hydrogen bonds
Molecular size
Shape Polarity of molecules
Affected by(1. Presence of lone pair electrons on F,O,N on one molecule2. 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 crystalsCrystals having an ordered arrangement of molecules are called molecular crystals.
table sugarice
Examples: ice, table sugar and iodine
Fig. 28.1 The crystal structure of iodine.
Iodine molecules are arranged orderly in iodine crystal.
These molecules are closely packed together, but they are still separate molecules.These molecules are held together by relatively weak intermolecular forces.
P. 56 / 15
The oxygen atom of each water molecule forms hydrogen bonds with two hydrogen atoms of nearby water molecules.
Structure and bonding of ice
a water molecule
hydrogenbond
hydrogenbond
hydrogen atom
oxygen atom
P. 57 / 15
2
1
3
4
The central oxygen atom of each water molecule has a tetrahedral arrangement of two lone pairs (forming hydrogen bonds) and two bond pairs.
Fig. 28.3A water molecule canform hydrogen bonds with four other water molecules.
P. 58 / 15
In solid ice, the tetrahedral arrangement repeats over and over again, resulting in a regular open network structure of water molecules.
Learning tipLearning tip
P. 59 / 15
hydrogen bond
empty space
a water molecule
Fig. 28.4 The structure of ice.
ice (ball)
ice (ball)
P. 60 / 15
ExplanationIn ice, water molecules are arranged in an orderly manner in an open network structure because of extensive hydrogen bonding.
Open network structure!
P. 61 / 15
liquid water
In this open structure, water molecules are further apart than they are in liquid water.
melts
open structure collapses
water molecules tend to pack more closely together
Think aboutThink about
ice
P. 62 / 15
2. Melting pointWater 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
P. 63 / 15
Ice
presence of extensive hydrogen bonding between water molecules
regular open network structure
low densityrelatively highmelting point
P. 64 / 13
Structure and bonding of fullerenes
Fullerenes are molecules composed entirely of carbon atoms, in the form of hollow spheres or hollow tubes.
P. 65 / 13
Buckminsterfullerene (or buckyball)The first fullerene discovered was buckminsterfullerene.
Fig. 28.10 (a) The structure of buckminsterfullerene. (b) A soccer ball.
P. 66 / 13
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.
P. 67 / 13
Do you know?Do you know?
Other related molecules composed of only carbon atoms were also discovered.
Class practice 28.2Class practice 28.2
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
P. 68 / 13
GraphiteGraphite
DiamondDiamond
insolublein all liquid
solvents
insolublein all liquid
solvents
C60C60
dissolves in organic
solvent
dissolves in organic
solvent
1. Solubility
P. 69 / 13
buckminsterfullerene (C60) is an electrical insulator.
2. Electrical conductivity
Substance Electrical conductivity
Graphite √ (with delocalized e-)
Diamond X
C60
X (simple molecular structure,No ions, no delocalized electrons
http://en.wikipedia.org/wiki/Diamond_cubic
P. 70 / 13
Substance Melting point (°C)
Graphite 3730 (Giant covalent structure)
Diamond 3550(Giant covalent structure)
C60
1070(Simple molecular structure)
C60 molecules are held together by weak van der Waals’ forces.
1. Melting point