Bonding:
Intermolecular Forces
Covalent Bonds
Covalent bonding leads to two types of molecules
Giant Structures
Eg. Diamond, sand
Strong covalent bonds
between atoms create a
large network resulting
in a high melting and
boiling point
Simple Molecular
Structures
Eg. Methane, hydrogen
chloride
Few covalent bonds per
molecule. Exist as gases
or low boiling liquids
therefore have low
melting and boiling
points.
How we know intermolecular forces
exist
Gases have large distances between molecules, short range
forces between molecules are insignificant
Cool and Compress
Distances between molecules decrease and the forces
between become significant. Gases liquefy (G->L)
Cool and Compress
Distances are further reduced. Forces become more
significant. Liquid solidifies. (L->S)
Types of Intermolecular Forces
London Forces (VDW
Forces)
• Due to random
movement of electrons
that lead to the
formation of an
instantaneous dipole.
Induced dipoles in the
molecule and induced
dipole-dipole force
between molecules
• Strength depends on the
molar mass of the
molecule
• Dependent on the
surface area of the
molecule
Dipole-Dipole Forces
Hydrogen Bonding
• Occur due to electrostatic
attraction between
molecules with
permanent dipoles
• Significantly stronger
than london forces in
molecules of similar size
• Molecule will not just
have london forces they
will have dipole-dipole
forces as well.
• Occurs in molecules that
contain H bonded to
N/O/F
• Non-bonding electron
pair on the N/O/F atoms
interact with the H atom
that carries a high δ+
character because it is
bonded to another of the
small very
electronegative atoms
• Strongest of all the
intermolecular forces
Examples to explain London Forces
Boiling points
of noble gases
•He- 4K
•Xe-165K
•Higher atomic
mass=higher
number of
electrons
Boiling points
of alkanes
•Methane111K
•Hexane: 341K
•As molar mass
increases,
london forces
increase
Boiling points
of Halogens
•As molar mass
of halogens
increases from
F2 to I2 the
boiling points
also increase
Boiling points
of straight
chains &
branches
Alkanes of the
same molar
mass
n pentane BP
309 K
neo pentane
BP 283 K
Plastics and
Polymers
They have very
high
Molar mass
and very high
surface area
Examples to explain Dipole-Dipole
Forces
The HCl molecule is polar and
has slight negative and slight
positive centres on it.
This is a permanent DIPOLE
and it interacts with other
dipoles also and this
electrostatic attraction will be
DP-DP forces.
Comparing the boiling points
of non polar noble gases and
polar hydrogen halides of
same molar masses
Boiling points of Non-polar Ar,
Kr and Xe will be lower than
those of Polar hydrogen
halides like HCl, HBr and HI
Because these are permanent
dipoles they will have DP-DP
forces along with VDW forces
Hydrogen Bonding
Hδ+-Fδ---------------Hδ+-Fδ• Strongest of the intermolecular forces
• Affects the physical properties of the
compounds
Examples of H-Bonding
Comparing BP of Hydrogen peroxide, Fluorine
and Hydrogen Chloride (Mr ~ 34-36)
• Hydrogen peroxide
• Hydrogen Chloride
• Fluorine
431 K
188 K
85 K
(polar, hydrogen bonded)
(polar)
(non polar)
Comparing BP of Propane, Ethanal and Ethanol
(Mr~ 44-46)
• Propane C3H8
only VDW
BP is 231 K
• Ethanal CH3CO-H VDW, polar
BP is 294 K
• Ethanol CH3CH2OH VVF, polar, Intermolecular H bonding—352 K
Hydrogen Bonding in diff. compounds
Examples of H-Bonding contd….
Comparing the boiling points of
hydrides of group 4,5,6,7
• gp 4 compounds lowest boiling points as they are
Tetrahedral, non polar, only VVF
• Methane (CH4) period 2 has lowest BP for the group
and as we move to periods 3,4,5 the boiling point also
increases. Why?
• Molar mass, Van der waals forces, so BP 
Examples of H-Bonding contd….
What is the trend for Boiling points in periods 3,4,5 for different
groups5,6,7?
For Periods 3, 4, 5 as molar mass increases the bp also increases.
But for the 1st member in period 2 for these groups the BP is
exceptionally high[ NH3<HF<H2O] . Why?
This is because they have in addition to VVF,H Bonding also .
The deviation in trend is very high for water. Why?
There are 2 H atoms and 2 non bonding e pairs in 1 molecule. So is
capable of 2 hydrogen bonds per molecule, extent of H bonding is the
greatest, hence BP is highest.
Trends in Boiling points of Groups
4,5,6 and 7
Examples of H-Bonding contd….
Explain this trend in boiling point of period 2
members of groups 5,6,7--[ NH3<HF<H2O]
• There are 2 H atoms and 2 non bonding e pairs in 1
molecule. So is capable of 2 hydrogen bonds per
molecule, extent of H bonding is the greatest, BP highest.
• HF has 1 H atom and 3 non bonding e pairs per molecule.
So extent of H bonding lesser than water so BP lesser
than water.
• NH3 has 3H atoms and 1 non bonding e pair per
molecule so the extent of hydrogen bonding lesser
Anomalous behavior of Water
H Bonds
• In water liquid - 2/molecule
• In ice - 4/molecule
Density
• Density of ice is less than that of water
• When ice melts from 0°C to 4°C then density of water increases
unlike other liquids
Molar enthalpy
of fusion
Molar enthalpy
of vaporization
• very high
• very high
Structure of ICE
Density of ice is less than water
• Each water molecule can form 4 H bonds to 4
other water molecules in a tetrahedral symmetry
• So the structure produced resembles that of
diamond but here there are H bonds holding the
tetrahedron and not covalent bonds
• Open structure like a cage with large empty spaces
in between so air is trapped in them, density is
less and it floats on water.
Hydrogen Bonds in Water, Ice
Compare H2O and H2S
Properties
Water
Hydrogen sulphide
Molar enthalpy of fusion
6.0 kJmol-1
2.4 kJmol-1
Molar enthalpy of vaporization
41.0 kJmol-1
19.0 kJmol-1
State at room temperature &
Pressure
Liquid
Gas
Electro negativity
Very high
Less
Size
O atom is small
S is larger than O
Hydrogen Bonding
2 H bonds per
No H Bonding
molecule of water
Effect of H bonding on Organic
compounds- case 1
Ethanoic acid
Butane
• (Mr =60)
• BP 391 K
• Liquid at rtp
• Due to H Bonds it can
exist as dimer and its
Mr seems ~ 120
• Can bond with water
molecules and is
miscible
• (Mr =58)
• BP 273 K
• Gas at rtp
• Only covalent bonds
• Immiscible in water
Dimer of Ethanoic acid
Effect of H bonding on Organic
compounds- case 2
• Propanone is miscible in water but it has no H
bonding between propanone molecules
• This is because its O atoms can bond to water
molecules.
Effect of H bonding on Organic
compounds- case 3
• p- nitro phenol or 4- nitro phenol is having
intermolecular H bonds , stronger forces of
interaction between molecules and so its BP is
279°C
• o- nitro phenol or 2- nitro phenol has
intramolecular H bonds, weaker forces of
interaction between molecules and so has
lower BP of 216°C
Biological importance of H bonding
• DNA pairing occurs due to H bonds.
• Secondary structure of proteins (α helix and
β pleated proteins)
• α helix-is an example of INTRA MOLECULAR H
Bonding
Hydrogen bonds in DNA Base Pairs
2 H bonds between Adenine and Thymine
3 H bonds between Cytosine and Guanine