Transcript 7 - Shapes of Molecules & Intermolecular Forces

7 – Shapes of Molecules &
Intermolecular Forces
Leaving Certificate Chemistry
Using the Valence Shell Electron Pair
Repulsion Theory
The shape of a molecule depends on the
number of pairs of electrons in the outer shell
of the central atom
2007
2005
2004
2003
Q5 (b) (6) Q4 (b) (6) Q4 (b) (6) Q5 (b) (12)
Using the VSEPR Theory
What is the shape of a BeCl2 (beryllium chloride)
molecule?
Cl
Be
Cl
2 bonding pairs
No Lone Pairs
2 Bonding pairs
Beryllium chloride
(BeH2) is a linear
molecule
Beryllium Chloride
Bond Angle = 180o
Negatively charged electrons repel each
other and arrange themselves as far
apart as possible
Using the VSEPR Theory
What is the shape of a BCl3 (boron trichloride) molecule?
3 bonding pairs
Cl
No Lone Pairs
Boron trichloride (BCl3)
is a triangular planar
molecule
B
Cl
Cl
Boron trichloride
Bond Angle = 120o
Negatively charged electrons repel each
other and arrange themselves as far
apart as possible
Using the VSEPR Theory
What is the shape of a CH4 (methane) molecule?
H
4 bonding pairs
H
H
C
H
No Lone Pairs
Methane (CH4) is a
tetrahedral molecule
Methane
Bond Angle = 109.5o
Negatively charged electrons repel each
other and arrange themselves as far
apart as possible
Using the VSEPR Theory
What is the shape of a NH3 (ammonia) molecule?
Lone Pair
of
electrons
3 bonding pairs
H
N
H
H
1 Lone Pair
Ammonia (NH3) is a
pyramidal molecule
Ammonia (NH3)
Bond Angle = 107o
The presence of the lone pair of electrons
distorts the tetrahedral shape to become a
pyramidal shape
Using the VSEPR Theory
What is the shape of a water molecule?
2 bonding pairs
2 Lone Pairs
O
H
H
Water (H2O) is a
V-Shaped Molecule
Water (H2O)
Bond Angle = 104.5o
The presence of the 2 lone pair of electrons
distorts the tetrahedral shape to become a
v-shaped molecule
Learn the following table off by heart
Bond
Lone
Molecule Shape
Bond Angle
Pairs
Pairs
2
0
Linear
180o
3
0
Triangular
120o
Planar
4
0
Tetrahedral
109.5o
3
1
Pyramidal
107o
2
2
V-Shaped
104.5o
1
3
Linear
180o
Today’s objectives
• To learn about the difference between a
polar bond and a polar molecule
• To predict whether a molecule will be
polar
Symmetry & Bond Polarity
Some molecules may have polar bonds
but they are not polar molecules
. . . .This is due to symmetry
∂―
∂―
O
∂―
Cl
∂ 2+
∂―
C
∂―
Cl
O
∂ 4+
∂―
C
Cl
∂ 3+
B
Cl
Cl
―
∂
Cl
∂―
∂―
Cl
Since the centre of the partial negative charges
coincides with that of the partial positive
charges, none of these molecules are polar
Since the centre of the partial negative charges
does not coincide with that of the partial
positive charges, none of these molecules are
polar despite the presence of polar bonds
Learning objectives
• Difference between intermolecular and
intramolecular forces
• 3 types of intermolecular forces and
relative strengths
• Relationship between intermolecular
forces and melting/ boiling points
Intramolecular Bonding
Water
O
H
H
Intramolecular bonding is the term used
to describe the bonding between atoms in
a molecule
Intermolecular Forces
Intermolecular forces
are
attractions
between
different
molecules
2008
Q5 (a) (5)
Intermolecular Forces
Hydrogen Bonding
Van Der Waals Forces
Dipole-Dipole
Interaction
Van Der Waals Forces
Let’s look at neon atoms. There are 10 electrons in
every neon atom.
+
Ne
Ne
+
+
This temporary force is
called a Van Der Waals
Force
Van Der Waals Forces
Intermolecular
Forces
Van Der Waals Forces
They result from the ebb and flow of electrons
within these molecules, which causes temporary
polarity in a molecule
These weak forces are the only main forces
between like non-polar molecules like N2, O2, H2,
I2, CH4.
The bigger the molecule the more electrons so
the greater the strength of the VDW forces.
Van Der
Waals Forces
– used by
Intermolecular
Forces
Gecko’s?
Dipole-Dipole Forces
A dipole-dipole interaction exists between polar
molecules. They are permanent and much
stronger than Van Der Waals forces.
δ+
δ-
H
δ-
Cl
Cl
δ+
H
Dipole-Dipole Forces Created
Cl
H
δ+
δ-
Dipole-Dipole Forces
Hydrogen Bonding
When molecules that have a hydrogen atom
bonded to nitrogen, oxygen or fluorine atom come
together a very strong dipole-dipole interaction is
setup due to the high electronegativity difference
between the atoms.
δ+ H
δ+
δ2-
H
O
δ+
H
δ2-
δ+
Hydrogen Bonding Created
O
O
H
H
δ+
δ2-
H
δ+
Hydrogen Bonding
Hydrogen bonding forces are permanent.
They are and much stronger than normal DipoleDipole Forces
δ+ H
δ+
δ2-
H
O
δ+
H
δ2-
δ+
Hydrogen Bonding Created
O
O
H
H
δ+
δ2-
H
δ+
Learning check…
• What is the difference between a
intermolecular force and an intramolecular
force?
• Name three different types of
intermolecular force
• Which is the strongest?
• Which is temporary?
Particle arrangements: solids
This animation shows a 2-D view of the motion of the
atoms in a 3-D solid.
Particle arrangements: liquids
This animation shows a 2-D view of the motion of the
atoms in a liquid. There is no order.
Particle arrangements: gases
Hydrogen Bonding
Hydrogen bonding
water
high
boiling point
in
Strong forces attract
molecules together and
a lot of energy is
needed to separate the
particles to turn it to a
gas
Hydrogen Bonding in Water
Hydrogen Bonding
Kevlar is a type of
fibre that is extremely
strong and flexible. It
is strong because long
chains of molecules line
up parallel to each
other and are held
together strongly by
hydrogen bonding
Changes of state
Check your learning..
• What is the relationship between boiling
point and intermolecular bonding?
Boiling Points & Forces
• If a substance has a very high boiling point
then the forces between molecules of that
substance are very strong – ionic, dipoledipole forces or hydrogen bonding.
• If a substance has a very low boiling point
then the forces between molecules of that
substance are very weak – usually Van Der
Waals forces
Why does oxygen (O2) have a much higher
boiling point than hydrogen (H2)?
• Both are non-polar
• Therefore both molecules only experience
weak Van Der Waals forces.
• But the Van Der Waals forces are stronger
in oxygen because it is a molecule with more
electrons (16 versus 2).
• More Electrons  Greater VDW Forces 
Higher Boiling Point for oxygen.
2004
Q4 (c) (6)
Why does methanal have a much higher boiling
point than ethene?
H
H
C
H
C
H
H
Ethene
δ+
δ-
C
O
H
Methanal
• Ethene is non-polar  Weak Van Der Waals
forces present
• Methanal is polar  Stronger dipole-dipole
forces present
• Therefore methanal has a much higher
boiling point then ethene.
Why does water have a much higher boiling
point than hydrogen sulfide (H2S)?
H
H
O
H
Water
Contains very strong
hydrogen
bonding
intermolecular forces
S
H
Hydrogen Sulfide
Contains very weak
Dipole-Dipole
intermolecular forces
2007
Q5 (c) (12)