Transcript HYDROGEN BONDING

1d – Intermolecular Forces
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To examine the effects of intermolecular forces
on properties of a substance
To know how Van der Waals forces arise and
their effect
To Know how permanent dipole/dipole
interactions occur and their effect on properties
To know how hydrogen bonding arises and it’s
importance for properties of some substances
He
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Noble gases have full outer electron shells
They do not need to combine with other atoms.
They are said to be monatomic.
Group 0 are all gases and
exist as individual atoms.
However, the monatomic gases do form weak inter-atomic
bonds at very low temperatures.
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Sometimes the electrons can end up on one
side of the atom, i.e. the electron cloud can wobble
This means that one side of the atom is more
negative than the other side. i.e. 2 ‘electric poles’ are formed, otherwise
called a dipole.
These charges are given the symbol δ ‘delta’
δ+
A temporary dipole is therefore formed.
δ-
δ+
δ-
δ+
A dipole can induce other atoms
to form dipoles, resulting in
dipole –dipole attraction.
δVan der Waals forces
180
160
166
140
120
121
100
b.p / K
80
87
60
40
20
4
Helium
Neon
Argon
Krypton
Xeon
27
0
B.p.’s increase as the size of the atom increases
This happens because the Van der Waals’ forces increases with increasing size of
atoms.
A SMALL MOLECULE
COVALENT SUBSTANCE
covalent bonds (strong)
intermolecular
attractions
(weak)
what causes
them?
covalent bonds (strong)
intermolecular
bonds
(weak)
=
van der Waal’s
forces
MUCH WEAKER
THAN
covalent bonds (strong)
boiling
point
boiling
point
WHY?
wee, light molecule
big, heavy molecule
weak attraction
strong attraction
lower boiling point
higher boiling point
the higher the boiling point.
the stronger the intermolecular attraction
the heavier the molecule
Fluorine atom
Fluorine molecule F2
9+
9+
9+
diatomic
A covalent bond is formed when a pair of electrons are shared. The atoms in
a covalent bond are held together by electrostatic forces between positively
charged nuclei and negatively charged electrons.
Strong covalent
bond
F
Weak Van der Vaals force
F
F
F
Strong intra-molecular bonding and weak intermolecular bonding exist in this diatomic
molecule.
F 2 m.p. -220o C or 53 K
Halogens b.p.’s
500
450
457
400
b.p./ K
Fluorine
350
Chlorine
300
332
250
200
238
150
100
50
85
0
As the size of the halogen atom increases, so does the size
of the van der waals’ forces between the halogen molecule.
Bromine
Iodine
The differing electronegativities of different atoms in a
molecule and the spatial arrangement of polar covalent bonds
can cause a molecule to form a permanent dipole.
-
+
No permanent dipole
Symmetrical molecule
Permanent dipole
Asymmetrical molecule
+
-
-
4 polar covalent C-Cl bonds in
CCl4 tetrahedral shape
NON-POLAR molecule
e.g. also CO2
-
-
3 polar covalent C–Cl bonds and
1 polar covalent C-H bond in CHCl3
POLAR molecule
e.g. also H2O
Both propanone and butane have the same formula mass of 58
however, butane boils at – 1 oC while propanone boils at 56oC
Propanone is a polar molecule as it has a permanent dipole, so has polarpolar attraction as well as Van der Waals’ forces between molecules.
H
H
C
H
-
O
H
C
C
+
H
b.p. 56 o C
H
Butane has no permanent dipoles, so only Van der Waals forces
between molecules. So has a lower boiling point.
H
H C
H
H
C
H
H
C
H
H
C H
H
b.p. -1 o C
Hydrogen bonding is a special type of dipole-dipole attraction in which hydrogen
atom acts as a bridge between two very electronegative atoms. It is the
strongest of the weak inter-molecular forces.
A -
H +
B -
F
H
+
-
The Hydrogen atom is in a straight line between A - and B - .
The bond strength is stronger than other forms of permanent dipole-permanent
dipole interaction but weaker than a covalent bond.
A and B are electronegative atoms, such as F, O or N.
Such atoms possess one or more lone pair of electrons.
Proteins consist of long chain atoms containing polar >C=O and H-N< bonds.
Hydrogen bonds help give proteins their shape.
Group
4 5 6 7
Group
4 5 6 7
1st row 2nd row 3rd row 4th row
The intermolecular attraction
is much stronger than you
would expect it to be.
This extra strong attraction
is given the name
THE HYDROGEN BOND.
But what causes it?
A
A
pure
covalent
bond
A
+
A
A
pure
covalent
bond
-
B
polar
covalent
bond
A special type of polar covalent bond
+
H
-
X
X = a highly electronegative element
covalent bonds
H+
X-
H+
X-
H+
X-
+
-
+
-
+
-
HYDROGEN BONDS
(HYDROGEN BONDING)
X = a very electronegative atom
Covalent substances
that have hydrogen bonding
than you would otherwise expect.
WATER
-
O
-
+
H
+
H
-
O
H
H
+
+
Oxygen has 2 lone pairs of electrons which can form
a hydrogen bonds with two hydrogen atoms.
Each water molecule, in theory, could be surrounded
by 4 hydrogen bonds
Pond Skater
Ice Skater:
Water has a high surface tension. The molecules on the surface
have in effect, hydrogen bonds. This has the effect of pulling
the surface molecules closer together.
Water has a its greatest density at a temperature of 4oC. When, as water cools
further, the molecules start to move further apart, due to the hydrogen bonding,
until a more open structure formed at its freezing point. So ice floats!!
sugars
H OH
H O
HO
HO
H
H
glucose
H
OH
OH
amino acids
H
H
O
N
CH C
H
an amino acid
OH
proteins
H
H
O
H
H
O
H
H
O
H
H
O
H
H
O
N
C
C
N
C
C
N
C
C
N
C
C
N
C
C
R1
R2
R3
R4
R5
DNA
NH 2
N
N
HO
N
N
O
H
H
O
H
O
P
O-
H
H
O3 billion
haemoglobin
DNA