Bonding forces and intermolecular forces
!–
!–
!+
!–
!+
!+
!–
!+
!–
Bonding forces and intermolecular forces
Bonding forces are attractive forces that occur within an individual
molecule
-- they hold the individual atoms or ions of a molecule together
(i.e., covalent and ionic bonds)
Intermolecular forces are attractive forces that occur between
different molecules
-- they determine how hard it is to separate individual molecules
from one another, which influences:
• whether a substance is a solid, liquid, or gas at a given
temperature
• the viscosity and volatility of a liquid substance
• the melting point and boiling point of a substance
Intermolecular forces are much weaker than bonding forces
Three types of intermolecular forces
• dipole – dipole forces
• London dispersion forces
• hydrogen bonding
Dipole – Dipole Forces
Dipole – dipole forces occur in polar substances
• they are the result of attractions between the positive
and negative ends of different molecules
Example: Chloromethane (CH3Cl)
!+
H
!Cl
H
C
!+ H
H
!+
!-
Cl
!+
H
C
!+H
!+
H
H
!+
!+
C
Cl
!-
H !+
Dipole – Dipole Forces
Dipole – dipole forces are relatively weak, but they have a
significant effect on boiling point
• polar
molecules have higher boiling points than nonpolar
molecules of comparable size
H
H
!O
H
H
C
C
H
C
H
C!
H
H
C
H
H
+
C
H
H
butane (C4H10)
non-polar
Molecular Wt: 58 amu
b.p. = – 0.5 °C
H
C
H
H
H
acetone (C3H6O)
polar
Molecular Wt: 58 amu
b.p. = 56.2 °C
London Dispersion Forces
London dispersion forces are short-lived attractive forces resulting
from the constant motion of electrons within molecules
Example: Diatomic fluorine molecule (F2)
–
–
–
–
–
–
++
+++
+ ++
+
–
–
++
+++
+ ++
+
–
–
–
–
–
–
–
–
–
–
Averaged over time, the distribution of electrons in symmetrical
London Dispersion Forces
London dispersion forces are short-lived attractive forces resulting
from the constant motion of electrons within molecules
Example: Diatomic fluorine molecule (F2)
–
–
!-
–
++
+++
+ ++
+
–
–
–
–
–
–
–
–
–
–
–
++
+++
+ ++
+
–
–
!+
–
–
But at any given moment, the electron distribution may be nonsymmetrical, resulting in temporary polarity
London Dispersion Forces
London dispersion forces are similar to dipole – dipole forces,
except:
• they are the result of temporary dipoles
• they are weaker than dipole-dipole forces
• they occur in both polar and nonpolar molecules
The strength of London dispersion forces increases with
molecular weight
• in
larger molecules, more electrons are moving around,
resulting in stronger temporary dipoles
The strength of London dispersion forces depends on molecular
shape
• the
larger the area of surface contact between molecules, the
stronger the interaction between molecules
London Dispersion Forces
2,2-dimethylpropane
C5H12
(less surface area)
b.p. = 9.5 °C
pentane
C5H12
(greater surface area)
b.p. = 36 °C
Hydrogen bonding
Hydrogen bonds are formed between polar molecules that contain
hydrogen covalently bonded to fluorine, oxygen or nitrogen.
-- Interaction between a positively polarized hydrogen atom and an
unshared electron pair on a N, O or F atom
-- Similar to dipole – dipole forces, but stronger
hydrogen bond
H
!-
O
H
!+
H
hydrogen bond
O
H
covalent bonds
!+
H–F
H – F !covalent bonds
The hydrogen bond
In water
• each hydrogen atom can form
one hydrogen bond
• each oxygen atom can form two
hydrogen bonds
H
H
O
O
H
H
H
• Ionic/covalent bonds are
intramolecular bonds (bonds
between atoms within the
same molecule)
O
H
H
O
H
H
O
O
H
H
• Hydrogen bonds are
intermolecular bonds (bonds
between atoms of different
molecules)
H
Physical properties of water
Formula
Molar mass
(g/mol)
Melting
point
(°C)
Normal boiling
point
(°C)
Heat of fusion
(kJ/mol)
Heat of
vaporization
(kJ/mol)
H 2O
18.02
0.00
100.0
6.04
40.7
H 2S
34.09
-85.5
-60.3
2.38
18.7
H2Se
80.98
-65.7
-41.3
2.51
19.3
H2Te
129.6
-49
-2
--
23.2
methane
CH4
16.04
-182.5
-161
1.1
8.19
ethane
C 2H 6
30.07
-182.8
-88.6
2.8
14.7
propaneC
3H 8
44.10
-187.6
-42.1
3.5
15.7
butane
C4H10
58.08
-138
-0.5
4.7
22.4
Different types of attractive forces between molecules
Between atoms or ions
Ionic bond
Covalent bond
(X = nonmetal)
Between molecules
Practice problems
Chapter 8 Problems:
8.90, 8.91, 8.92, 8.93, 8.116