chapter 6-Intermolecular Forces-2012,4,10

forces or dispersion forces.
London Dispersion Forces
On the average, the charge distribution in the helium atom is spherical. At a particular instant, the helium atom is polar, with an excess electron on the left side and a shortage on the right side.
Relative strength of forces:

Chapter 6 Intermolecular Forces
Dipole-dipole interactions London dispersion forces Dipole-induced dipole force Hydrogen bonding
Intermolecular Forces
These intermolecular forces as a group are referred to as van der Waals forces.
van der Waals Forces
• • • • Dipole-dipole interactions London dispersion forces Dipole-induced dipole force Hydrogen bonding
The covalent bond present within HCl molecules is much stronger than the forces present between the neighbouring molecules, which exist when the molecules are sufficiently close to each other.
Dipole-Dipole Interactions
The more polar the molecule, the higher is its boiling point.
Of the following substances, predict which has the highest boiling point based on intermolecular forces.
1. Propane, C3H8 2. Dimethyl ether, CH3OCH3 3. Methyl chloride, CH3Cl 4. Acetaldehyde, CH3CHO 5. Acetonitrile, CH3CN
Correct Answer:
1. Propane, C3H8 2. Dimethyl ether, CH3OCH3 3. Methyl chloride, CH3Cl 4. Acetaldehyde, CH3CHO 5. Acetonitrile, CH3CN
dipole moment
1. Dipole moment (μ) ： A quantitative measure of the polarity of a polar molecular.
Q d
+ -
Unit：C · m
d +Q -Q
q: the charges on both poles; d: the distance between the charges
Dipole Moment of Some Molecules μ(×10－30 C·m)
Molecule H2 N2 CO2 CS2 CH4 CO CHCl3 H2 S Dipole moment 0 0 0 0 0 0.40 3.50 3.67 molecule SO2 H2O NH3 HCN HF HCl HBr HI Dipole moment 5.33 6.17 4.90 9.85 6.37 3.57 2.67 1.40
There are both repulsive interactions between like charges and attractive interactions between unlike charges, but the attractive interactions predominate. Molecules that have permanent dipoles are attracted to each other. For example HCl and chloroform (CHCl3) Dipole-dipole attractions.
Intermolecular Forces
Intermolecular forces are forces of attraction or repulsion which act between neighboring particles: atoms, molecules or ions. They are weak compared to the intramolecular forces, the forces which keep a molecule together.
Lecture
Section I: the Structure of Matters Chapter 6
Intermolecular Forces
2012, 04, 10
Summary: intramolecular forces and intermolecular forces
Bond type Covalent bonds Ionic bonds Metallic bonds Hydrogen bond Dipole–dipole forces Dipole-induced dipole force London dispersion forces.
Intermolecular Forces
• The intermolecular attractions are weaker than covalent bonds; • They are, however, strong enough to control physical properties such as boiling and melting points, vapor pressures, and viscosities.
London Dispersion Forces
London Dispersion Forces
London dispersion forces (LDF, also known as dispersion forces, London forces, instantaneous dipole– induced dipole forces) is a type of force acting between atoms and molecules. The LDF is named after the German-American physicist Fritz London. The LDF is a weak intermolecular force arising from quantum induced instantaneous polarization multipoles in molecules.
Dispersion forces.
(a) On the average, the charge distribution in the helium atom is spherical. (b) At a particular instant, however, there can be a nonspherical arrangement of the electrons. The nonspherical electron distributions produce momentary dipoles and allow momentary electrostatic attractions between atoms that are called London dispersion
1) Of the molecules below, only __________ is nonpolar. A) CO2 B) H2O C) NH3 D) HCl 2) The molecular geometry of the BCl3 molecule is __________, and this molecule is __________. A) trigonal pyramidal, polar B) trigonal pyramidal, nonpolar C) trigonal planar, polar D) trigonal planar, nonpolar E) trigonal bipyramidal, polar
Answer: A , D
Dipole-Dipole Interactions
Dipole-Dipole Interactions
Dipole–dipole interactions are electrostatic interactions of permanent dipoles in molecules. These forces are only important when the molecules are close to each other.
Each of these molecules has almost the same molecular weight; however, acetonitrile has the largest dipole moment (3.9 D) and hence the largest dipole-dipole forces. Thus it has the highest boiling point.