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Intermolecular Forces
Chapter 12
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
The Kinetic Molecular Theory The Kinetic Molecular Theory explains the forces between molecules and
the energy that they possess. This theory has 3 basic assumptions.
Matter is composed of small particles (molecules). The measure of space that the molecules occupy (volume) is derived from the space in between the molecules and not the space the molecules contain themselves.
The molecules are in constant motion. This motion is different for the 3 states of matter.
Gas - The kinetic energy of the molecule is greater than the attractive force between them, thus they are much farther apart and move freely of each other. Lack strong forces of attraction between molecules.
Liquid - Molecules will flow or glide over one another, but stay toward the bottom of the container. Motion is a bit more random than that of a solid. Significant forces of attraction.
Solid - Molecules are held close to each other by their attractions of charge. They will bend and/or vibrate, but will stay in close proximity. Vibrate around fixed position. Significant forces of attraction.
When the molecules collide with each other, or with the walls of a container, there is no loss of energy.
Gas Liquid Solid
From: http://www.psinvention.com/kinetic.htm
A phase is a homogeneous part of the system in contact with other parts of the system but separated from them by a well-defined boundary.
2 Phases
Solid phase - ice
Liquid phase - water
12.1
Observable Properties
Takes shape and volume of its container
Low density
Very compressible
Takes shape of its container, has definite volume
High density
Difficult to compress
Has definite shape and volume
Density slightly higher than for liquid
Highly incompressible
Gas Liquid Solid
Intermolecular Forces Intramolecular Forces
Forces of attraction between molecules
Much weaker than intramolecular forces
Boiling points and melting points of a substance generally increase with the strength of intermolecular forces
Hold atoms together in a molecule
Intermolecular Forces
12.2
Intermolecular vs Intramolecular
• 41 kJ to vaporize 1 mole of water (inter)
• 930 kJ to break all O-H bonds in 1 mole of water (intra)
Generally, intermolecular forces are much weaker than intramolecular forces.
“Measure” of intermolecular force
boiling point
melting point
Hvap
Hfus
Hsub
Types of Intermolecular Forces
Dipole-dipole
Ion-dipole
Dipole-induced dipole
Ion-induced dipole
Dispersion forces (London forces) (Induced dipole-induced dipole)
Intermolecular Forces
Dipole-Dipole Forces - Attractive forces between polar molecules (molecules with dipole moments)
Orientation of Polar Molecules in a Solid
12.2
• The larger the dipole moment greater the force
Intermolecular Forces
Ion-Dipole Forces - Attractive forces between an ion and a polar molecule
12.2
Ion-Dipole Interaction
Strength of interaction is dependent on:
• charge and size of the ion • magnitude of dipole moment • size of molecule
12.2
Intermolecular ForcesDispersion Forces (London forces) (Induced dipole-induced dipole) - Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules
12.2
ion-induced dipole interaction - Attractive interaction between an ion and induced dipole
dipole-induced dipole interaction - Attractive interaction between a polar molecule and induced dipole
• Attractive interaction in non-polar substances
• Also present in polar molecules
Induced dipole: dipole in a non-polar molecule or atom
• Caused by separation of + and – charges in an atom or non-polar molecule due to proximity of an ion or a polar molecule
What affects if dipole moment is induced?• Charge on ion• Strength of dipole• Polarizability of atom or molecule
Induced Dipoles Interacting With Each Other
12.2
Only type of intermolecular forces present in non-polar molecules are dispersion forces
All molecules (both polar and non-polar) have dispersion forces
**However, while polar molecules may have other types of intermolecular forces present, non-polar molecules have ONLY dispersion forces
Polarizability – ability to distort electron cloud (change shape of electron cloud)
The more electrons in a molecule the more polarizable it is (easier to distort electron cloud) larger molar mass stronger dispersion forces
The more electrons in a molecule the higher the melting point
SO
O
What type(s) of intermolecular forces exist between each of the following molecules?
HBrHBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules.
CH4
CH4 is nonpolar: dispersion forces.
SO2SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules.
12.2
Types of Intermolecular Forces
Dipole-dipole Hydrogen bonding *** (see next slide)
Strong type of dipole-dipole interaction
Dipole-induced dipole
Dispersion forces (Induced dipole-induced
dipole)
These types of intermolecular forces are referred to as van der Waals forces
Other types of Intermolecular Forces that are NOT van der Waals forces
Ion-dipole
Hydrogen bonding Placed in a separate category since only
certain elements can take part in formation of hydrogen bonding
Hydrogen bonding
Special type of dipole-dipole interaction between hydrogen atom in a polar bond (N-H, O-H, or F-H) and an electronegative O, N, or F atom
Strong type of intermolecular attraction
However, hydrogen bond is still much weaker than covalent (intramolecular) bonds
12.2
A H … B A H … Aor
A & B are N, O, or F
Hydrogen bonding
Hydrogen Bond
12.2
Boiling points of NH3, H2O, HF are much higher than expected Why? due to hydrogen bonds
Boiling points
Usually increase as molar mass increases, for similar compounds composed of elements in the same group on periodic table Why?
More electrons in a molecule increase in dispersion forces increase in boiling point
Figure 12.6 in textbook (see next slide)
Hydrogen compounds of Group 4A (increase molar mass increase boiling point) CH4 (lightest compound) lowest boiling point SnH4 (heaviest compound) highest boiling point
Hydrogen compounds of Groups 5A, 6A, 7A (NH3, H2O, HF) do NOT follow this trend In these groups, the lightest compound heaviest
boiling point this is due to hydrogen bonding (NH3, H2O, HF have
stronger intermolecular attractions (hydrogen bonding) than other molecules in the same groups)
Decreasing molar massDecreasing boiling point
12.2