CHM 11200 Lecture Notes - Lecture 2: Intermolecular Force

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8 Feb 2017
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Lecture #2 1/12/2017
Monday, January 09, 2017
3:54 PM
Chapter 11….Intermolecular Forces
Properties of liquid, solid, and gas are determined by cohesive forces that hold them together
Sometimes VERY strong…other times pretty weak
Intermolecular forces- Sometimes the forces holding atoms/molecules together are not as strong as a
bond but strong enough to cause the atoms and molecules to be attracted to each other
Recall: polar bonds
Polar covalent bonds- uneven distribution of electrons between the atoms in a bond
Dipole moment
Combine effect of 3-D shape with bond polarity to get the dipole moment of the molecule overall
Polar bond polar molecule
In solids and liquids composed of covalent molecules the forces between molecules (intermolecular
forces) much smaller than bonding forces within each molecule (bonds)
Arise from interaction of positive and negative changes
Intermolecular forces- attractive forces between molecules
Intramolecular forces- hold atoms together in a molecule
Less energy (41 kJ) to vaporize 1 mole of water (inter)
More energy (930 kJ) to break all O-H bonds in 1 mole of water (intra)
Intermolecular forces are weaker than intramolecular forces
London dispersion forces
These are weakest of the intermolecular forces but occur between ALL atoms and molecules
Get stronger as the size of the atom increases
Even though electrons repel each other but occasionally are in the same area
These forces are interactions between an instantaneous dipole and an indeed dipole
Present between ANY two molecules
The ONLY forces between nonpolar molecules (and between noble gas atoms)
Increase as size of an atom or molecule increases
Molecular shape and polarizability
More compact
Less easily polarized
Lower boiling point
More elongated
More easily polarized
Higher boiling point
Within groups on PT boiling points of nonpolar substances increase with the size of the molecule or
atom
Dipole-dipole forces
Generally stronger interactions than London dispersion
Occur between molecules that are a permanent dipole
Positive end of one dipole is attracted to the negative end of the other and vice-versa
Only important when molecules are REALLY close together
The stronger the intermolecular interactions the harder it is to break the interactions (takes more
energy)
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