CHM102 Lecture Notes - Lecture 9: Hydrogen Fluoride, Chemical Polarity, Heat Capacity
Intra- and Intermolecular Forces
- Intermolecular forces are forces of attraction between molecules.
- Chemical changes are related to breaking and formation of intramolecular forces
(i.e., covalent bonds).
- Physical changes are governed by intermolecular forces.
Intermolecular Forces
- Non-polar molecules with evenly distributed electron cloud.
- An instantaneous dipole in one molecule induces an instantaneous dipole in the
adjacent molecule.
- Instantaneous dipole-induced dipole forces are present throughout the surface.
- think of a molecules as a big balloon- the electron cloud about the nucleus is
poofy, and can be distorted somewhat
- The larger the atom, the more easily the electron cloud is distorted. This
distortion is called polarizability: an easily distorted electron cloud is very
polarizable.
- The more easily the electron cloud is polarized (dispersed), the stronger the
dispersion forces between molecules. Since we just decided that polarizability
depends upon atomic size, than as molecular weight increases, strength of
dispersion forces increases, and so boiling and melting points increase.
Bond Polarity
- Based on a previous analysis of the density, specific heat capacity, equilibrium
vapour pressure, etc… of water, one can conclude that water has relatively
strong intermolecular forces.
- Experiments have confirmed that water molecules have a dipole moment, a force
that aligns them toward the poles of an electric field.
- Water molecule is an electric dipole, with a positive charge at one end and
negative at the other.
- Two atoms that share a pair of electrons in a covalent bond may have different
ability to attract bonding electrons.
- This ability to attract bonding electrons is called electronegativity.
- Net negative charge resides on the atom with greater electronegativity.
- The greater the difference between the electronegativity of bound atoms, the
greater the charge separation.
Document Summary
Intermolecular forces are forces of attraction between molecules. Chemical changes are related to breaking and formation of intramolecular forces (i. e. , covalent bonds). Physical changes are governed by intermolecular forces. Non-polar molecules with evenly distributed electron cloud. An instantaneous dipole in one molecule induces an instantaneous dipole in the adjacent molecule. Instantaneous dipole-induced dipole forces are present throughout the surface. think of a molecules as a big balloon- the electron cloud about the nucleus is poofy, and can be distorted somewhat. The larger the atom, the more easily the electron cloud is distorted. This distortion is called polarizability: an easily distorted electron cloud is very polarizable. The more easily the electron cloud is polarized (dispersed), the stronger the dispersion forces between molecules. Since we just decided that polarizability depends upon atomic size, than as molecular weight increases, strength of dispersion forces increases, and so boiling and melting points increase.
