- Physical Chemistry I
Spring 2012, Unique 52135
Lecture Summary, 23 January 2012
Forces: The ideal gas law assumes no interaction
between species. However, intermolecular forces do occur and
change the observed properties and behavior of real gases. The
strength and distance scale over which intermolecular forces operate
is a function of 1/r^n, where r is the distance
between two species and n is an integer. The larger the value
of n, the shorter the length scale at which the intermolecular force
operates. In decreasing order of length scale, some important
intermolecular forces are:
Electrostatic (Coulombic) Interactions: 1/r^2. These are long distance interactions that can be either attractive or repulsive, depending on the permanent charge of the species.
Dipole-Dipole Interactions: 1/r^3. These are shorter length scale interactions that can be either attractive or repulsive depending on the direction of the dipole moments.
van der Waals Interactions: -1/r^6 + 1/r^12. These are very short range interactions that include an attractive term (-1/r^6) and a repulsive term (1/r^12). All materials have van der Waals forces, regardless of chemical identity.
The idea of attractive and repulsive forces is put together in the van der Waals equation, a state function for a real gas:
P(vdW) = (nRT/(V-nb)) - a(n/V)^2
a and b are constants that depend on the identity of the gas and which determine the relative importance of attractive and repulsive interactions. Although there are many other real gas state functions, we will stick with the van der Waals equation for any non-ideal system.