- Physical Chemistry I
Spring 2013 Unique 52575
Lecture Summary, 18 January 2013
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.
When comparing a real gas to an ideal gas, a very helpful tool is compressibility, Z = VmP/RT. If Z = 1, the gas is behaving identically to an ideal gas. If Z > 1, the gas is dominated by repulsive forces, and if Z < 1, the gas is dominated by attractive forces.