1. Real Gases: Deviations from Ideal Behavior
All real gases deviate, to
some degree, from
PV = nRT. The deviations
are most pronounced at...
high P and/or low T.
Real gas particles... 1) 2)
DO occupy space
DO experience
attractive forces

2. ideal gas (ideal gas equation):
real gas (van der Waals equation):
correction for
particle attraction
(attraction makes P smaller
than ideal gas law predicts)
correction for
particle volume
(particles w/SOME vol.
exert a bigger P than
those w/zero vol.)
Johannes Diderik
van der Waals
(1837 – 1923)

3. Effect of Particle Volume
ideal gas particles
mass m, speed v, ~NO volume
real gas particles
mass m, speed v, YES volume
shorter mean free path = more collisions/sec.
**GREATER pressure than PV = nRT predicts

4. The constants a and b are unique for each gas.
-- a is large when interparticle
attractions are large
-- b is large for large gas particles
a and b both tend to increase with mass
e.g., both are larger for Xe than for Ne