This weekend, Daylight Saving ends. I get an hour of
sleep, you get an extra hour to study for the final.
Bizarro By Dan Piraro
Read
sections 5.5, 5.6, 12.3
Practice
PS #5 is posted
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Real gases
Ideal gas law
useful for ordinary conditions.
See table 5.3 from Silberberg
Remember:
Ideal gas law is accurate to ~1.5% for gases under 20
atm AND 50C above their boiling points.
Ideal gas law
PV = nRT
Molar volume at STP
Vm = 22.414 L
STP = standard temperature and pressure (for gases)
0C and 1 atm
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See fig 5.23
PV PVm
Compressibility factor Z = nRT = RT
Essentially, we are plotting the deviation from the
ideal gas law.
Temperature effect
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The closer a gas is to the liquid state the more it will
The most famous equation is the van der Waals
deviate from the ideal gas law.
equation.
an2
(P + V2 )(V – nb) = nRT
Corrections to describe the behavior of gases are
an2
P + V2 adjusting measured P up to account for
called equations rather than laws. These equations
are good for limited ranges but within those ranges
interactions
are better than the ideal gas law.
larger a  stronger attractions
V – nb
Most corrections account for the finite volume of the
adjusting V down to account for the volume
of molecules
molecules and for the attractive forces between the
larger b  larger molecular size
molecules.
a, b are parameters specific and different for each gas
molecule
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See figure 5.25
Compare the pressures predicted for 0.8 L of Cl2
weighing 17.5g at 273.15 K using (a) the ideal gas
equation and (b) the van der Waals equation.
First find
17.5 g
n = 70.9 g mol–1 = 0.247 mol
a) Using ideal gas law
PV = nRT
P=
nRT
V =
0.247 mol 0.08206 L atm mol-1 K-1 273.15K
=
=
0.8 L
= 6.92 atm
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b) Using van der Waals (vdW) equation
atm L2
a = 6.49 mol2
For chlorine,
L
b = 0.0562 mol
an2
(P + V2 )(V – nb) = nRT
 Gases
 Intermolecular forces
 Liquids
 Solids
nRT
an2
P = V – nb – V2
P=
States of Matter
 Changes of state
L atm
0.247 mol 0.08206 mol K 273.15K
L
0.8 L – (0.247 mol 0.0562 mol)
 Intermolecular forces in solutions
–
Unit 2 – bonding within a molecule
2
atm L
6.49 mol2 0.2472 mol2
0.82 L2
When do the interactions between molecules
P = 7.04 atm – 0.62 = 6.42 atm
become significant?
What holds molecules together ?
Doesn’t look like a lot, but the difference of 0.5 atm
from the ideal gas law in this case is about 10 % !
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Ionic attractions
Intermolecular forces

Molecules
 bonding
INTRAmolecular forces
2nd unit
Ion – ion
Ion –dipole
 attraction between different parts of a large molecule
Condensed states 
INTERmolecular forces
Dipole attractions
rather weak forces
Dipole–dipole attractions
Intra = within
inter = between
Hydrogen bonding
London dispersion forces (LDF)
Properties that depend/relate to IMF
All the weak attractions come under the general name
 mp, bp
 solubility
 vapor pressure
Demo
van der Waals forces