States of Matter
Gas: Properties, Laws, KMT
Liquid: Intermolecular Forces, VP,
Phase Diagrams
Solid: Crystal Structure, Types of
Solids
A Comparison
• Gas: low density (d), compressible, takes shape
and volume of container. Weak intermolecular
forces (IMF).
• Liquid: high d, incompressible, takes shape of
container but has its own volume. Strong IMF.
• Solid: higher d, very incompressible (rigid), takes
its own shape and volume. Stronger IMF.
Figure 10.1 The Schematic
Representations of the Three States
of Matter
Intermolecular Forces
• We will focus on attractive forces between
molecules.
• These forces are much weaker than
chemical bonds that hold atoms together.
• And they are nearly nonexistent between
gas molecules at room temp. and pressure.
• However, they are important in liquids and
solids.
Properties of a Gas
•
•
•
•
State of Matter
Compressible since molecules are far apart.
Takes the shape and volume of container.
Forms homogeneous mixtures with other
gases.
• Pressure is a gas property which tells us
about the amount of gas present.
Gas Laws
• These are empirical laws (based on expts
rather than derived from theory) that define
mathematical relationships between any two
gas properties (P, V, T, n).
• For example: If T and n are held constant,
what happens to V if you increase P?
• V will decreases: Boyle’s Law relates V vs
P: V α 1/P or PV = k at constant n and T.
Figure 5.15 Increased Pressure due
to Decreased Volume
Gas Laws (2)
• If P and n are held constant, what happens
to V if you increase T?
• V will increase: Charles’ Law relates V vs
T (K): V α T or V/T = b at constant n and P
• If P and T are held constant, what happens
to V if n increases?
• V will increase: Avogadro’s Law relates V
vs n: V α n or V/n = a at constant P and T.
Figure 5.17 The Effects of
Increasing the Temperature of a
Sample of Gas at Constant Pressure
Why Does an Egg Crack upon
Boiling? (p 5)
• Charles’ Law: As T increases, V increases.
• So what defines the volume that increases
with T? V = air sac between porous shell
and membrane around egg contents.
• As egg ages, the air sac increases. If T
increases too quickly, the gas trapped in the
sac increases rapidly and cracks the shell.
Figure 5.18 Increased Volume due
to Increased Moles of Gas at
Constant Temperature and Pressure
Ideal Gas Law
PV = nRT
• Combine Boyle, Charles and Avogadro’s Laws
• Equation of state for ideal gas; hypothetical state
• Note universality of equation; I.e. identity of the
gas is not needed.
• Limiting law (in the limit of high T and low P~1
atm); this means that as T increases and P
decreases, real gases start to behave ideally.
Kinetic Molecular Theory of
Gases (1)
• Gas molecules are far apart form each other
and their volumes are
• They move constantly, rapidly and
randomly in all directions and at various
speeds.
• There are no intermolecular forces between
gas molecules except when they collide.
Collisions are elastic.
Figure 5.19 Collisions with Walls
and other Particles Cause Changes in
Movement
Figure 5.20
A Plot of the Relative Number of O2 Molecules
that Have a Given Velocity at STP
Kinetic Molecular Theory of
Gases (2)
• Measured pressure of gas is due to
collisions with walls.
• Collisions are elastic.
• The average kinetic energy of the gas is
proportional to T (K).
• KMT explains measurable properties like P,
T, V, v and the empirical gas laws.
Kinetic Molecular Theory of
Gases (3)
• Average kinetic energy = [(3/2) RT] α T
– KE depends on T only
– i.e. KE does not depend on identity of gas (M)
• Root mean square velocity
– urms = √(3RT/M) where R = 8.314 J/(K-mol)
– As T increases, urms [dec, stays the same, inc]
– As M increases, urms [dec, stays the same, inc]
Figure 5.21
A Plot of the
Relative
Number of
N2 Molecules
that Have a
Given
Velocity at 3
Temperatures