Chapter 7 States of Matter and Changes in State
7.2
KINETIC-MOLECULAR THEORY AND THERMAL ENERGY
The behavior of ideal gases* is explained by
kinetic-molecular theory
, which is based on
the following postulates:
- The volume of the molecules is negligible co
mpared to the volume of their container. This
means that, on average, the distances be
tween the molecules of the gas are large
compared to their size.
- The particles undergo constant, random motion.
3. There are no attractive
forces between the particles.*
- The average kinetic energy (energy of motion)
of the particles is proportional only to the
absolute temperature.
The fourth postulate states that the temperatur
e of a system is a measure of the average
kinetic energy (energy of motion) of the molecules in the system, so hotter atoms and molecules have more kinetic energy than do
colder ones. The only motion available to
single atoms is translation,
† so hotter atoms mover faster than do colder atoms. For
example, the average speed of a He atom on a cold day (0
oC = 32
oF) is about 1.3x10
3
m/s, while it is close to 1.4x10
3 m/s on a hot day (32
oC = 90
oF). Thus, a He atom travels
12 football fields per second! However, motion is more complicated for molecules, because they can also rotate, and vibrate (atoms vibrate against their bonds). Thus, hotter molecules move faster, spin faster, and vibrate faster than cooler molecules.
* Most gases do interact at high pressures and/or low temperatures.
The interactions affect their pr
essure and volume to produce
deviations PV = nRT. Such gases are no longer ideal gases. Only non-ideal gases can condense to become liquids.
† Translation is motion in a straight line. Atoms and molecules can both
translate, but molecules can also rota
te and vibrate. We will consider
the different ways molecules can di
stribute their kinetic energy in
more detail in Chapter 9.
The average kinetic energy of the molecules
in a system is often referred to as the
thermal energy
of the system. Thermal energy is th
e average energy, so some molecules
have more energy and are moving faster, while others have less energy and are moving slower. All of the energies related to molecules
or ions that we have encountered to this
point have been potential energies arising from electrostatic interactions. The thermal energy of the molecules is energy that can be
used to overcome these interactions and to
drive chemical reactivity. For example, if
you supply enough thermal energy (get the
molecule hot enough), the motions become so
energetic that bonds break, which is what
happens when you char meat under a broiler. In addition, as we explain later in this chapter, when the thermal energy exceeds the potential energy
that holds the molecules in
a crystal, the crystal melts. The thermal energy
of the molecules also dictates how fast they
react with one another. When the temperature of a reaction is increased, the rate at which it occurs also increases as the molecules collid
e more frequently and with more energy.
Thermal energies of solids and liquids are not easily defined, but
chemists usually
approximate the thermal energy as RT
, where R is the ideal gas law constant. However, in
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