Chemistry - A Molecular Science

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:

1. 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.

2. The particles undergo constant, random motion.
   3. There are no attractive

forces between the particles.*

4. 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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