Transcript Unit 4 Section A.15-A.16 - Welcome to Westford Academy

UNIT 4
SECTION A.15-A.16
In which you will learn about:
•Ideal
•Real
gases
gases
A.15 NON-IDEAL GAS BEHAVIOR

All gas relationships considered up to now have
related to ideal gases.
An ideal gas is a gas sample that behaves under all
conditions as the kinetic molecular theory predicts
 Most gas behavior approximates that of an ideal gas
 Such gas behavior is satisfactorily explained by the
kinetic molecular theory.


At very high gas pressures or at very low gas
temperatures, real gases do not behave ideally

The gas laws you have considered do not accurately
describe gas behavior under such extreme conditions
REAL GASES

On average, gas molecules move slowly at very low
temperatures




As their average kinetic energy decreases, the weak
intermolecular attractive forces among molecules may
become such a significant factor (when compared to their
relative motions) that the gas condenses to a liquid.
At very high gas pressures, if the temperature is not
too high, gas molecules becomes so close together that
these same weak forces of attraction may also cause
the gas to condense to a liquid.
These extreme temperature and pressure conditions
are well beyond normal values for atmospheric gases.
In short, real gases DO exhibit IMFs, and DO take up
volume. We assume ideal gases do not exhibit IMFs
and do not take up any volume (compared to the
container).
A.16 UNDERSTANDING KINETIC
MOLECULAR THEORY

Another way that matter can be modeled is
through the use of analogies.

An analogy can help you relate certain features of an
abstract idea or theory to a situation that is familiar
to you.
Imagine that you cause several highly elastic,
small “super-bounce balls” to bounce around
inside a box that you steadily shake; this serves
as an analogy for gas molecules randomly
bouncing around inside a sealed container.
 The balls bounce randomly inside the box.

HOMEWORK QUESTIONS
1)
Decide which of these four gas variables - volume,
temperature, pressure, or number of molecules – best
matches each of the following factors, and explain each
choice:
a)
b)
c)
d)
2)
The number of super-bounce balls inside the box
The size of the box
The vigor with which you shake the box
The number and force of collisions with the box walls of the
randomly moving super-bounce balls
How does each of the following changes relate to what you
have learned about gases and KMT?
a)
b)
c)
The vigor of shaking and the number of super-bounce balls
remain the same, but the size of the box is decreased.
The size of the box and the number of super-bounce balls
remains the same, but the shaking becomes more vigorous.
The size of the container and the vigor of shaking are kept
the same, but the number of super-bounce balls is increased.
MORE HOMEWORK QUESTIONS
3) Suggest another situation similar to those in
Question 2 that can serve as an analogy for the
behavior of gases. Explain.
4) All analogies have limitations. For example, the
super-bounce ball analogy fails to represent certain
characteristics of gases. Gas molecules travel at very
high velocities (on the order of 6000 km/h, which
nearly equals 10 000 mph). Suggest two other
characteristics of actual gases that are not properly
represented by this super-bounce ball analogy.
5) Describe your own analogy that might be useful for
modeling gas behavior.
a) Identify features of your analogy that relate to
features of the kinetic molecular theory and T-V-P
relationships for gases.
b) Point out some key limitations of your analogy.