Unit 4 Section A.15-A.16

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Unit 4Section A.15-A.16

• In which you will learn about
• Ideal gases
• Real gases

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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

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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).

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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.

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Homework Questions

• 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
• 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?
• 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.

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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.