Unit 4 Section A.15-A.16

1. Unit 4Section A.15-A.16 In which you will learn about: Ideal gases Real gases

2. 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

3. 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).

4. 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.

5. 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.

6. 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.