Lecture 9/15/14

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khaengel
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291329
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Lecture 9/15/14
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2014-12-11 15:30:35
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BME 221
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BME 221
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  1. Two assumptions for ideal gas:
    • The gas molecules possess negligible volume.
    • The gas molecules do not interact with each other.
  2. When do ideal assumptions work well for non-ideal gases (all gases are non-ideal)?
    • Relatively low pressure (≤ 10 atm)
    • Relatively high temperature (Room temperature is high enough)
  3. Two assumptions for real gas: What does this mean for pressure, not considering attractive forces?
    • Molecules of real gases possess volume and interact with each other.
    • Gases have less free space to move due to the presence of the volume of the gas molecules, which increases the pressure.
  4. Explain how pressure decreases for real gases
    • Under attractive forces, the gas molecules are slowed down (think at equivalent energy levels,some energy is stored as potential energy). This reduces the number of collisions and the momentum transfer during these collisions between gas molecules, as well as the collision between gas molecules and the container wall. Since pressure is nothing but a measure of the collisions between the gas molecules and the container wall, the pressure decreases.
  5. What is the significance of the compressibility factor Z? What is Z for an ideal gas or a gas that is easier to compress than the ideal gas? Explain with an equation.
    • Another measure of how ideal the gas behaves.
    • Ideal gases have a compressibility factor of 1, since for ideal gases, P V = nRT.
    • If Z < 1, the gas is easier to compress than ideal gas and vice versa.
  6. Name 3 consequences of interactions between gases (non-ideal behavior), explain.
    • Decrease in pressure, attractive forces due to interactions slow the gas molecules down before hitting the container walls. This reduced speed reduces both the transferred momentum from each collision and the frequency of collisions (pressure is a consequence of collisions of gas molecules with the wall of the container).
    • Some gases are easier or harder to compress than ideal gases.
    • Condensation, seeing a liquid phase is a consequence of the finite volume of molecules.
  7. Describe supercritical fluids and name one practical use for it
    • Single phase
    • T>Tcritical
    • Exhibit properties of both fluid and gas
    • Can not be condensated
    • It is useful in extraction: supercritical CO2is used to extract caffeine from coffee beans to make decaffeinated coffee
  8. Explain the process of phase transformation for a gas that condensates at a low temperature
    • At the low temperature, as the volume is decreased, initially the pressure increases, but then the pressure remains constant although the volume is further decreasing.
    • At this state, some gas condenses and forms liquid in the container.
    • During this condensation process, the pressure remains constant until all gas is condensed.
    • Then, further reduction in volume will quickly increase pressure due to the "incompressibility" of liquids
  9. what are the 5 assumptions associated with the kinetic theory of gases?
    • Gases are made up by a large number of molecules, which are separated by a large distance from each other.
    • The gas particles are in constant random motion.
    • All collisions are elastic (no transformation from kinetic energy to other forms of energy).
    • Ideal gas: the molecules have mass but negligible volume
    • Ideal gas: no interactions between gas particles.
  10. What to picture/consider when understanding pressure for ideal gases..
  11. Describe a reversible process for gas expansion
    • Consider a process that the gas expands with an external pressure infinitesimally smaller than the internal pressure. The gas expands in infinitesimal steps until it reaches the final volume.
    • We can consider the external pressure to be equal to the internal pressure in this scenario. The system can be considered at equilibrium at any time point.
  12. Which one does more work: reversible or irreversible gas expansion
    Reversible expansion does more work than irreversible expansion. Reversible processes do the maximum work with the same initial and final states.
  13. Is work path independent or dependent?
    Work is a path function. Even with the same initial and final states, youmay still find that the work done is different.
  14. How to approximate a reversible expansion
    • A reversible expansion can be approximated by many irreversible expansions
  15. What is work a measurement of?
    Work is a measure of energy transfer
  16. Define heat
    Heat is the transfer of energy between two bodies with different temperature
  17. Where do work and heat occur?
    Work and heat only appear at the boundary of the systems.
  18. Is heat path dependent or independent?
    Heat is path dependent

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