Lecture 9/22/14

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Lecture 9/22/14
2014-12-11 12:31:15
BME 221

BME 221
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  1. Name two examples that demonstrate that there are always some molecules with very high speed/energy, as depicted with a Maxwell's distribution graph (# molecules vs. molecular speed)
    • Water evaporating at room temperature
    • Paper combusts at room temperature (very slow reaction)
  2. Explain what happens to the energy/speed distribution at different temperatures and why this is so. Note what the Maxwell distribution would look like for at higher temperatures
    • As temperature increases, a greater portion of the molecules will gain higher molecular speed. This is because temperature is proportional to kinetic energy and also to velocity squared.
    • At higher temperatures, the will be a bigger spread in the distribution, as well as more symmetry.
  3. Explain what happens to the energy/speed distribution at different molar masses and why this is so. Note what the Maxwell distribution would look like for at a larger molar mass.
    • As molar mass increases, the distribution range of molecular speed decreases. This is because molar mass is inversely proportional to velocity squared.
    • The larger the molar mass, the smaller the spread of distribution of molecular speeds.
  4. Describe the situation (in terms of molecular speed of the water) in which you have a pot of boiling water, you take off the lid for one second and then place it back on the pot
    Initially, there was steam contained inside the closed pot. Once the lid was removed, the steam escaped. This causes a reduction in the number of molecules in the system as well as a loss in kinetic energy. Placing the lid back on, the water will re-equilibriate. This will cause the average molecular speed of the system to decrease. Also consider that when the energy is lost from escaping steam, the temperature goes down for the system as well, since energy is proportional to temperature.
  5. Describe what would happen to the speed distribution (Maxwell's distribution) if a high speed particle were to be shot into a system of particles.
    • The overall kinetic energy would increase and the distribution would the reform to reach a new equilibrium.
    • Also consider that the maximum number of particles in the distribution (the highest point in the average of the curve) would decrease due to raising the speed of particles in the system, so that the distribution flattens out a little bit (more molecules now occupying higher molecular speeds, distribution shift to the right and down some).
  6. When has a system reached equilibrium, with respect to the speed distribution?
    When the distribution of speed/energy does not change.
  7. What is the first law of thermodynamics?
    Energy can only be converted from one form to another. It cannot be created or destroyed. Or, the total energy in the universe (an isolated system) is a constant.
  8. Internal energy is a state function, what does that mean?
    It only depends on the initial and final states of a system
  9. Consider irreversible gas expansion: In an isolated system, if adiabatic expansion will occur, how is temperature effected? Name an example of an item that works on these principles.
    • The temperature of the gas will drop since the system does work in the surroundings and hence loses energy in the form of work.
    • Some refrigerators use such cooling methods.
  10. Consider an irreversible gas expansion: what is necessary to achieve an isothermal expansion of gas (what will you need to provide to the system). What would be the additional requirement of this system performing reversible gas expansion? why?
    • To get an isothermal expansion of gas, we will have to provide energy to the system (e.g. by using a water bath).
    • A reversible expansion will require more heatĀ from the reservoir than an irreversible expansion since it does more work