Lecture 10/29/13

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Lecture 10/29/13
2014-12-06 18:19:08
BME 221
BME 221
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  1. Draw an example of a 1 component system where 2 phases are in equilibrium and explain the molecular phase dynamic.
    • Dynamic: some water turns into ice and some ice turns into water simultaneously. Delta G = 0. T=0c and P = 1 atm
  2. Describe the result of temperature increasing in a 2 phase system of water (1 component), if initially at 0c and 1 atm (phase equilibrium)
    • When temp increases, so does the Gibbs energy of both phases.
    • Because there is more entropy in the liquid than there is in the solid, if we increase the temp, the Gibbs energy comes down more for the liquid -> driving force for melting
    • The molar entropy is larger for the liquid now -> to achieve equilibrium again the ice would need to also lower it's entropy, causing it to melt spontaneously.
    • the vice-versa goes for lowering the temperature of the system --> lower temp of water and it will turn into ice.
  3. What is the sequence of increase in molar entropy of a substance, in terms of phases.
  4. What does entropy determine about which phase is occurring?
    • Entropy determines which phase is stable at a temperature.
    • Thermodynamically, the most stable form of a substance has the lowest molar Gibbs free energy.
    • At a higher temp, liquid has lower Gibbs energy than the solid phase.
    • At a lower temp, solid has a lower Gibbs energy than the liquid phase.
  5. what defines the most thermodynamically favorable phase?
    • The phase with the lower Gibbs energy.
    • Note: even if there is the option to lower G, you have no idea how long it will take for it to occur.
  6. When would the most stable phase be the vapor phase?
    When would the most stable phase be the solid?
    • At high temps (T>Tb), the most stable phase is the vapor (or gas).
    • At low temps (T<Tf), the most stable phase is the solid.
  7. How is Gibbs energy related to increases in pressure (for solid, liquid, and vapor phases)?
    Is there any difference in Gibbs energy between any of the phases during the increase in pressure? Explain.
    • Because volume and molar volume are always positive, the Gibbs energy always increases with pressure (eqn. dG=VmolardP).
    • This increase is greatest for Vapor gases, because their Vmolar is larger than the molar volume of condensed phases.
  8. how does the melting point and boiling point change with pressure?
    Increase the pressure and the melting and boiling points increase also.
  9. Rank the molar volumes by phases for regular compounds and compare it to that for water.
    • usually: Vs<Vl<<Vg (molar volumes)
    • water: Vl<Vs<<Vg (molar volumes)
  10. How can you explain why when you increase pressure of water, it becomes harder to freeze it and why ice floats?
    • Draw graph for G vs. T for two different pressures.
    • Assumption: molar Vsolid > molar Vliquid --> the density is smaller for ice that that of water.
  11. Freeze drying vs. other methods of protein purification.
    • Freeze drying: for purifying proteins/heat unstable molecules/foam-forming molecules, freeze drying could be used. Freeze-dried proteins are stable can be stored for a long time. Freeze-drying is basically a two-step process: a shock freeze followed by a large pressure drop.
    • Shock freeze: if you freeze water slowly, the formation of ice crystals can damage the proteins. This is proved by experimental data of thawed frozen protein. Therefore, shock-freezing is preferred.
  12. Define solution:
    Solution is a homogeneous mixture of two or more components that form a single phase.
  13. Why is molality preferred as a concentration unit over molarity?
    The reason that molality is preferred in physical chemistry is that the molality of a solution is independent of temperature. However, in actual practice, molarity and molality is generally similar for aqueous solutions.
  14. question: What is the approx. molar volume of water at 25C at 1atm?
  15. Explain why when you mix 50ml of water and 50 ml of ethanol together that the result is not 100ml, it is less than 100ml
    The main reason is that the intermolecular forces between water and ethanol molecules are greater than each of the molecules within themselves.