Physical Chemistry Question - non-mcat-related.. just need hlep

Discussion in 'Pre-Medical - MD' started by kreno, Sep 29, 2002.

  1. kreno

    kreno Candy Man

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    Okay, so i'm taking physical chemistry but missed a couple classes 'cuz of interviews... and now I am having trouble with what seems should be a simple concept. Thing is, my book doesn't explain it, so I came here.

    Stupid thing... degree of freedom or something. It's like an equestion F(degrees of freedom) = c - p +2 or something like that. Where c stands for something, and i guess p for Phases or something maybe?

    I'm really not an idiot! but if you could help me out, and explain from scratch i'd apprecaite it. thanks
     
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  3. FROM MY TEXBOOK, F = 2 - C + N
    c is the number of phases
    n is the number of chemical species

    hope this helps
     
  4. Chemguync

    Chemguync Senior Member

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    Problem is, there are different types of degrees of freedom. We have degrees of freedom for ways a polyatomic can move. (translational, rotational, vibrational). like nonlinear polyatomics have 3n-5 degrees of freedom, where n = number atoms (H2O --> n=3), and linear polyatomics have 3n-6.. you can prove this to yourself, I think our professor made us prove it.. but I wouldn't recommend it.. I think there are other types, but I left p-chem behind last spring and vowed not to retain any more than I had to. Good luck, and enjoy those nice long study weekends.
    -Chem
     
  5. kreno

    kreno Candy Man

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    Thanks, but what i'm talking about is

    f = c - p +2 where c is the "number of compondents" and p is the "number of phases." This is for like phase diagram stuff. So... can someone explain to me how you determine C and P and once you do, you get "f" but what does that TELL you (obviously degree of freedom, but so what?
     
  6. CD

    CD Senior Member

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    The degrees of freedom tell tell you the number of intensive variables that must be specified to describe the state of the system completely. For example, in a one component system F=c - p + 2 so F=1 - p +2 for F=3-p. Let's say our component is H20. That means that if we only have water we are in the liquid portion of the phase diagram and our degrees of freedom are F=3-1 or F=2. Therefore two intensive variables must be specified to describe that system. ...or said another way....Temperature and Pressure have to both be designated in order to describe EXACTLY where in the liquid phase we are. If however, you have steam and water present you are in another place on the phase diagram (actually along the gas/water interface line) and F=1 - 2 +2 for F=1 . This means that you only have to specify one intensive variable. Looking at the diagram it becomes clearer because on the gas/liquid interface line if you specify temperature, then the pressure is known. Okay, now if you have gas/liquid/solid all existing at the same time you have three phases and F=1 - 3 +2 or F=0. This is because there is only one place on the diagram where all three coexist...the triple point. If all three exist, the pressure, temperature are all known. You don't have to specify anything. I hope this makes sense!
     
  7. CD

    CD Senior Member

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    Ooops I just realized I didn't answer the first part of your question. C stands for the number of components. That means the number of specific species present. For example. H20 is a component and ethanol would be another component. THe component is independent of the phase that it is in so if you have water steam and ice that is just ONE component. However, if you have H2O, and Ethanol present, you have TWO components.

    P is the number of phases present. So if you have gas only you have 1 phase. If you have liquid only, you have one phase. But if you have liquid AND gas you have TWO phases. Let's suppose you have H2O and Ethanol both as a liquid. In that case you have ONE phase. You just want to specify how many phases are present in the ENTIRE system you are dealing with.

    Wow....I surprised myself....can't believe I remembered all this stuff!
     
  8. poloace

    poloace Senior Member

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    wow... its amazing how fast i've forgotten something i used to be really solid at.
     

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