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Zaara

can anyone pls suggest the reason why,CO2 and SiO2 are not isostructural at 300K???

  • one year ago
  • one year ago

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  1. Frostbite
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    When comparing molecules and ions that are isoelectronic, only with the respect to their valance electrons, the expectation of isostructural behaviour may not hold. To take your situation: CO2 and SiO2 at room temperatur and pressure, CO2 is a linear molecule but SiO2 has an extended strcture containing silicon atoms in terahedral environments. An extended solid phase form of CO2 has been made at about 1900 K and 40 gigapascal pressure (If I remember right). This has a quartz-like structure, quartz being a 3-dimensional polymorph of SiO2. In the gas phase however we find SiO2 as linear triatomic molecules. A reason for this can be explained saying the vibrational modes may be thermally excited (in a classical interpretation one expresses this by stating that “the molecules will vibrate faster”), but they oscillate still around the recognizable geometry of the molecule.

    • one year ago
  2. Zaara
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    does geometry of these molecules takes the major part in this case???

    • one year ago
  3. Zaara
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    overall and simply why they do not exist isostructural at this temperature? is it b'coz of the structural change or any other???

    • one year ago
  4. Zaara
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    uni...

    • one year ago
  5. Frostbite
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    kk. Had your lectures about temperature, internal energy, molecular energy and vibrational modes? (perhaps morse potential aswell)

    • one year ago
  6. Zaara
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    nw im going through the lectures of internal energy :)

    • one year ago
  7. Frostbite
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    Alrightie :) Anyway you know that the energy for the distance between atoms looks like this right?|dw:1356793711039:dw|

    • one year ago
  8. Zaara
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    yeah!!!

    • one year ago
  9. Frostbite
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    Well same idea aplies to angles between atoms - beside they are diffrence from atom to atom (in some cases)

    • one year ago
  10. Zaara
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    fine explain more pls....

    • one year ago
  11. Frostbite
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    |dw:1356793835811:dw|

    • one year ago
  12. Frostbite
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    The energy levels of the two molecules are diffrence... CO2 is more stable in its linare form at 300 K (room tempture), while SiO2 is not.

    • one year ago
  13. Frostbite
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    Sorry my drawings suck..

    • one year ago
  14. Zaara
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    is SiO2 having a constant graph?

    • one year ago
  15. Zaara
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    nt abt the drawings it s about the matter we r discussing:)..... i can undestand :) looks gud anyway...

    • one year ago
  16. Frostbite
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    |dw:1356794123184:dw|It would (without being sure) maybe look like this:

    • one year ago
  17. Frostbite
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    Else I can suggest you read about vibrational modes of molecules, I sure belive I will becuase I'm a bit unsure (molecular geometry is not my best :P) :)

    • one year ago
  18. Zaara
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    i hav no idea abt vibrational modes.... suggest any reading materials... if u can :)

    • one year ago
  19. Frostbite
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    What textbook do you use? (might be I have it)

    • one year ago
  20. Zaara
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    open university books...

    • one year ago
  21. Frostbite
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    In that case I see if I can find some lecture notes here from the University of Copenhagen.

    • one year ago
  22. Zaara
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    ya pls....

    • one year ago
  23. Frostbite
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    That are not in Danish :P

    • one year ago
  24. Zaara
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    in English???

    • one year ago
  25. Frostbite
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    What I ment ;)

    • one year ago
  26. Zaara
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    haaa...

    • one year ago
  27. Zaara
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    Bt u explained me some basic ideas... :) thanx alot 4 dat

    • one year ago
  28. Frostbite
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    Well I wish i could explain it better, mostly only work with the equations and then make conseqences from that.. Anyway, I can find some good reading in my open lectures, however else take a look at the wiki: http://en.wikipedia.org/wiki/Molecular_vibration

    • one year ago
  29. Zaara
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    im on wiki nw thanx :):)

    • one year ago
  30. Frostbite
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    No prob, have a good day and if you are good with organic chemistry please see if you can answer my question :P

    • one year ago
  31. Zaara
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    i tried bt couldnt... im jst a beginner:).... if i find anywhere... sure i'll let u knw... :)

    • one year ago
  32. Carl_Pham
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    Carbon is a lot smaller than silicon. Being that small, it can more readily involve the closer-in p valence orbitals to form pi bonds to neighboring oxygen atoms -- which gives you the CO2 molecule. Silicon, on the other hand, has a harder time involving the 3p orbitals in pi bonding to its neighbors, so it generally prefers to stick to sigma bonds. That means to satisfy its valence it forms bonds to four -- not two -- neighboring oxygens, which results in the extended diamondoid structure of silica at 300K. This is one of the illustrations of how the first element in each group often behaves nontrivially different from the others. One of the more significant is that the second row elements, being much smaller, form pi bonds much more readily than the elements in Period 3 and higher. You can see a similar effect in Group 6A: oxygen readily forms the double-bonded O2 molecule and exists as a gas at STP, while sulfur, just below it, instead prefers to form extended rings and chains of single bonds, and exists in the solid state at STP. Same with Group 5A: nitrogen forms the triply bonded N2, while phosphorus exists as the pyramidal P4 and is a solid at STP. No idea what molecular vibrations has to do with anything.

    • one year ago
  33. Zaara
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    do u mean the different types bonding of these two plays the key role???

    • one year ago
  34. Carl_Pham
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    No, I mean the size of the atoms plays an important role in the types of bonding each prefers, and that, in turn, explains why they form two different types of oxides.

    • one year ago
  35. Zaara
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    fine.... size might be factor... i didnt think abt the size...thanx alot :)

    • one year ago
  36. Frostbite
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    Carl-Farm, your way is way more specific, and also giveing a more actural reason, but the reason i used molecular vibrations is just a matter of approach, however your explination is more specific, what I wrote is only what I have experinced and choosed to see as energy that changes the molecule's 3-dimensional geometry.

    • one year ago
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