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Some difficult questions

  1. Dec 4, 2004 #1
    1. Explain why you would expect a BeF2 molecule to be linear but an SF2 molecule to be angular?

    2a) How would you designate the hybrid orbitals formed by "mixing" one d, one s and two p orbitals?

    b) How many of the hybrid orbitals would be formed by the mixing?

    3. Explain why C20H40 is a solid at 25degcel, while C4H8 is a gas at 25degcel?

    Thanks
     
  2. jcsd
  3. Dec 5, 2004 #2

    chem_tr

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    Show your work first. You will find that there is an important difference between [tex]BeF_2[/itex] and [itex]SF_2[/itex], look for a non-bonding electron pair on central atom if there is a deviation from ideal geometry (here, linear geometry is the ideal one), as required by VSEPR theory.

    [itex]dsp^2[/itex] hybrid orbitals are found most commonly in square-planar systems like ethylene, benzene, or metal compounds like [itex]Na_2[PtCl_4][/itex]. Look in carbon's and platinum's electronic configuration for a change with added atoms.

    I have not understood 2b, sorry.

    This is an easy question, you will find the solution by some more brain-storming, I am sure. Look for a principal difference and the reason is just this.
     
  4. Dec 5, 2004 #3

    Gokul43201

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    BH2O : As chem_tr has pointed out, you have to make an attempt at each question before we can help you.
     
  5. Dec 5, 2004 #4
    The last question is interesting. could someone give me a hint? Could the weight of the molecule of C20H40 be the reason it's a solid or should i look at the problem in terms of bonds formed?

    c20h40 and c4h8 are both cyclic compounds from what i drew.
     
  6. Dec 5, 2004 #5

    Gokul43201

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    They could also both be alkenes (but I doubt that's very important). The weight of the molecule is related to the reason, but I'd like to allow BH2O to respond before giving away anything more.
     
  7. Dec 11, 2004 #6
    Ok, well....

    1) For BeF2, Be has 4 electrons so it doesn't have any lone pairs left after forming bonds with F. (which would make it linear) While SF2 has lone pairs around its, so the electrons want to stay away from each other. (making it angular) ???

    2. a and b) I really dont have an idea. I dont get even what the question asks.

    3. I would think the mass is obviously important since its the same elements but forming different compounds..suggesting that the mass is the key. However, My simple explanation would be that C20H40 is heavier compound, therefore, it would be harder to break down. I dont have a great explanation though..or if thats the only factor.

    Thanks
     
  8. Dec 11, 2004 #7

    Gokul43201

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    1) This is a little more complex...but I'm not sure to what level you are expected to answer this. You are partially there.

    BeF2 is more likely to be ionic (draw the Lewis Dot Structure to see why) with 2 single bonds. The farthest that two bonding electrons can get from each other is 180 deg apart, making BeF2 linear.
    S has 6 valence electrons which comprise two pairs and two unpaired electrons. These unpaired electrons form covalent bonds with each F atom (again draw the Dot Structures), resulting in a total of 4 electron pairs around the S atom. The farthest that 4 electron pairs can get from each other is when they are directed towards the corners of a tetrahedron, or about 109 degrees apart.

    This explanation gets a lot more evident if you just draw the dot structures.

    2) Surely your text must tell you how to desgnate hybridized orbitals...it's very simple. In this case, you would have : [itex]dsp^2 [/itex]. The total number of orbitals you get is qual to the number of orbitals you put in, ie : 4.

    3) There are multiple reasons for this. But you want to think about what boiling and melting involve. Thing about intermolecular forces. There is also a second order effect coming from the mass (involved in the BP), but I'm not sure that you are expected to consider this too.
     
  9. Dec 12, 2004 #8
    Thanks Gokul, as always.

    This leads me to solubility and melting point.

    For example, just to take some compunds..lets say NaF, NaCl and Nai. Why would it have increasing solubility in water? (from NaF to Nai) and decreasing melting point? (in that same order)

    Would you use the force of attraction (and the Inverse Square Law) to find out?

    Does this tell you in words that the NaF substance since it takes more for it to melt is harder to seperate the ions?

    So, wondering, how would you explain the increasing solubility when it comes to those compounds (in that order) and also the decreasing solubility (in the same order)?
     
  10. Dec 12, 2004 #9
    oh, and another question for you (or anyone).

    If you have a elements melting, boliling and molar volume, you can decipher if its a metal, gas, liquid, non-metal solid and I think even conduction??

    If an element has both negative melting and boiling points (-189degcel and -186degcel) but a 22.4molar volume(L)..at first I thought it must be a gas right? but since its 22.4molar volume..I dont know. Doesn't that mean that its atomic mass is large (since you get the molar volume through the elements atomic mass and density? but it also means since we got a positive molar volume that its not very dense..suggesting a gas too?

    also..how would you know which elements are likley conduct well in the solid form from that data?

    Thanks..if you can clarify that for me.
     
    Last edited: Dec 12, 2004
  11. Dec 12, 2004 #10

    Gokul43201

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    What is the volume occupied by 1 mole of any gas at STP.

    Use the Gas Law : PV = nRT, and plug in P = 1 atm, n = 1 mole, R = 0.0821 lit/K-mol and T = 273 K. V = ?


    Is it possible for something to have a negative molar volume ? In fact, is it possible to have a negative volume, at all ?

    The melting point can sometimes give you a rough idea of metallic nature. Typically non-metals have low melting points with the exception of Si & C. Hg is the only transition-metal with a negative (C) MP; all other transition-metals have pretty high MPs.

    Conductivity can also be crudely guessed. This is only a hand waving argument, so don't expect it to be rigorous. A more rigorous explanation would be beyond the scope of high school or early college. Conductivity comes from free (conduction) electrons. The more free the electrons, the greater the conductivity. Interatomic bonding comes from valence electrons. The more tightly bound the electrons are, the stronger will be the bonding and hence, the higher will be the MP. So , the higher MP transition-metals will be poorer conductors.

    Here's a very rough guideline that many solids obey :
    MP < 1000K => non-metal; 1200K < MP < 1500K => highly conductive transition-metal; MP > 1500K => poorly conducting transition-metal.
    Notable exceptions are C, Si, Hg and alkali/alkaline earth metals.
     
  12. Dec 13, 2004 #11
    Alright, its 24.4l/mol. Since the volume occupied by 1mol of any gas at STP is 24.4.

    And yeah, apparently is it possible to have a - molar volume (since I see them here in the book)..so, what does that mean? how can it be a negative? does that mean it solid?
     
    Last edited: Dec 13, 2004
  13. Dec 14, 2004 #12
    it must be a mistake with the negative volume..how about a very small molar volume..what does that tell us?
     
  14. Dec 14, 2004 #13

    Gokul43201

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    1) What is the definition of molar volume ?
    2) What is a mole ?
    3) From the above two, can you relate the molar volume to some characteristic property of an element. (Hint : think of the reciprocal)
     
  15. Dec 18, 2004 #14
    ok, thanks.

    I dont know if you answered this question for me directly:

    I always thought that it would make sense that if something is more soluble, it would have a lower melting point for that reason too. (since it breaks down easier)

    but what is it that I have to look at here for this question?

    Thanks
     
    Last edited: Dec 19, 2004
  16. Dec 19, 2004 #15

    Gokul43201

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    It's not possible to correlate the two as you just did. Solvation and melting are two different processes.

    Solvation involves ionization and hydration while the melting point is related to the lattice energy. While it may be possible to draw some crude conclusions from percentage ionic character, that won't always work.

    Look at this.
     
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