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Melting Points

  1. Apr 30, 2008 #1
    If you have a question such as "Which substance has the highest melting point?" & say these are the substances CH4, HF, He, CL2, how would you determine an answer? I'm not really looking for an answer to this particular question, but in general, there must be something I've forgotten in my chemistry classes... any links or just an explanation of the concept would be very helpful.
     
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  3. May 1, 2008 #2

    Borek

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    What is responsible for melting and boiling points? What do they depend on?
     
  4. May 1, 2008 #3
    I'm not trying to be rude, but I thought that was what I was asking. Its ok though, I'll just run a few google searches & hopefully something will pop up. Again, I'm not looking for an answer to the "specific" question asked, rather general information in the subject.
     
  5. May 1, 2008 #4
    Ionic compounds melt at higher temperatures, because it takes more energy to break the crystal lattice structures they form.

    In general, the stronger and more polar the bonding for the molecule, the higher the melting point.
     
  6. May 1, 2008 #5

    jambaugh

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    That doesn't sound right. Covaliant bonds are usually stronger than Ionic.
     
  7. May 1, 2008 #6

    Borek

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    No, you were asking which substance has the highest metling point, and I am trying to push you in the right direction :smile:

    Why do molecules in crystal keep together? Why do molecules in liquid keep together?


     
  8. May 1, 2008 #7
    Nope. Ionic bonds are stronger than covalent bonds. [​IMG]
     
  9. May 1, 2008 #8

    GCT

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    Depends on the context - covalent bonds are superior in aqueous solutions for example - be sure to remember this when taking a biology class.

    Kuahji you need to consider the different types of intermolecular attractions for each molecule - please list them for each of the molecules.
     
  10. May 1, 2008 #9
    After doing a good bit a research, this appears to be correct in most cases. Thank you everyone who took the time to reply. ^_^
     
  11. May 1, 2008 #10

    jambaugh

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    Well, I was thinking of say silicon dioxide vs. say sodium silicate. In particular the covalant bond between carbons in diamond. And I seem to remember hearing the opposite of what you say many years ago when I took chemistry.

    Let me ask you to name some of the weaker covalent bonds...I'm drawing a blank.

    EDIT:[Well now that I think on it, metals bond covalently...and you have the full range of strengths from tungsten to mercury.
    But it doesn't sit quite right in my mind to say ionic bonds are stronger in general. The hardest substances with the highest melting points are covalently bonded crystals.... Diamond, Sapphire, Silicon-Carbide, etc]
     
    Last edited: May 1, 2008
  12. May 1, 2008 #11

    GCT

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    Ask yourself this question - is there such a thing as an ionic molecule ? The original question pertains to intermolecular attractions between covalent molecules.

    Most of you are discussing ionic compounds in a lattice and diamond itself is a network of intramolecularly bonded carbon. Both are products of intramolecular attractions and bonding.

    That is be certain to note the difference between intermolecular and intramolecular phenomena. Ionic lattices would be considered an interspecies phenomena - the cation and anion species all possess equal attractions based on the charges and the distance between the charges (with all due respect to the quantum physics perspective) unlike CH4 for example which has intramolecular covalent bonds between Carbon and Oxygen to make up the Methane molecule and intermolecular Van Der Waals attractions between the Methane molecules.
     
    Last edited: May 1, 2008
  13. May 1, 2008 #12

    jambaugh

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    GTC,
    I see by what you are saying that I am making a less than ideal comparison between crystiline salts and covalent crystals such as gems. Even when you consider regular lattices of e.g. metal ions and hallide ions the neighborhood weakens a specific ionic bond to some extent. Is that correct?

    Anyway, I've been commenting outside the scope of the OP and should cease... thusly.
     
  14. May 1, 2008 #13

    GCT

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    Yeah we're not really answering the OP's question however the subject that you bring up relates to it somewhat. Ionic crystals are considered to be neutral so there is no "specific ionic bond" within such crystals surrounded by higher or lower charged areas for instance. We're considering electrostatic attractions versus intramolecular covalent bonds - at the moment I'm pretty certain its the covalent crystals that would have higher melting points.
     
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