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Math Help - Intermolecular Forces

  1. #1
    Member rtblue's Avatar
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    Intermolecular Forces

    Hey guys, I had a quick chemistry question:

    The melting point of H_{2} is -129 degrees celcius. The melting point of C_3H_8 is -190 degrees celcius. Explain why H_2 has a higher melting point than C_3H_8.


    When i read this question, I was under the impression that C3H8 would have a higher melting point, due to the fact that it has a higher molecular weight than H2, thus increasing the London forces. I would appreciate it if anyone could help me out with this problem.

    Thank You.
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  2. #2
    MHF Contributor Amer's Avatar
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  3. #3
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    Quote Originally Posted by rtblue View Post
    Hey guys, I had a quick chemistry question:

    The melting point of H_{2} is -129 degrees celcius. The melting point of C_3H_8 is -190 degrees celcius. Explain why H_2 has a higher melting point than C_3H_8.


    When i read this question, I was under the impression that C3H8 would have a higher melting point, due to the fact that it has a higher molecular weight than H2, thus increasing the London forces. I would appreciate it if anyone could help me out with this problem.

    Thank You.
    Things have really advanced since my days of taking college chemistry (as London forces weren't taught back then). I suspect the answer lies with the geometry of the differing molecules.
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  4. #4
    MHF Contributor Unknown008's Avatar
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    The mass have nothing to do with the intermolecular bonding, which however is directly associated to melting/boiling points. In non-polar substances (like in your question), the predominating form of intermolecular forces is London forces, and the latter depends on the number of electrons present in each molecule (it's a induced dipole interaction). Therefore, propane (C3H8) has a higher melting point.

    Note that the melting point of hydrogen is -259 C and not -129 C and has boiling point -253 C.
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  5. #5
    Member rtblue's Avatar
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    Well, Unknown008, increasing mass, generally, means that the number of electrons is increasing (I don't see how we can only add protons and neutrons to a compound). That is essentially what I was saying when I referred to mass. I believe there was a typo in the question, since, from your data the melting point of H2 should be -259 degrees Celsius.

    Thanks for pointing that out.
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  6. #6
    MHF Contributor Unknown008's Avatar
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    Quote Originally Posted by rtblue View Post
    Well, Unknown008, increasing mass, generally, means that the number of electrons is increasing (I don't see how we can only add protons and neutrons to a compound). That is essentially what I was saying when I referred to mass. I believe there was a typo in the question, since, from your data the melting point of H2 should be -259 degrees Celsius.

    Thanks for pointing that out.
    Not totally true, since neutrons also affect the mass, while they won't affect the number of electrons. In that sense, you can end up with two substance with identical mass, yet one has more electrons that the other and has hence a greater melting/boiling point.
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