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Thread: covalence degree

  1. #1 covalence degree 
    Forum Freshman almirza's Avatar
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    We know that ionic substances have very high melting and boiling temperatures due to their strong ionic bonds and also their large sizes

    I read that it also depends on the amount of ionic character of the bonding...
    I understood that it is about the polarising and covalence dgree but what i want to ask is..
    Does this mean that substances with high covalence dgree and therefore low ionic degree have lower boiling and melting temperatures ???
    If yes why??


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  3. #2  
    Forum Senior silkworm's Avatar
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    The high melting and bioling points of ionic substances is atributed to their crystaline structures. Instead of having a hydrogen bonds, or any other weak bonds between molecules, you have actual ionic bonds between molecules. These bonds take more energy to break than the intermolecular forces of covalent substances.

    Long story short, phase change is dominated by intermolecular forces. Covalent bonds are strong, but only within each molecule.

    Check out intermolecular forces for my clarity, I'm too sleep deprived right now to give a short and sweet explaination.


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  4. #3  
    Forum Freshman almirza's Avatar
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    Ofcourse it dominated by intermolecular forces

    but here there r only ionic bonds
    My question is that
    How the covalence degree caused by polarisation affect the boiling and melting temperature?
    Does it make the Tb and Tm decreases or what?
    ALMIRZA
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  5. #4  
    Forum Senior silkworm's Avatar
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    Ionic character is generally determined by how electronegative the heteroatom is in the organic molecule (and the degree of unsaturation in the carbons). This helps keep the molecules together due to "electronegativity to burn." It's not that the structure of the solid is similar to that of a crystalline solid, it's just there's more pull on each molecule by each molecule.
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  6. #5  
    Forum Freshman almirza's Avatar
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    As an example:

    MgCl2 has more covalence degree than CaCl2
    because Mg is smaller than Ca so the attraction to the outer shell electrons of Cl will be more( i.e the distortion of the electrons in Cl will be more)
    okey dear?????

    So now which one has the higher boiling and meltin temperature , MgCl2 or CaCl2 ???????
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  7. #6  
    Forum Senior silkworm's Avatar
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    I'm pretty sure it's calcium chloride. I can't think of an exception that would change that.

    Is that okay honey?
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  8. #7  
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  9. #8  
    Forum Freshman almirza's Avatar
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    Sorry if i am asking a lot
    But why?
    Is it because ionic bonding is stronger than covalent????
    ALMIRZA
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  10. #9  
    Forum Senior silkworm's Avatar
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    I has to do with how the molecules interact when their together. In the words of the wikkipedia article:

    "Ionic compounds in the solid state form a continuous ionic lattice structure in an ionic crystal."

    "In an ionic bond, the atoms are bound by attraction of opposite ions, whereas, in a covalent bond, atoms are bound by sharing electrons. In covalent bonding, the molecular geometry around each atom is determined by VSEPR rules, whereas, in ionic materials, the geometry follows maximum packing rules."

    Melting and boiling do not break ionic or covalent ponds. When something melts, what hold its solid structure is no broken by energy. When something vaporizes, a substance is broken into invidual molecules.

    Both are dominated by intermolecular forces. Covalently bonded materials are subject to Hydrogen bonding, Van deer Waals, and dipole-dipole interactions. In ionic materials, intermolecular forces are more ionic, which is a stronger attraction. Ionic attactions are 10 times greater than hydrogen bonding, the second strongest intermolecular force.

    Also, when concerning the mp or bp of a substance, the size of the atoms of the molecule must also be considered. The heavier the atoms of the molecule, the higher the bp and mp.
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