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Thread: How does the structure of ethane give it the properties described: colourless gas at room temperature with a low boiling point (-89)?

  1. #1 How does the structure of ethane give it the properties described: colourless gas at room temperature with a low boiling point (-89)? 
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    In my understanding,

    So ethane is C2H6, forming single covalent bonds. I understand that gasses have a weak vaan der waals attraction but i'm unsure of how to link this to the structure. The larger the structure, the stronger the attraction is, but why? Why is ethane considered non polar? Are the hydrogens delocalised?


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    Bullshit Intolerant PhDemon's Avatar
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    OK, whenever you are asked about melting and boiling points in chemistry the key concept is the intermolecular forces. The strongest of these are hydrogen bonds, these don't exist in ethane as for this type of force you need a hydrogen atom attached to a nitrogen, oxygen or fluorine atom. The second strongest force are dipole-dipole forces, these require a permanent dipole in the molecule, as ethane is symmetrical and there are no strongly electronegative atoms ethane does not have these. The third and weakest intermolecular force is van der Waals forces. Every molecule has these so these are the ones of interest for ethane. The strength of van der Waals forces depend on the number of electrons in the molecule as they are caused by temporary dipoles due to the movement of electrons around the molecule. Ethane is quite a small molecule with no heavy atoms so the number of electrons is low and the van der Waals forces are weak.

    Now we have identified the type of intermolecular force (van der Waals) and its relative strength (quite weak) we can think about the boiling point. If the attractive force between the molecules is weak (as we have established) it will take very little energy to separate the molecules, because of this the kinetic energy the particles have even at low temperatures will be enough to overcome the forces of attraction, leading to a low boiling point.

    Hope this helps...


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    Thanks PhDemon, I understand the whole concept now!
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