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    What is the difference in structure depending on if the molecule is hydrated or not?

    In particular I'm looking at sodium thiosulphate and iron(III) chloride. Are the water molecules just in between the other molecules? Or does the structure actually change?

    Cheers m'dears.
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    (Original post by SmileyGurl13)
    What is the difference in structure depending on if the molecule is hydrated or not?

    In particular I'm looking at sodium thiosulphate and iron(III) chloride. Are the water molecules just in between the other molecules? Or does the structure actually change?

    Cheers m'dears.
    Sodium thiosulphate has a giant ionic structure...

    ... the water molecules are used to build the lattice and surround the ions where there is room to do so.

    The same is true for hydrated iron(III) chloride
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    (Original post by charco)
    Sodium thiosulphate has a giant ionic structure...

    ... the water molecules are used to build the lattice and surround the ions where there is room to do so.

    The same is true for hydrated iron(III) chloride
    Right so is this right: they both have a giant ionic structure with and without the water. And when it is hydrated the water molecules fill in any gaps?

    thanks for the help!
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    (Original post by SmileyGurl13)
    Right so is this right: they both have a giant ionic structure with and without the water. And when it is hydrated the water molecules fill in any gaps?

    thanks for the help!
    Yes, but beware, anhydrous iron(III) chloride is covalent
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    (Original post by charco)
    Yes, but beware, anhydrous iron(III) chloride is covalent
    What I'm confused
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    (Original post by SmileyGurl13)
    What I'm confused
    Why? some substances are covalent and exist as molecules...

    Well Iron(III) chloride is one of them.
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    (Original post by charco)
    Why? some substances are covalent and exist as molecules...

    Well Iron(III) chloride is one of them.
    But iron is a metal and chlorine is a non-metal so doesn't that make it ionic? And how does is change from covalent to ionic depending on whether it is hydrated?
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    (Original post by SmileyGurl13)
    But iron is a metal and chlorine is a non-metal so doesn't that make it ionic? And how does is change from covalent to ionic depending on whether it is hydrated?
    Like I said, there is potential for confusion here...

    FeCl3 is covalent

    It's an exception to the metal/non-metal rule as is aluminium chloride for the same reasons. That is, the high charge density (high charge, low volume) of the hypothetical 3+ charged metal ion prevents formation of an ionic structure with the highly polarisable chloiride ions.

    In the hydrated form however, the iron ions can form the hexaaqua complex ion stabilising the ionic structure.
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    (Original post by charco)
    Like I said, there is potential for confusion here...

    FeCl3 is covalent

    It's an exception to the metal/non-metal rule as is aluminium chloride for the same reasons. That is, the high charge density (high charge, low volume) of the hypothetical 3+ charged metal ion prevents formation of an ionic structure with the highly polarisable chloiride ions.

    In the hydrated form however, the iron ions can form the hexaaqua complex ion stabilising the ionic structure.
    So six water ligands bond to the iron ions but what happens to the chlorine atoms? Are they still bonded to iron?

    Ta
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    (Original post by SmileyGurl13)
    So six water ligands bond to the iron ions but what happens to the chlorine atoms? Are they still bonded to iron?

    Ta
    In an ionic structure the chloride ions are not 'bonded' to any particles as such. They are the balancing ions in the structure.
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    Life is never too simple. The answer you get in this case depends on how much detail you want to know, and adding water to ionic salts can create a huge range of possible structures.

    FeCl3 in the anhydrous state has 6 chlorines bonded to each iron atom, and each chlorine is shared between two iron centers to give a continuous sheet. Layers of these sheets then stack to form the 3D crystal. The best picture i could find is on wiki: http://en.wikipedia.org/wiki/Indium%28III%29_chloride

    When hydrated, you get a system with [Fe(OH2)4(Cl)2]+ cations, chloride anions and 2 extra water molecules per iron center floating around. [http://jcp.aip.org/resource/1/jcpsa6...Authorized=no] Change the metal ion, the ligands, anions or solvent and the potential for variety is huge!
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    (Original post by charco)
    In an ionic structure the chloride ions are not 'bonded' to any particles as such. They are the balancing ions in the structure.
    Eh? :| so what would it look like drawn out and thiosulphate too?
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    (Original post by SmileyGurl13)
    Eh? :| so what would it look like drawn out and thiosulphate too?
    Read Blocker's post above - pretty much summarises it.

    In ionic structures the lattice is made of ions.

    The ions themselves may be held together by covalent bonds, as in for instance complex ions or ammonium, BUT the actual lattice is a giant structure of electrostatic attractions.

    The possible degrees of complexity make it not a simple question, and one I most certainly cannot just 'draw out'
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    (Original post by charco)
    Read Blocker's post above - pretty much summarises it.

    In ionic structures the lattice is made of ions.

    The ions themselves may be held together by covalent bonds, as in for instance complex ions or ammonium, BUT the actual lattice is a giant structure of electrostatic attractions.

    The possible degrees of complexity make it not a simple question, and one I most certainly cannot just 'draw out'
    Okay well do you think you could draw a dot and cross diagram for me for Anhydrous FeCl3 I'm having a few problems :/

    Thank you
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    You'll strugle to draw it in dot and cross format since each iron has 6 chlorides around it in an octahedral structure. If you draw the octahedron as a solid block, you then find that to get each chlorine bonded between two more irons, you have to draw 3 octahedra sharing edges with a central one, and then each of those is sharing an edge with 3 more etc. Its very difficult to represent in any way.

    In electronic terms, a dot cross diagram isnt really the way to go either. Just remember each iron has lost three electrons, and each chloride has gained one so overall the charge is balanced. Then the negative chloride ions surround the iron in specific sites such that you get the structure described earlier.

    http://en.wikipedia.org/wiki/Octahed...cular_geometry
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    (Original post by Blocker)
    You'll strugle to draw it in dot and cross format since each iron has 6 chlorides around it in an octahedral structure. If you draw the octahedron as a solid block, you then find that to get each chlorine bonded between two more irons, you have to draw 3 octahedra sharing edges with a central one, and then each of those is sharing an edge with 3 more etc. Its very difficult to represent in any way.

    In electronic terms, a dot cross diagram isnt really the way to go either. Just remember each iron has lost three electrons, and each chloride has gained one so overall the charge is balanced. Then the negative chloride ions surround the iron in specific sites such that you get the structure described earlier.

    http://en.wikipedia.org/wiki/Octahed...cular_geometry
    Okay this is way more confusing than I first though lol... So is an fe with 3 Cls coming off is that not a very gd representation? But they are still bonded covalently right? This is the ANHYDROUS form I'm talking about just to check ta
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    Anhydrous

    This is the awkward part. While the molecular formula works out as FeCl3 (as of course your expecting), what you have is a combination of irons bound to 6 chlorides, and chlorides bound to 2 irons. So to start with you might draw one iron in the middle of your page with 6 chlorines around it. But then you would have to add 3 more iron atoms, one for each adjacent pair of chlorines. These are then surrounded with more chlorines, and more irons, so on and so forth until overall you have FeCl3, but made up of [FeCl6] and [ClFe2] units. Think of it like a jigsaw puzzle.

    Hence the iron with 3 chlorines coming of it is not an accurate representation of the structure.

    As to the bonding, it is an awkward mixture of covalent and ionic. To explain it properly requires a bit more detail than the typical a-level syllabus. Essentially you have these small, hard Fe3+ cations, surrounded by fairly large, soft Cl- anions. Hence there is an ionic attraction between the two. But there is also a degree of covalency, with those easily polarizable Cl- ions donating a lone pair towards the metal center, forming a complex. The bonding is not true covalent as the electron pair is much more towards the chlorine than it is the iron, but it is not true ionic either, as it is partly shared between chlorine and iron.

    Since a picture is worth a thousand words:

    True Covalent:
    Fe-----:-----Cl

    True Ionic:
    Fe----------:Cl

    Actual bonding:
    Fe-------:---Cl

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    (Original post by SmileyGurl13)
    Okay this is way more confusing than I first though lol... So is an fe with 3 Cls coming off is that not a very gd representation? But they are still bonded covalently right? This is the ANHYDROUS form I'm talking about just to check ta
    This is perfectly acceptable:

    Anhydrous FeCl3 (for what it's worth...)

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    I found this app useful to know the Lewis and VSEPR shapes of molecules. check it on iTunes.
    http://itunes.apple.com/us/app/molec...1747?ls=1&mt=8
 
 
 
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