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can someone help me with london despersion force? watch

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    someone please tel me why london despersion force a temporary . and which molecule posses these forces?
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    (Original post by Amankhanhussain)
    someone please tel me why london despersion force a temporary . and which molecule posses these forces?
    This seeems long but if u read through it properly and slowly and read my summary it will make sense


    a london dispersion force is also known as an induced dipole-induced dipole attraction this is because it is formed when two non polar molecules(molecules with no dipoles as there electronegativities(ability to attract an atom)are the same e.g. H2 induce a dipole on each other

    this occurs because electrons move instantaneously with in a molecule in all different directions hence at one point in time the electron distribution is unbalanced causing an instantaneous( temporary for a short moment ) dipole on each side

    this instantaneous dipole will induce a dipole on the neighbouring molecule because- if we take the slighlty negative side for example and imagine it to be closest to the neighbouring molecule- it will induce a dipole on the neighbouring molecule by repelling electrons away creating a slighlty positive charge
    this induced dipole on the neighbouring molecule will in return induce a dipole on the original molecule causing a small weak intermolecular force called london forces

    so as a summary (calling the 2 molecules as m1 and m2)

    movement of electons in m1 causes instantaneous dipole

    this induces a dipole on m2

    m2 then has an induced dipole and the dipole induces a dipole on m1

    m1 and m2 both have dipoles

    dipoles attract each other forming london dispersion forces
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    I've explained it in a longer and more convulted way before, but the easiest way to think of it is that if at any particular point an electron (or group of electrons) isn't spread out evenly over a molecule, then there would definitely be areas of partial charge (both positive and negative). These are temporary because electrons keep moving about all the time.

    It acts in exactly the same way a normal dipole would, except it lasts for an incredibly small fraction of a second and is usually considered the weakest of the intermolecular forces (per atom, can become very strong with extremely big atoms). It is also affected by shapes of molecules.

    All molecules with electrons have London/Dispersion forces, because it's a property of electrons behaving as non-particles - but for the purposes of A Level or GCSE it's only referred to when stronger dipole-dipole or hydrogen bonding is not present.
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    (Original post by youreanutter)
    This seeems long but if u read through it properly and slowly and read my summary it will make sense


    a london dispersion force is also known as an induced dipole-induced dipole attraction this is because it is formed when two non polar molecules(molecules with no dipoles as there electronegativities(ability to attract an atom)are the same e.g. H2 induce a dipole on each other

    this occurs because electrons move instantaneously with in a molecule in all different directions hence at one point in time the electron distribution is unbalanced causing an instantaneous( temporary for a short moment ) dipole on each side

    this instantaneous dipole will induce a dipole on the neighbouring molecule because- if we take the slighlty negative side for example and imagine it to be closest to the neighbouring molecule- it will induce a dipole on the neighbouring molecule by repelling electrons away creating a slighlty positive charge
    this induced dipole on the neighbouring molecule will in return induce a dipole on the original molecule causing a small weak intermolecular force called london forces

    so as a summary (calling the 2 molecules as m1 and m2)

    movement of electons in m1 causes instantaneous dipole

    this induces a dipole on m2

    m2 then has an induced dipole and the dipole induces a dipole on m1

    m1 and m2 both have dipoles

    dipoles attract each other forming london dispersion forces

    thank you so much it helped me a lot..
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    (Original post by Rather_Cynical)
    I've explained it in a longer and more convulted way before, but the easiest way to think of it is that if at any particular point an electron (or group of electrons) isn't spread out evenly over a molecule, then there would definitely be areas of partial charge (both positive and negative). These are temporary because electrons keep moving about all the time.

    It acts in exactly the same way a normal dipole would, except it lasts for an incredibly small fraction of a second and is usually considered the weakest of the intermolecular forces (per atom, can become very strong with extremely big atoms). It is also affected by shapes of molecules.

    All molecules with electrons have London/Dispersion forces, because it's a property of electrons behaving as non-particles - but for the purposes of A Level or GCSE it's only referred to when stronger dipole-dipole or hydrogen bonding is not present.
    thanks . you mentioned good points..
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    (Original post by Rather_Cynical)
    I've explained it in a longer and more convulted way before, but the easiest way to think of it is that if at any particular point an electron (or group of electrons) isn't spread out evenly over a molecule, then there would definitely be areas of partial charge (both positive and negative). These are temporary because electrons keep moving about all the time.

    It acts in exactly the same way a normal dipole would, except it lasts for an incredibly small fraction of a second and is usually considered the weakest of the intermolecular forces (per atom, can become very strong with extremely big atoms). It is also affected by shapes of molecules.

    All molecules with electrons have London/Dispersion forces, because it's a property of electrons behaving as non-particles - but for the purposes of A Level or GCSE it's only referred to when stronger dipole-dipole or hydrogen bonding is not present.
    whys it affected by molecule shapes
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    (Original post by youreanutter)
    whys it affected by molecule shapes
    If you consider an alkane that's a long, straight chain like long tubes - you can pack that into a box pretty tightly and there's very few airgaps in between. They're close together, so the forces of attraction are stronger.

    If you consider an alkane that's branched up like a huge stickman made of the same tubes taking up all 3 dimensions, they can't quite pack as tightly. They're far apart, so the forces of attraction are weaker.

    The general rule therefore becomes "the more branched (with the same molecular formula) the less attraction between the molecules"
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    (Original post by Rather_Cynical)
    If you consider an alkane that's a long, straight chain like long tubes - you can pack that into a box pretty tightly and there's very few airgaps in between. They're close together, so the forces of attraction are stronger.

    If you consider an alkane that's branched up like a huge stickman made of the same tubes taking up all 3 dimensions, they can't quite pack as tightly. They're far apart, so the forces of attraction are weaker.

    The general rule therefore becomes "the more branched (with the same molecular formula) the less attraction between the molecules"
    how can u tell by a molecular or structural or skeletal formula how branched a molecule is?
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    (Original post by youreanutter)
    how can u tell by a molecular or structural or skeletal formula how branched a molecule is?
    It's the akyl-group that's responsible for branching - think of it as replacing a hydrogen in the middle of an alkane with CnH2n+1. It wouldn't be possible to know with the molecular formula alone, but the structural formula would show you.

    The different isomers of pentane is a great example:

    Name:  pentane isomers.JPG
Views: 112
Size:  21.2 KB

    EDIT - meant akyl-group!
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    (Original post by Rather_Cynical)
    It's the carbonyl-group that's responsible for branching - think of it as replacing a hydrogen in the middle of an alkane with CnH2n+1. It wouldn't be possible to know with the molecular formula alone, but the structural formula would show you.

    Pentane is a great example:

    Name:  pentane isomers.JPG
Views: 112
Size:  21.2 KB
    so the more alkyl groups attactched the more branched?and is it only alkyls that effect branching
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    (Original post by youreanutter)
    so the more alkyl groups attactched the more branched?and is it only alkyls that effect branching
    I meant akyl-group, not carbonyl-group - sorry.

    It's could be any group really, as long as it makes the shape take up more space. Feel free to look up shapes of molecules http://chemed.chem.purdue.edu/genche...ch8/vsepr.html. Akyl-groups are more commonly cited because they can be quite long and therefore branch out more.

    It's a steric effect because you can't get the electrons of those branches getting too close to the ones in another branch without getting some repulsion (like charges repel).
 
 
 

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