FSc Notes Chemistry Part 1 Chapter 4 Liquids and Solids Lecture 2
(3) London Dispersion Forces:
(Instantaneous dipole – induced dipole Interaction)
In 1930 a German physicist, Fritz London offered or
presented a simple explanation for inter molecule attractive forces among
“non-polar molecules”.
The inter molecular attractive forces among the polar
molecules can easily be understand (as in case of Dipole-dipole interaction and
Hydrogen bonding) but it is not an easy task to understand the inter molecular
attractive forces among the non-polar molecules like he molecules or H2
molecules etc because they do not have any +ve or –ve pole. Then how can they
attract each other. But its a fact that such non-polar molecules have the
forces of attraction among them because gases like H2, He (having non-polar
molecules), can be liquefied. Thus they can remain in liquid state only, if
there are some attractive forces among their molecules. Thus forces of
attraction which operate among non-polar molecules are explained by Fritz
London as follow. In case of non-polar molecules of a gas, the electrons are
equally distributed around the nuclear and these molecules move freely in
random direction. When these molecules come close to each others, their
electrons repel each other. Thus both the molecules (which collide) get their
electrons aside due to which +ve and –ve charges separate upon the molecule
temporarily and dipoles are generated but as soon as the molecules get away
from each other the electrons come back to their original position and the
polarity of the molecules no longer remains. Therefore such dipoles are named
as “Instantaneous dipoles” because they are short lived. Now when the volume of
the gas is decreased by increasing pressure and decreasing temperature the
molecules come close to each other. Now the +ve pole of an instantaneous dipole
(nearer to another non-polar molecule) attracts all the electrons of the other
non-polar molecule and repel its nuclei. As a result another dipole is created
which is known as “induced dipole” because here the polarity is generated
through induction. Similarly the –ve pole of the instantaneous dipole repels
the electrons of the nearer non-polar then another induced dipole is generated.
Then the +ve pole of an instantaneous dipole attracts the –ve pole of an
induced dipole while –ve pole of an instantaneous dipole attracts the +ve pole
of attraction are known as or London dispersion forces. Thus London dispersion
forces or short range forces can be defined as, “The force of attraction b/w
the +ve or –ve pole of an instantaneous dipole with the –ve or +ve pole of an
induced dipole. London dispersion forces are fond in all types of molecules
whether polar or non-polar. However in case of polar molecules, they are
dominated by Dipole-dipole interaction or by Hydrogen bonding but in case of
non-polar molecules like H2, cl2, He etc. London dispersion forces are
dominant.
Factor Affecting London Dispersion Forces:
London dispersion forces are affected by the following
factors.
1) Molecular Size:
London dispersion forces are greatly affected by molecular
size. Larger the molecular size of the molecules stronger will be the London
dispersion forces among them and vice versa.
Reason:
In case of molecules which have larger size the valence
electrons are far away fro their nuclei and thus these electrons can easily be
disturbed. When these molecules come close to each other, their electrons,
being almost free of the influence of nuclei, easily repel each other, and thus
prominent instantaneous dipoles are created i.e. these instantaneous dipoles
have prominent –ve and positive pole. Thus these prominent –ve and +iv poles
of an instantaneous dipole have greater ability of induction. Thus prominent
induced dipoles are generated. As a result the London dispersion forces will be
stronger because both the instantaneous dipoles and induced dipoles have
stronger poles.
For example elements of Group VIII A (Noble gases) are all
non-polar mono-atomic molecules. As we go down the group, the boiling point
increases. It is because of the fact that down the group the size of atoms
increases and thus London dispersion forces become more and more stronger.
On the other hand molecules which have smaller size do not
produce prominent instantaneous & induced dipole and thus the London
dispersion forces among them are weaker. When the molecular size is smaller
then their valence electrons are closer to their nuclei and thus these
electrons are held firmly by the nuclear and cannot be disturbed easily. Thus
when these molecules come close to each other, their electrons do not get
disturbed Considerably and thus very weak instantaneous dipoles are created
which in turn produce very weak induce dipole and therefore very weak London
dispersion forces are established among these molecules.
For example both he and SO3 are gases with non-polar
molecules. The boiling point of SO3 is much higher than he. It is because of
the fact that molecular size of SO3 is larger than force operates among SO3
molecules than He molecules.
(2) Molecular Shape:
Molecular shape affects the strength of dispersion forces
(London dispersion forces) considerably. Molecules whose shape is “Long and
thin”, have stronger dispersion forces. It is because of the fact that such
molecules can generates stronger temporary dipoles due to electron moment and
hence stronger attractive forces generate and thus such substances have high
boiling point. on the other hand molecules whose shape is short and fat have
weaker London dispersion forces. It is because of the fact that in such
molecules, smaller no of sites are available where dispersion can occur, thus
weaker temporary dipoles are generated and hence weaker London dispersion
forces are established among them.
For example let us consider two substances i.e. n-butane and
2-methyl propane. Both these molecules have same chemical formula i.e. C4H10.
Thus both have same atoms and same no of electrons n-butane with longer and
thin shape has high boiling point (- 0.5 degree Celsius) and 2 – methyl Propanol and with
shorter and fat shape has lower boiling point (-117degree Celsius). Thus it is proved that
longer and thin shaped molecules has stronger London dispersion forces while
shorter and fat shaped molecules have weaker London dispersions force.
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