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Monday, 28 September 2015

FSc Notes Chemistry Part 1 Chapter 4 Liquids and Solids Lecture 2

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.

Written by: Asad Hussain

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