dipole moments and its application

7/27/2019 Dipole Moments and Its Application

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dipole moments and its application

Molecules having two equal and opposite charges separated by certain distance are saidto possess an electric dipole. In the case of such polar molecules, the centre of negativecharge does not coincide with the centre of positive charge. The extent of polarity in

such covalent molecules can be described by the term Dipole moment. Dipole moment can be defined as the product of the magnitude of the charge and thedistance of separation between the charges.It is represented by the Greek letter 'm'. Mathematically it is equal todipole moment (m) = charge (e) x distance of separation (d).It is expressed in the units of Debye and written as D(1 Debye = 1 x 10-18e.s.u cm)Dipole moment is a vector quantity and is represented by a small arrow with tail at thepositive centre and head pointing towards a negative centre.For example, the dipole moment of HCl molecule is 1.03 D and that of H2O is 1.84 D.The dipole of HCl may be represented as:

Dipole Moment and Molecular StructureDiatomic moleculesA diatomic molecule has two atoms bonded to each other by a covalent bond. In such amolecule, the dipole moment of the bond gives the dipole moment of the molecule.Thus, a diatomic molecule is polar if the bond formed between the atoms is polar.Greater the electronegativity difference between the atoms, more will be the dipolemoment.The dipole moment of hydrogen halides decreases with decreasingelectronegativity of halogen atom.Polyatomic moleculesIn polyatomic molecules the dipole moment not only depends upon the individualdipole moments of the bonds but also on the spatial arrangement of the various bondsin the molecule. In such molecules the dipole moment of the molecule is the vector sumof the dipole moments of various bonds.For example, Carbon dioxide (CO2) and water (H2O) are both triatomic molecules butthe dipole moment of carbon dioxide is zero whereas that of water is 1.84 D. This isbecause CO2 is a linear molecule in which the two C=O (m=2.3D) bonds are oriented inopposite directions at an angle of 180. Due to the linear geometry the dipole moment ofone C = O bond cancels that of another. Therefore, the resultant dipole moment of themolecule is zero and it is a non-polar molecule.


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Water molecule has a bent structure with the two OH bonds oriented at an angle of104.5. The dipole moment of water is 1.84D, which is the resultant of the dipolemoments of two O-H bonds.Similarly in tetra-atomic molecules such as BF3 and NH3, the dipole moment of BF3molecule is zero while that of NH3 is 1.49 D. This suggests that BF3 has symmetricalstructure in which the three B-F bonds are oriented at an angle of 120 to one another.Also the three bonds lie in one plane and the dipole moments of these bonds cancel oneanother giving net dipole moment equal to zero.NH3 has a pyramidal structure. The individual dipole moments of three N-H bonds givethe resultant dipole moment as 1.49 D.Thus, the presence of polar bonds in a polyatomic molecule does not mean that themolecules are polar.Importance of dipole momentDipole moment plays very important role in understanding the nature of chemicalbonds.Importance of dipole moment and problems The measurement of dipole moment helps in distinguishing between polar and non-

polar molecules. Non-polar molecules have zero dipole moment while polar

molecules have some value of dipole moment.

For example:Non-polar molecules: O2, Cl2, BF3, CH4Polar Molecules: HF (1.91 D), HCl (1.03 D), H2S (0.90 D) Dipole moment measurement gives an idea about the degree of polarity in a diatomic

molecule. The greater the dipole moment the greater is the polarity in such amolecule.


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Dipole moment is used to find the shapes of molecules. This is because the dipolemoment not only depends upon the individual dipole moment of the bonds but alsoon the arrangement of bonds.

It is possible to predict the nature of chemical bond formed depending upon theelectronegativities of atoms involved in a molecule. The bond will be highly polar if

the electronegativities of two atoms is large. However, when the electron iscompletely transferred from one atom to another, an ionic bond is formed (ionicbond is an extreme case of polar covalent bonds). The greater the difference inelectronegativities of the bonded atoms, the higher is the ionic character. When theelectronegativity difference between two atoms is 1.7, then the bond is 50% ionic and50% covalent. If the electronegativitv difference is more than 1.7, then the chemicalbond is largely ionic (more than 50% ionic character) and if the difference is lessthan 1.7, the bond formed is mainly covalent.

The percentage of ionic character can be calculated from the ratio of the observed dipolemoment to the dipole moment for the complete electron transfer (100% ioniccharacter).In HCl molecule, the observed dipole moment is 1.03 D and its bond length is 1.275.Assuming 100% ionic character, the charge developed on H and Cl atoms would be 4.8 x10-10e.s.u.Therefore, dipole moment for 100% ionic character will be= q x d = 4.8 x 10-10e.s.u x 1.275 x 10-8cm=6.12x 1O-18e.s.u.cm= 6.12 D (1D = 10-18 e.s.u. cm.)

Problem12. Calculate the ionic character of HCl. Its measured dipole moment is 3.436 x 10 -30coulomb meter. The HCl bond length is 2.29 x 10-10 meter.SolutionDipole moment corresponding to 100 % ionic character of HCl= 1.602 x 10-19 C x 1.29 x 10-10 m= 20.67 x 10-30 CmActual dipole moment of HCl = 3.436 x 10-30 Cm

13. The C-Cl bond is polar but CCl4 molecule is non-polar. Explain.SolutionThe C-Cl bond is polar because the chlorine atom being more electronegative pulls theshared electron pair towards itself. In CCl4, there are four C-Cl bonds. Since these polarbonds are symmetrically arranged, the polarities of individual bonds cancel each other


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resulting in a zero dipole moment for the molecule. The net result is that CCl4 moleculeis non-polar.

dipole moments and its application

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