Dipole Moment

DIPOLE MOMENT As we know in polar molecules like H2O and HCl the center of negative charge does not coincide with the center of positive charge. These types of molecules which have two equal and opposite charges separated by certain distance are said to possess electric dipole. The term dipole moment gives an idea about the extent of polarity in polar covalent molecules. It is defined as the product of the magnitude of the charge and the distance of separation between the charges. Mathematically it is represented as: Dipole moment (μ) = charge (e) x distance of separation (d) Unit to measure dipole moment is Debye. It is expressed as D (1Debye= 1x10-18e.s.u. cm). 1D = 3.33564x10-30 cm Dipole moment is a vector quantity and represented by a small arrow (→). Tail of the arrow is pointed towards positive center and head towards negative center.

Dipole Moment and Molecular Structure: 1) Diatomic Molecules - It is formed by the combination of two atoms bonded together by covalent bond. Diatomic molecule will be polar if bond formed between the atoms is polar. e.g. Dipole moment of HCl molecule is the same as that of H —Cl bond, (μ=1.03D)

The dipole moment increases with the increase in electronegativity.

*The negative or positive signs for (μ) shows that H is the negative or positive end of the dipole respectively.

2) Polyatomic Molecules - Dipole moment of polyatomic molecules not only depends upon the indvidual dipole moment of the bonds but also on the spatial arrangement of the various bonds in the molecules. So the dipole moment of such molecules is the vector sum of the dipole moments of the various bonds. For example, CO2 and H2O both these molecules are triatomic but have different dipole moments. CO2 (zero), H2O (1.84D) why it is so that can be explained on the basis of their structures.

CO2 molecule has a linear structure in which two C=O bonds are oriented in the opposite directions at an angle of 1800. Each C=O bond has 2.3 D dipole moment but due to linear structure the dipole moment of one C=O bond cancels that of another. In this way resultant dipole moment of CO2 is zero.

Hence CO2 is non-polar molecule. But in case of water molecule the two O—H bonds are oriented at an angle of104.5º and have a bent structure. The dipole moment of H2O is1.84 D that is the resultant of the dipole moments of two bonds.

Take the case of tetra atomic molecules like BF3 and NH3. Their dipole moments are zero and 1.49 D respectively. This shows that BF3 molecule is symmetrical and all the three bonds are directed at an angle of 120º. All the three bonds are in the same plane and dipole moments cancel one another resulting net dipole moment equal to zero. While ammonia molecule is pyramidal structure, the resultant dipole moment of NH3 molecule given by all the three N-H bonds is 1.49D.

So we can say that the presence of polar bonds in a polyatomic molecule does not mean that the molecule as a whole will always have a dipole moment. There are some molecules where molecules have dipole moment zero because of the arrangement of the individual bonds in the molecule. For example, CO2, BF3, CCl4 all these molecules have symmetrical structure. But in case of CHCl3 , dipole moment is not equal to zero because all the bonds are not same. The resultant of bond dipole moments of three C-Cl bonds is not cancelled by bond dipole moment of C-H bond.

Interesting case of NH3 and NF3 molecules in relation to net dipole moment: Both these molecules are pyramidal in shape having lone pair of electrons on nitrogen atom. The N-F bonds are more polar than N-H bonds because of the presence of the most electronegative fluorine atom. As a result, the resultant dipole moment of NF3 is larger than that of NH3. But actually the dipole moment of NF3 (μ = 0.24 D) is lesser than that of NH3 (μ = 1.46 D). This is due to the presence of lone pair of electron on nitrogen atom. In NF3, the orbital dipole is in the opposite direction to the resultant dipole moment of the three N-F bonds. Thus, the lone pair moment cancels the resultant N-F bond moments. But in case NH3 it is just opposite to NF3. The orbital is in the same direction as the resultant dipole moment and therefore adds on the resultant dipole of N-H bonds.

Applications of Dipole Moment : To understand chemical bond, “dipole moment” plays an important role. For example 1) It helps in distinguishing polar and non - polar molecules. Non -polar molecules have zero dipole moment while polar molecules have some value. 2) It also helps in measuring the degree of polarity in diatomic molecules. Greater the dipole moment, greater the polarity. For example: – HF (1.91 D) is more polar than HCl (1.03D).

3) Dipole moment is used to find the shapes of the molecule. In Be F2 molecule if the dipoles of two Be-F bonds cancel each other, then its dipole moment will be zero. Or the two bonds must be oriented in opposite directions. This is possible if the molecule is linear.

Dipole Moment of Some Molecules:

4) Dipole moment also gives an idea about the ionic character in a bond or a molecule. Experimentally it is found that the dipole moment of HCl is 1.03 and its bond length is 1.257A0. For 100% ionic character the charge developed on H and Cl atoms would be 4.8x10-10es.u.Therefor dipole moment for 100% ionic character can be calculated as; µionic = 4.8x 10-10 x 1.275x 10 –8 e.s.u. -cm = 6.12x 10-18 e.s.u.–cm = 6.12D The observed dipole moment (µobs.) =1.07 D Therefore percentage ionic character= µobs / µionic x100 =1.03 /6.12x100 =16.83% In general, larger the value of dipole moment more will be the ionic character.

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