Dipole moments and its application

Molecules having two equal and opposite charges separated by certain distance are said to possess an electric dipole. In the case of such polar molecules, the centre of negative charge 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 the distance of separation between the charges.

It is represented by the Greek letter 'm'. Mathematically it is equal to

dipole 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 the positive 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 Structure

Diatomic molecules

A diatomic molecule has two atoms bonded to each other by a covalent bond. In such a molecule, 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 dipole moment.

The dipole moment of hydrogen halides decreases with decreasing

electronegativity of halogen atom.

Polyatomic molecules

In polyatomic molecules the dipole moment not only depends upon the individual dipole moments of the bonds but also on the spatial arrangement of the various bonds in the molecule. In such molecules the dipole moment of the molecule is the vector sum of the dipole moments of various bonds.

For example, Carbon dioxide (CO₂) and water (H₂O) are both triatomic molecules but the dipole moment of carbon dioxide is zero whereas that of water is 1.84 D. This is because CO₂ is a linear molecule in which the two C=O (m=2.3D) bonds are oriented in opposite directions at an angle of 180°. Due to the linear geometry the dipole moment of one C = O bond cancels that of another. Therefore, the resultant dipole moment of the molecule is zero and it is a non-polar molecule.

Water molecule has a bent structure with the two OH bonds oriented at an angle of 104.5°. The dipole moment of water is 1.84D, which is the resultant of the dipole moments of two O-H bonds.

Similarly in tetra-atomic molecules such as BF₃ and NH₃, the dipole moment of BF₃ molecule is zero while that of NH₃ is 1.49 D. This suggests that BF₃ has symmetrical structure 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 one another giving net dipole moment equal to zero.

NH₃ has a pyramidal structure. The individual dipole moments of three N-H bonds give the resultant dipole moment as 1.49 D.

Thus, the presence of polar bonds in a polyatomic molecule does not mean that the molecules are polar.

Importance of dipole moment

Dipole moment plays very important role in understanding the nature of chemical bonds.

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, CH₄

Polar 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 a molecule.

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

• It is possible to predict the nature of chemical bond formed depending upon the electronegativities of atoms involved in a molecule. The bond will be highly polar if the electronegativities of two atoms is large. However, when the electron is completely transferred from one atom to another, an ionic bond is formed (ionic bond is an extreme case of polar covalent bonds). The greater the difference in electronegativities of the bonded atoms, the higher is the ionic character. When the electronegativity difference between two atoms is 1.7, then the bond is 50% ionic and 50% covalent. If the electronegativitv difference is more than 1.7, then the chemical bond is largely ionic (more than 50% ionic character) and if the difference is less than 1.7, the bond formed is mainly covalent.

The percentage of ionic character can be calculated from the ratio of the observed dipole moment to the dipole moment for the complete electron transfer (100% ionic character).

In HCl molecule, the observed dipole moment is 1.03 D and its bond length is 1.275Å. Assuming 100% ionic character, the charge developed on H and Cl atoms would be 4.8 x 10^-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.)

Problem

12. Calculate the ionic character of HCl. Its measured dipole moment is 3.436 x 10^-30 coulomb meter. The HCl bond length is 2.29 x 10^-10 meter.

Solution

Dipole 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 Cm

Actual dipole moment of HCl = 3.436 x 10^-30 Cm

13. The C-Cl bond is polar but CCl₄ molecule is non-polar. Explain.

Solution

The C-Cl bond is polar because the chlorine atom being more electronegative pulls the shared electron pair towards itself. In CCl₄, there are four C-Cl bonds. Since these polar bonds are symmetrically arranged, the polarities of individual bonds cancel each other resulting in a zero dipole moment for the molecule. The net result is that CCl₄ molecule is non-polar.