Saturday, May 14, 2011

Applications of Dipole Moment

Some of the applications of dipole moment are given below:
1.     DISTINCTION BETWEEN POLAR AND NON-POLAR MOLECULES
        The molecules having dipole moment are called polar molecules whereas molecules having zero dipole moment are said to be non-polar molecules.
    For example:
(i).    Molecules such as H2, N2, O2, Cl2 etc. have non-polar bonds and zero value of dipole moment.
(ii).     Molecules such as CO2, BF3, CH4, etc., have polar bonds but zero value of dipole moment.   
(iii).    Molecules such as HF, HCl, NH3, H2O, have polar bonds and their dipole moment is greater than zero. In other words, they are polar molecules.
2.     IONIC CHARACTER IN A MOLECULE
        Dipole moment gives an idea about the ionic character in a bond or a molecule. For example, let us calculate the percentage of ionic character in HCl molecule. Experiments have shown that the dipole moment of HCl is 1.03 D and its bond length is 1.275Å. Now for 100% ionic character the charge developed on H and Cl atoms would be 4.8 ´ 10–10 e. s. u. Therefore, dipole moment in case of 100% ionic character is given as:
        µ = 4.8 × 10–10 × 1.275 × 10–8 e. s. u.-cm
              = 6.12 × 10–18 e. s. u.-cm = 6.12 D
            The observed dipole moment
                µ = 1.03 D
        Therefore, Percentage Ionic character = 1.03/6.12 × 100 = 16.83%
    In general, larger the value of dipole moment is, more will be the ionic character.
3.     SHAPES OF THE MOLECULES
        The value of dipole moment helps to predict the shape of the molecule. For example, experiments show that the dipole moment of BeF2 is zero. This is possible only if bond dipoles of two Be – F bonds cancel each other. In other words, the two bond dipoles must be oriented in opposite directions. This is possible if the molecule is linear. Similarly, water has a dipole moment of 1.83 D. Thus, its molecule cannot be linear because the bond dipoles do not cancel each other. Thus, the molecule must have an angular shape.

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