Dipole Moment (µ) definition:
In simple words, the dipole moment is the product of charge +ve or –ve and the distance between the centres of charges.
Dipole moment is the measure of separation of charges and also the measure of the polarity of bond and polarity of the molecule.
Dipole moment Formula:
µ = q x r
where
µ = dipole moment
q = charge +ve or –ve
r = distance between the centre of charges i.e bond length.
Dipole moment is a vector quantity having magnitude “q x r” and direction from +ve pole to –ve pole. e.g.
Units of dipole moment:
The S.I unit for dipole moment is Coulomb-metre (C-m) beside it other units are electrostatic unit-centimetre (esu-cm) and debye (D). Their relationship is:
1 D = 3.336 x 10-30 C.m
1 D = 1 x 10-18 esu-cm
1 esu-cm = 3.336 x 10-12 C.m
How do you find the dipole moment?
Measurement of dipole moment:
The dipole moment is measured by an electric condenser (Dipole meter). Parallel plates of the condenser are charged by the battery.
Polar molecules are dipoles. When they are placed between plates with an electric field off they are randomly oriented but when an electric field is on then the polar molecules are oriented such that their +ve poles are towards –ve plate and their –ve poles are towards +ve plates. The molecules hung between the two plates. The electric potential drops. A drop in potential is proportional to the polarity of the molecule. So dipole moment is measured in terms of the drop in potential.
Applications of Dipole Moment:
- Prediction of Polar nature
- Measure of Polarity
- Calculation of percent ionic Character
- Molecular Shapes and Bond Angles
Prediction of Polar nature:
It is used to predict whether a molecule is polar or non-polar. e.g. dipole moment of CH4, CO2, and Cl2 is Zero so they are non-polar. on the other hand dipole moments of CHCl3, H2O, and HCl are not Zero so they are polar molecules.
Measure of Polarity:
It is the measure of the polarity of the bond as well as that of the molecule. More is the dipole moment more is the polarity of molecule e.g. Consider halogen acids their order of polarity is
HF > HCl > HBr > HI
Dipole moment 1.91D 1.03D 0.79D 0.38D
Calculation of percent ionic Character:
Percent ionic character of a compound can be calculated as follows
µ obs = It is the observed dipole moment which is measured by the dipole meter
µ cal = It is the dipole moment which is calculated by the formula µ = q x r
considering molecule to be 100% ionic.
e.g.
For H – F
µ obs = 1.91 D or 1.91 x 3.336 x 10-30 C.m
r = Bond length of H – F = 9.17 x 10-11 m
q = 1.6 x 10-19 C consider molecule to be 100% ionic, then complete transfer of electron take place and charge will be equal to the charge of an electron.
µ cal = q x r = 1.6 x 10-19 x 9.17 x 10-11 = 1.467 x 10-29 C.m
Molecular Shapes and Bond Angles:
The dipole moment helps in deciding the molecular shape as well as bond angles.
The dipole moment of a molecule is obtained by vector addition of its bond moments. Molecular geometry is set such that the calculated dipole moment of the molecule should match the observed dipole moment of the molecule.
CO2 molecule
- Dipole moment, µCO2 = 0, this is only possible when CO2 has linear geometry. So two C=O bond moments on vector addition cancel each other and the resultant vector is zero.
CS2 molecule
Dipole moment, µCS2 = 0, this is only possible when CS2 has linear geometry. So two C=S bond moments on vector addition cancel each other and the resultant vector is zero.
H2O molecule
Dipole moment, µH2O = 1.84D, this is only possible when H2O has an angular or bent structure with a bond angle of 104.5 o. So two O─H bond moments, on vector addition give the resultant vector as 1.84D.
SO2 molecule
Dipole moment, µSO2 = 1.62D, this is only possible when SO2 has an angular or bent structure with a bond angle 118 o.
So S─O and S=O bond moments, on vector addition give the resultant vector as 1.62D.
BF3 molecule
Dipole moment , µBF3 = 0, this is only possible when BF3 has a trigonal planner structure with a bond angle 120 o. So three B─F bond moments, on vector addition give the resultant vector zero.
NH3 molecule
Dipole moment , µNH3 = 1.47D, this is only possible when NH3 has a pyramidal structure with a bond angle 107.5o . So three N─H bond moments, on vector addition give the resultant vector 1.47D.
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Frequently Asked Questions – FAQ
What is meant by dipole moment?
Answer
It is the separation of charge that causes a dipole moment. Whether between ions in an ionic bond or atoms in a covalent bond, dipole moments result from changes in electronegativity. The greater the disparity in electronegativity, the more pronounced the resulting dipole moment.
How do you find the dipole moment?
Answer
When two charges are opposite one another, the electric dipole moment between them is calculated using the formula p = qd, where q and d are the magnitude of the charges and the distance between them, respectively.
What dipole moment is polar?
Answer
It is the direction of the vector quantity that the dipole moment acts in. H2O is a good example of a polar molecule. Since H2O has a bent structure (according to VSEPR theory), the dipole moments of its bonds are not cancelled out. This is because of the oxygen lone pair. Therefore, water exhibits polar properties.
What is the unit of a dipole moment?
Answer
Electromagnetic dipole moment is measured in coulomb-meters (Cm) according to the International System of Units (SI). In the fields of atomic physics and chemistry, the debye (D) is another unit of measurement.
What is the charge of a dipole?
Answer
A pair of electric dipoles, q and -q, are two charges that are diametrically opposed to one another and are separated by a distance d. Electric dipoles always point from a negative charge (-q) to a positive charge (+q) in space. The centre of the dipole is defined as the location midway between q and -q.
Why is dipole moment a vector?
Answer
The direction of the electric dipole moment, which is specified to go from negative to positive charge, makes it a vector quantity (having both magnitude and direction). The centre of a dipole is found at the point where two like charges, one positive and one negative, meet.
Is CO2 a dipole moment?
Answer
Carbon dioxide has no dipole moment.
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