Application of Dipole Moment

A different application of dipole moment is taken one after another.
  • Determination of Partial Ionic Character and Residual Charge on the Constituent Atoms in a Molecule:
    Let us consider a molecule AB having the dipole moment μobs and the bond length l cm. if the shared electron pair lies at the midpoint of the atoms, the bond will be purely covalent and the present of ionic character is zero.
    But if the bond is 100% ionic, and B is more electronegative then A, B will carry a unit negative charge and A uni-positive charged respectively. In that case, the dipole moment of the molecule would be,
    μionic = e × ℓ
    = (4.8 × 10⁻¹⁰) ℓ esu cm
    But the dipole moment of AB is neither zero or nor μionic.
Application of dipole moment for calculation of partial ionic character
Partial Ionic Character

Induced polarization (Pi):

    The induced polarization (Pi)= (4/3) π N₀ αi
    But when the substance is in the gaseous state,
    Pi = {(D₀ - 1)/(D₀ + 2)} M/ρ
    (for the covalent substance)
    The value of D൦ is close to unity under this condition,
    Hence, {(D0 - 1)/3} × 22400
    = ( 4/3 ) π N₀ r³
    At NTP, M/ρ = molar volume = 22400cc/mole and αi= r³ taking the spherical shape of the molecule.
    or, r³ = (22400/4πN₀ ) (D₀ - 1)
    = 2.94 ×10⁻²¹ (D₀ - 1)
    Hence, the radius of the molecule r can be determined by measuring D൦ of the substance at NTP.

Determination of molecular structure :

  • Mono-atomic Molecules (A):
    The mono-atomic inert gases are non-polar and it indicates the symmetrical charge distribution in the molecule.
  • Di-atomic Molecules (AX):
    The homonuclear diatomic molecules are largely non-polar such as N₂, O₂, Cl₂ indicating that there are symmetrical charge distributions. The bonding electron pair is equally shared by the two bonding atoms. However, Br₂, I₂ have non zero values of dipole moment and this indicates the unsymmetrical charge distribution,
    H+I-
    Heteronuclear diatomic molecules are always polar due to the difference of electronegativity of the constituent atoms. The example is, HCl, HBr, HF, etc. this indicates that electron pair is not equally shared and shifted to the more electronegative atom.
Application of Dipole Moment
Dipole Moment of HCl, HBr, HI, HF
  • Tri-atomic Molecules (AX₂):
    The molecules, like CO₂, BeCl₂, BeF₂, SnCl₂, etc have zero dipole moment indicating that the molecules have a symmetrical linear structure.
    For example, CO₂ has structure, The electric moment of one C - O bond ( known as bond moment ) cancel the electric moment of the other CO bond.
Application of Dipole moment for Tri-atomic Molecules
The dipole moment of CO₂ and BeCl₂
    The electric moment associated with the bond arising from the difference of electronegativity is called the bond moment(m). In molecules, the vectorial addition of the bond moments (m) gives the resultant dipole moment(μ) of the molecule.
μ2 = m12 + m22 + 2m1m2Cosθ
    Where m₁ and m₂ are the bond moments.
    These help to calculate the bond angle (θ) in the molecule. Thus in the carbon dioxide molecule, μ = 0 and m₁ = m₂
    Hence, 0 = 2m²(1 + cosθ)
    or, θ = 180°
    that is the molecule is linear.
    For Non-Linear Structure:
    Another type of molecules such as H₂O, H₂S, SO₂᠌᠌, etc. have μ ≠ 0 indicating that they have non-linear structure. The bond angle can be calculated from the bond moments of the molecules.
  • Tetra Atomic Molecules (AX₃):
    The molecules like BCl₃, BF₃, etc have dipole moment zero indicating that they have regular planar structure. Their halogen atoms are on a plane at the corner of the equilateral triangle and B atom is at the intersection of the molecules.
Dipole moment of BF3 in application of dipole moment
Application of dipole moment of BF₃
  • Problem 1:
    The displacement of HCl is 1.03 Debye and its bond length is 1.27 A. Calculate the (a) charge on the constituent atom and (b) the % of the ionic Character of HCl.
  • Answer:
    (a) Charge on the constituent atom(q), = 0.8 × 10⁻¹⁰ esu
    (b) Percentage of the ionic Character of HCl, = 16.89%
    For Solution Follow the link
Questions and Answers
  • Problem 2:
    The Difference between the electronegativity of Carbon and Oxygen Is large but the dipole moments of carbon monoxide is very low - Why?
  • Answer:
    However, in CO, there is a large difference of electronegativity between C and O but the molecule is the very low value of dipole moment. This suggested that the charge density in O-atom is somehow back-donated to C-atom.
    This explains by forming a coordinate covalent bond directing towards C-atom.
  • Problem 3:
    H₂O molecule has a dipole moment-Explain. Does it invalidate a linear structure?
  • Answer:
Bond Moment of H2O and H2S in application of Dipole Moment
Dipole Moment of H₂O and H₂S
    For Water (H₂O), μ = 1.84 D
    and mOH = 1.60 D
    Thus, μ² = 2 m² (1+ cosθ )
    or, (1.84)² = 2 (1.60)² (1+ cosθ )
    or, θ = 105°
    The contribution of non-bonding electrons towards the total dipole moment is included within the bond moment.
  • Problem 4:
    The bond angle in H₂S is 97° and dipole moment = 0.95 D. Find the bond moment of the S - H link. ( Given, Cos97° = -0.122)
  • Answer:
    We have, μ = 0.95 D and θ = 97°.
    From the equation,
    μ² = 2 m² (1+ cosθ )
    Putting the value we have,
    (0.95)² = 2 m² (1 + cos97° )
    here m = mS-H
    or, 0.9025 = 2 m² (1-0.122)
    or, m² = 0.9025/ (2 × 0.878)
    or, m² = 0.5139
    or, m = 0.72
    Thus the bond moment of the S - H link is 0.72 D
  • Problem 5:
    Show that the bond moment vectors of NF₃ molecule adds up to zero.
  • Answer:
    While other types of the molecule such as NH₃, PH₃, AsH₃ are polar (μ≠0) indicating that that the molecule has a pyramidal structure in which the three H-atoms are on the plane and N-atom at the apex of the pyramid in NH₃. 
    But NF₃ has very small dipole moment though there is a great difference of electronegativity between N and F atoms and similar structure of NH₃.
    A low value of μ of NF₃ is explained by the fact that resultant bond moment of the three N - F bonds are acting in the opposite direction to that of the lone pair placed at the N-atom. But in NH₃, the resultant bond moment is acting in the same direction as that of the lone pair electrons.
Application of dipole moment of NH₃ and BF₃
Dipole Moment of NH₃ and BF₃

Application of Dipole Moment, Determination of Partial Ionic Character, Residual Charge and molecular structure related problems and its solutions

Inorganic Chemistry

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