Fajan's Rule, Types of Interactions & Dipole Moment | Inorganic Chemistry PDF Download

Application of Fajan’s Rules (i) solubility :

(i) Solubility

• Ag₂S is less soluble than Ag₂O in H₂O because Ag₂S is more covalent due to bigger S^2– ion.
• Fe(OH)₃ is less soluble than Fe(OH)₂ in water because Fe⁺³ is smaller than Fe⁺² and thus charge

is more. Fe(OH)³ is more covalent than Fe(OH)².

(ii) Colour of compounds :

• The colour of some compound can be explained on the basis of polarisation of their bigger negative  ions. For ex : AgCl is white AgBr, Agl, Ag₂CO₃ are yellow.

The bigger anions are more polarised and hence their electrons get excited by partial absorption of visible light.

• Similarly, SnCl₂ is white but SnI₂ is black
• PbCl₂ is white but PbI₂ is yellow.

(iii) Variation of M.P. [M.P. of covalent < M.P. of ionic]

M.P. : BeCl₂ < MgCl₂ < CaCl₂ < SrCl₂ < BaCl₂
M.P. : CaF₂ > CaCl₂ > CaBr₂ > CaI₂

(iv) Nature of oxides :

• Covalent character  acidic nature  basic nature

Na₂O     MgO    Al₂O₃    SiO₂ P₂O₅ SO₃ Cl₂O₇
Acidic nature Li₂O Na₂O K₂O Rb₂O basic

Non-Bonding interactions (Intermolecular forces)

Intermolecular attractions hold two or more molecules together. These are weakest chemical forces and can be of following types :
(i) Hydrogen Bonding    (ii) Ion-Dipole interaction   (iii) Van-der Waal’s interaction

(i) Hydrogen Bond

It is electrostatic force of attraction which exist between covalently bonded H-atom of one molecule and the most electronegative element of another molecules.

Conditions of H-bond

(i) Molecules should have more electronegative atom (F, O, N) linked to the H-atom
(ii) Size of electronegative atom should be smaller.
(iii) A lone pair should be present on electronegative atom.

Nature of H-Bond

It is a very weak electrostatic attraction between positive pole of and lone pair of an electronegative atom which can i.e. F, N or O (But in some cases Cl and C are also found to form H-bonds)

Strength of H-Bond

The strength of H-bond is usually very low (5-10 kJ/mol) but in some cases this value may be as highest 50kJ/mole or more. The strongest H-bonds are fomred by F atoms. D is more positive than H therefore it also form stronger bonds. Besides (F), the Strength of H bonds depends on :
(a) Magnitude of (+) pole of H (For this atom X must be more and more electronegative)
(b) Availability of lone pair on the atom Y.

Types of H-Bonds

(a) Inter Molecular

Exists between the negative and positive ends of different molecules of the same or different substances

(1) In H-F solid 

The solid KHF₂ is an ionic compound consisting of K⁺ and [HF₂]^– The anion [HF₂]^– consists of H-bonds

(b) Intra molecular

Exists between hydrogen and an electro-negative element of the same molecule. Example are :

(i) O-nitrophenol :

(ii) Chloral hydrate :

(Important example conaisn 2(OH) groups on one C atom)

(a) Ion-dipole attraction 

Exists between an ion and a polar molecule, its strength depends on (i) size of ion (ii) charge on the ion (iii) dipole moment of the polar molecule.

 This force is responsible of hydration

(b) Dipole-dipole attraction  

Electrostatic attractions between the oppositively charged ends of permanent dipoles. Exists between polar molecules and due to this force gas can be liquified.

(c) Ion-induced dipole attraction  

Exists between ion and non-polar molecules.

(d) Dipole-induced dipole attraction  

Exists between polar and non-polar molecules.

(e) Instantaneous dipole-induced dipole attraction 

Exists among the non-polar molecules like H₂, O₂, Cl₂ etc. in solid or liquid states

Relatrive strength of interactions
Ion-dipole > dipole-dipole > ion-induced dipole > dipole-induced dipole  instantaneous dipoleinduced
dipole

• Strength of vander waal force ∝ molecular mass
• van der Waal’s force ∝ Boiling point.

Polarity in covalent compounds (Dipole moment)
In diatomic molecule μ = δ x d
A molecule will have a dipole moment of 1 Debye (1D) if charges of 4.8 × 10^–10 esu are separated by
a distance of 1 Å.

Thus, 4.8 × 10^–10 esu cm = 1 D

but in polyatomic molecule with angle θ, resultant dipole moment is the vector summation of the vector moments.

where μ_AB is the vector moment of the side AB.

Also, 

Thus, greater the value of θ, smaller the value of μ. 

Example(θ =104.5⁰) >

Application of dipole moment

(1) To distinguish between polar & non polar molecules: The molecules having some value of dipole moment are polar like HCN, NH₃, OCS, PF₃, H₂O, SO₂, NO₂, O₃ etc.

While those having zero dipole moment are non polar like CO₂, BeF₂, H₂, N₂, CH₄, SF₆, SO₃, etc.

(2) To calculate the percentage of ionic character in the polar covalent bond in a given molecule:

Greater is the electronegativity difference between the bonded atoms in a polar molecule, greater is the value of dipole moment.

Percentage of ionic character in A – B bond in AB molecules = 

(3) To predict the shape of a given molecule: The dipole moment value of a given molecule can be used to predict whether the given molecules has symmetrical or unsymmetrical shape.

1. In a molecule which has symmetrical shape like dumbbell, (e.g. C₂, H₂, N₂, O₂, Cl₂, Bl₂ etc.), linear (e.g. CO₂, BeCl₂(g), HgCl₂), trigonal planar (e.g. BF₃, BCl₃, SO₃ etc.), regular tetrahedral (e.g. CH₄, CCl₄, CF₄ SiF₄ etc.), trigonal bipyramidal (e.g. PF₅, PCl₅(g)), square planar (e.g.XeF₄), regular octahedral (e.g. SF₆, WF₆), pentagonal bipyramidal (IF₇) the dipole moments of the vaious bonds present in the molecule cancel each other and hence the net dipole moment of the molecule becomes equal to zero. Thus a molecule which has a symmetrical shape has no (zero) dipole moment.
2. In a molecule which has unsymmetrical shape like angular (e.g. H₂O, H₂S), trigonal pyramidal (e.g. NH₃, NF₃, PH₃ etc.) the dipole moments of the various bonds present in the molecule are not able to cancel each other and hence the molecule has some value for its dipole moment.

Thus a molecule which has unsymmetrical shape has a certain value of dipole moment.

(4) To Calculate the bond angle between two bonds in a molecule: The angle subtended by two axes of overlaps of orbitals or bond is called bond angle.
Net dipole moment of the molecule =

in which m₁ and m₂ are the bond moments of various bonds present in the molecule.

Examples:
(i) Symmetrical molecules without lone pair of electrons will have μ = 0.

In this case resultant μ₂ of H - B - H on the left is cancelled by μ of the right.
O = C = O            θ =180⁰            μ = 0

 , planar , μ = 0

(ii) Symmetrical molecules with lone pair do not have dipole moments equal to zero.

(iii) Dipole moment in ring systems

if  Dipole moment = μ, Then, by using 

  Dipole moment = √3 μ , α = 60°

 Dipole moment = μ , α = 120°

    Dipole moment = 0 , α = 180°

 Dipole moment = μ 

Theoretical μ = 6.30D       μ = 3.80D         μ = 0.00D

Experimental μ = 6.00D    μ = 3.79           μ = 0.00D
In this case μ(o-isomer) > μ(m-isomer) > μ(p-isomer) but experimental value of o-isomer < theoretical value.

This is due to dipole-dipole repulsion in o-isomer that increases bond angle greater than 60° and μ decreases.

(iv) Dipole moment in Geometrical Isomers

(v) Dipole moment in conjugated system

As charge separation increases in conjugated systems, dipole moment is extremely high in them.