|1 Crore+ students have signed up on EduRev. Have you?|
Application of Fajan’s Rules (i) solubility :
is more. Fe(OH)3 is more covalent than Fe(OH)2.
(ii) Colour of compounds :
The bigger anions are more polarised and hence their electrons get excited by partial absorption of visible light.
(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 :
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
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 μAB is the vector moment of the side AB.
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.
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.
(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.