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Dipole Moment
Dipole Moment

The product of the magnitude of one of the charges ( positive or negative ) and the distance between their centers is called dipole moment “.

Explanation

The bond between two atoms of different electronegativities is polar i.e. have positive and negative poles. This produces polarity in the molecule.

The separation of charges on bonded atoms is called a dipole. It is measured in terms of f dipole moment defined as F is the magnitude of one of the charges and the distance between them, then

    µ = q x r

The dipole moment is a vector quantity. It is represented by an arrow, directed from the electropositive end to the electronegative end of the dipole

For polyatomic molecules, the net dipole moment is the resultant of the vector addition of individual bond moments.

Units

SI unit of dipole moment is mC ( meter column ).

Thus for a unit negative charge at a distance of 100 pm from a unit positive charge, we have

µ = ( 1.6022 x 1 0⁻¹⁹C ) x ( 100 x 10⁻¹²m ) = 1.6022 × 10⁻²⁹ mC    Equation ( 1 )

 mC is a bigger unit. Generally, a smaller unit Debye ( D ) is used.

          1D = 3.336 10⁻³⁰ mC     Equation ( 2 )

Thus from en ( 1 ) and ( 2 )      μ =1.6022 × 10⁻²⁹ /3.336 × 10⁻³⁰ = 4.8D

APPLICATIONS

Dipole Moments and Molecular Structure

Dipole moment measurements help to determine the angles between bonds or the geometry of molecules

Dynamic molecules

       Diatomic molecules are always linear, e.g. CO has a linear structure and as dipole moment is 0 12 D.

Trinomic Molecules

       Both CO₂ and CS have zero dipole moment. Thus these have a near structure, in which one dipole cancels the effect of the other.

       The dipole moment of SO, and H₂O are 1.64 D and 1.84 D respectively. It shows that SO₂ and H₂O have angular structures because one dipole does not cancel the effect of the other.

Tetratomic Molecules

       NH3 has a dipole moment, which shows that the individual bond moments do not cancel the effect of each other. It is because NH has Pyramidal Structure.

       BF3 has zero dipole moment that the individual bond moments cancel the effect of each other. So, BF3 has a triangular planar structure

Pentatomic Molecules

       Both CCl4, and CH4, have no dipole moment which suggests a regular tetrahedral structure for them.

Dipole Moment
Dipole Moment

Percentage logic Character

Experimental dipole moments can be used to determine the age ionic character of the bond

The formula is %age ionic character   = µobs / µionic × 100

Where µobs = Experimental dipole moment, µ ionic = Dipole moment calculated based on 100 % charge separation.

Since units positive charge = 1.6022 × 10⁻¹⁹C therefore

Charge  = q = 1.6022 × 10⁻¹⁹C          Bond length = r = 0.971× 10⁻¹⁰m

µobs  = 1.90D                                      µionic = ?

%ionic character =?

                µionic = q × r = 1.6022 × 10⁻¹⁹C × 0.971× 10⁻¹⁰m

                                          =1.469 × 10⁻²⁹ mC  

Since 1 D =   3.336 × 10⁻³⁰ mC

Therefore µionic = 1.469 × 10⁻²⁹ mC  /  3.336 × 10⁻³⁰ = 43.2%

Hence, 43% of HF bond is ionic and 57% is covalent. The bond is mainly covalent.

EFFECTS OF BONDING ON THE PHYSICAL PROPERTIES OF COMPOUNDS

The properties of substances depend upon the types of bonding present in them.

SOLUBILITY

•  ionic compounds are soluble in water but insoluble in non-aqueous solvents.

•  It is because polar water molecules detach cations and anions from the crystal lattice by electrostatic attractions. Thus, ionic compounds are dissolved by the hydration of ions.

•  The energy released during hydration is used to break the lattice.

•  Only those crystals are dissolved in water, for which, the energy produced during hydration is greater than office energy so that it can break the lattice

•  Many ionic crystals do not dissolve in water because the attraction of water molecules cannot overcome the attraction between ions.

•  ionic compounds are not dissolved in non-polar solvents like benzene and hexane.

Covalent Compounds

•  Compounds Generally covalent compounds dissolve in non-polar organic solvents e.g. benzene, ether, etc.

•  In this case attraction of solvent molecules with the compounds breaks the intermolecular forces.

•  Most covalent compounds are insoluble in H₂O. Few compounds are dissolved by hydrogen bonding.

 Directional Nature and Isomerism

ionic compounds do not show isomerism because ionic bonds are non-rigid and non – directional. Covalent compounds

Covalent compounds show isomerism because covalent bonds are rigid and direction

eg CH₂O shows structural isomerism

Compounds having the same molecular formula but different structures and properties are called isomers and the phenomenon is called isomerism

Reaction Kinetics

Ionic compounds

The reaction of ionic compounds is very fast

It is because ionic compounds exist as ions in aq, solution. Chemical reactions between ions occur rapidly. E.g By adding AgNO, solution to NaCl solution, white ppt of AgCl are produced at once. In this reaction, both AgNO, and NaCl are in ionized form. Therefore no bond is to be broken. Only a few bonds are formed. Thus reaction occurs rapidly.

covalent compounds

The reaction of covalent compounds is generally slow. Because no electrical forces are present in them to speed up the action and energy is required to break bonds and form new bonds. The molecules undergo a chemical change as a whole.

Covalent compounds show many types of reactions that depend upon the way of reaction and kind of reaction.

Read more about Bond Length

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