97.CHEMICAL BONDING (44)- Secondary bonding(1)- Dipole dipole interactions.

We have already seen in post # 54 that the main difference between primary and secondary bonding is that in secondary bonding THERE IS NO SHARING OR TRANSFER OF ELECTRONS.

Primary bonding

Secondary bonding

sharing or transfer of electrons.

NO sharing or transfer of electrons.

Strong force of attraction

Weak force of attraction

Short range

Long range

e.g.- Ionic,Covalent ,Co-ordinate covalent,Metallic bonds

e.g.- Hydrogen bonds, vander Waal’s forces, dipole interactions.

From this post onwards we will discuss secondary bonding in detail.

SECONDARY BONDING 

Secondary bonding arises in covalently bonded molecules due to the attraction of electric dipoles in atoms/molecules.This bonding is used to describe aggregation in molecules.

As mentioned in post # 54 there are three types of secondary bonds –

S1. Hydrogen bonds
S2. Vander Waals bonds / London dispersion force
S3. Dipole- Dipole interactions.

We shall begin by discussing the dipole interactions first , Vander waals bonds second  and then the hydrogen bonding at last.

S3.DIPOLE-DIPOLE INTERACTIONS

This kind of secondary bonding is seen in polar molecules on account of asymmetric charge distribution. In polar molecules, the more electronegative atom pulls the shared electron cloud towards itself and thus it has a partial negative charge(-δ).A partial positive charge is developed on the other atom. These partial charges are permanent dipoles, which are responsible for the aggregation in such molecules. The positive end of one molecule gets attracted to the negative end of the other molecule and vice versa.This is the secondary bond – an intermolecular bond.

(Intermolecular ⇒ Between the molecules
Intramolecular ⇒ within the molecule).

The aggregate can be shown as follows –

967.jpg

 

In the above figure – A⇒Electropositive atom & B ⇒ Electronegative atom.
All the positive and negative ends are attracted towards each other and this dipole interaction is shown by a red dotted line.
e.g. HCl , CO, HF  molecules.

More the dipole moment (µ) , more is the interaction i.e these two quantities are directly proportional to each other.The energy of this interaction is directly proportional to the fourth power of dipole moment as shown below.It is inversely proportional to the distance (r) and temperature (T).At higher temperatures, the thermal energy acts as the disruptive force , thus decreasing the strength of dipole interactions.

968.jpg

Both ICl and Br2 have the same number of atoms and approximately the same molecular weight, but ICl is a solid whereas Br2 is a liquid at 0oC. Why?

Br is a homonuclear molecule and so it is not polar.Thus, there are no dipole interactions.However in ICl molecule, the electronegativities of iodine and chlorine atoms are significantly different and thus it is a polar molecule. Being a polar molecule, there are dipole interactions(cohesive force), which bring the adjacent molecules closer and at 0oC there are no disruptive forces (thermal energy ) too. Thus ICl exists in liquid state.

In the next post we shall discuss the vander waals bonds.Till then ,

Be a perpetual student of life and keep learning …

Good day !

 

References and Further Reading –

1.https://www.chem.purdue.edu/gchelp/liquids/dipdip.html
2.https://www.youtube.com/watch?v=2Q_fna3TTbs&t=128s

 

 

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