Intermolecular forces: Explain Dipole-Dipole forces

Polar molecules,like nonpolar molecules,are attracted to one another by dispersion forces.In addition,they experience dipole forces.The dipole-dipole force is an attractive intermolecular force resulting from the tendency of polar molecules to align themselves such that the positive end of one molecule is near the negative end of another .

Recall that polar molecule has a dipole as a result of the electronic structure of the molecule.For example;HCl,is a polar molecule because of the difference in electronegativties of the H and Cl atoms.  

The figure below shows the aligment of polar molecules in the case of HCl.Note that this alignment creates a net attraction between molecules.This attractive force is partly responsible for the fact that hydrogen chloride,which is a gas at room temperature becomes liquid when cooled to -85 oC .

 

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