Effect of Polarity on Molecular Properties

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Effect of Polarity on Molecular Properties

Every chemist should understand the meaning of polarity. It is the measure of the electronegativity differences between two bonded atoms. Thus, polarity arises as a result of the dipole moment existing between the molecules or atoms. The
dipole moment is a quantitative measure of the degree of charge
separation in a molecule. The polarity of
a bond, such as that in HCl, is characterized by a separation of
electric charge. We can represent this in HCl by indicating partial charges, ԃ+

 and ԃ- on  the atoms.

Any molecule that has a net
separation of charge, as in HCl, has a dipole moment. A molecule in which the
distribution of electric charge is equivalent to ԃ+

 and ԃ-   separated by a distance d has a
dipole moment equal to ԃd.
Whether
a molecule has a dipole moment (that is, whether it is polar or nonpolar)
can
affect its properties. We will explore this issue in detail in the next tutorial
guide. Briefly, though, we can see why cis- and trans-1,2-dichloroethene, 
Effect of Polarity on Molecular Properties
mentioned
in the chapter opening, have different boiling points. The COCl bonds are quite
polar, because of the relatively large electronegativity difference between C
and Cl atoms (0.9); the HOC bonds have moderate polarity (electronegativity
difference of 0.4). The bond dipoles (light arrows) add together in cis-1,2-dichloroethene
to give a polar molecule (the total dipole moment is indicated by the heavy
arrow). (Note that the electronegative Cl atoms are on the same side in the cis
molecule.)
In
the trans molecule, the bond dipoles subtract to give a nonpolar
molecule. In a liquid, polar molecules tend to orient so that positive
ends of molecules are attracted to negative ends.
This
results in an attractive force between molecules (an intermolecular force),
which increases the energy required for the liquid to boil, and therefore
increases the boiling point. Recall that the trans compound boils at 48oC,
whereas the cis compound boils at a higher temperature, 60oC.
PROBLEM
TO SOLVE
1. Two
molecules, each with the general formula AX3, have different dipole moments.
Molecule Y has a dipole moment of zero, whereas molecule Z has a nonzero dipole
moment. From this information, what can you say about the geometries of Y and
Z?
2. Each of the following molecules has a nonzero dipole moment.
Select the molecular geometry that is consistent with this information. Explain
your reasoning.
a. SO2 linear, bent
b. PH3 trigonal planar, trigonal pyramidal
Problem Strategy:  Because the dipole moment is a vector quantity, it can
happen that nonzero bond dipoles cancel one another to give a molecule with
zero dipole moment. Look for bonds that are symmetrically arranged to oppose
one another (think of a tug of war between bonds).Click here for summarized relationship between geometry and dipole moment.
Solution:  a. In the linear geometry, the SOO bond contributions
to the dipole moment would cancel, giving a zero dipole moment. That would not
happen in the bent geometry; hence, this must be the geometry for the SO2
molecule. b. In the trigonal planar geometry, the bond contributions to the
dipole moment would cancel, giving a zero
dipole moment. That would not occur in the trigonal pyramidal geometry;
hence, this is a possible molecular geometry for PH3.

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