Video Transcript
Which of the molecules Br2, HF,
CHCl3, CCl4, and NH3 contains polar bonds but is not a polar molecule?
Polar bonds exist between some
atoms that have different electronegativity values. The electron density is high around
the more electronegative atom and low around the other bonded atom.
Chemists usually calculate the
absolute difference of electronegativity values to determine if a bond will be polar
or not. 𝐸 one and 𝐸 two are the
electronegativity values of the atoms engaging in a bond. Bonds are polar when their atoms
have a difference of electronegativity values higher than 0.4 and less than
approximately 1.7 or 1.8.
The bromine molecule does not have
any polar bonds. Because it has atoms with the same
electronegativity value, its electron density is shared equally about the two
covalently bonded atoms. Bromine cannot be the correct
answer to this question because it does not have any polar bonds.
Hydrogen fluoride has a polar bond
because it has atoms of different electronegativity values. The bond has an absolute
electronegativity difference of 1.78, which falls approximately between the range
for polar bonds. Because this molecule contains only
one bond and that bond is polar, the molecule is polar overall and is not the answer
to this question.
We can calculate that CHCl3
contains three polar bonds. NH3, or ammonia, has three polar
bonds as well. We can use dipole arrows to show
the polarity of the bonds. These molecules are polar because
the dipole moment of one polar bond does not cancel out the dipole moment of other
polar bonds. CHCl3 and NH3 are polar
molecules.
CCl4, or carbon tetrachloride,
contains only polar bonds. The molecule, however, is nonpolar
because it has symmetrical geometry. These four equivalent
carbon–chlorine bonds generate four dipole moments. These dipole moments cancel each
other, and the molecule has no overall polarity. Carbon tetrachloride is the only
listed molecule that has polar bonds but is nonpolar. Therefore, the correct answer is
CCl4.
