Chemical Bonding: Intermolecular and Intramolecular
Question 8 From the 2001 AP Exam
Q8. Account for each of the following observations about pairs
of substances. In your answers, use appropriate principles of chemical bonding
and/or intermolecular forces. In each part, your answer must include references
to both substances.
a) Even though NH3 and CH4 have similar
molecular masses, NH3, has a much higher normal boiling point (-33
degrees C) than CH4 (-164 degrees Celsius).
NH3 is polar and has hydrogen-bonding
and dispersion forces. CH4 is nonpolar and only has dispersion
forces. The hydrogen bonding forces in NH3 are much stronger
compared to the dispersion forces in CH4.
b) At 25 degrees Celsius and 1.0 atm, ethane (C2H6)
is a gas and hexane (C6H14) is a liquid.
The most important intermolecular
attractive force in C2H6 and C6H14
are London dispersion forces. C6H14 has
a higher boiling point because hexane has more electrons and is a larger
(has a greater volume) compared to C2H6.
C6H14 is more polarizable, has the higher boiling
point, because it has more electrons than C2H6.
Since the C2H6 and C6H14
have the same intermolecular attractive forces and involve atoms from the
same period, the more polarizable the higher the boiling point.
c) Si melts at a much higher temperature (1,410 degrees Celsius)
than Cl2 (-101 degrees Celsius).
A sample of Si consists of Si
atoms covalently bonded together in a network solid (macromolecule). A sample
of Cl2 consists of nonpolar Cl2 molecules.
The Cl2 molecules are attracted by weak London dispersion
forces. The energy required to overcome strong covalent bond to melt Si
is much greater compared to the weak intermolecular dispersion forces in
Cl2.
d) MgO melts at a much higher temperature (2,852 degrees Celsius)
than NaF (993 degress Celsius).
Both MgO and NaF are ionic solids.
The attractive forces holding the ions together are ionic. The electrostatic
attractive force between ions with a +2 and -2 charge is much greater compared
to the electrostatic force between ions with a +1 and -1 charge.