CHEM104
Spring 2009
QUIZ 2 - ANSWERS
1. Both NaCl and MgO crystallize in the same cubic structure where the anions
occupy the sites that define a fcc lattice and the cations occupy all the
octahedral sites. The unit cell for NaCl is shown in Fig. 1.9.
How will the lattice energy U of NaCl compare to U of MgO?
The lattice energy U of NaCl will be much smaller than that for U of MgO due
to the difference in charges on the ions. The (Z+)(Z-) charge product for NaCl
is -1 while the same for MgO is -4, which will make U(MgO) about 4x larger
than U(NaCl). There may be additional difference due to the ion separation
distance. T might be expected that Mg—O will be shorter than NaCl making
U(MgO) still arger.
2. The melting point of NaCl is 801 deg C while the m.p. of NaI is 660 deg C.
Assuming that these solids have the same structure, explain the difference in
melting points.
The lattice energies of NaCl as compared to NaI will have the same (Z+)(Z-)
charge product and the same geometrical factor since they have the same
structure. The difference will mainly be due to the distance between the ions
where Na—Cl will be shorter than Na—I. Distance affects U inversely, so
U(NaI) is smaller than U(NaCl) and consequently can be destroyed by
melting at a lower temperature.
3. Dr. B. tells you that she is doing a reaction whose synthesis procedure
uses LiCl. However, all she can find in her lab is KCl. She asks you if you
think substituting KCl for LiCl will work exactly the same. You pull out your
trusty chemistry text and point to the data in Table 14.1 and say “But Dr. B.,
these numbers show that (what do you say here?)!”
. …. The dissolution of LiCl is EXOTHERMIC by about 37 kJ/mol but KCl is
endothermic by about 17kJ/mol. So if your procedure needs the heat
generated or needs to easily dissolve the LiCl, the KCl may not completely
dissolve or it might become too cool to do the right reaction.
4. The atmospheric pressure on other planets is not necessarily 1 atmosphere
as it is on Earth. Imagine a planet whose “normal” atmospheric pressure is 8
atmospheres. Now imagine the behavior of CO2 on this planet: would the
term “dry ice” be a misnomer on this planet? Explain why.
Look at Figure 13.19, the phase diagram for CO2. If one starts on the left at a
pressure of about 8 atm and moves right ( increasing the T), the solid CO2
will melt and go into a liquid phase above -60 deg C then will evaporate into a
gas at about ~ -30 deg C. So the frozen CO2 will indeed become “wet” and
produces a liquid phase, unlike what it does on Earth at 1 atm pressure.
So yes, the term “Dry Iice” would really confuse the locals!