Exam 3
December 5, 2000
Open book, open notes
1. Estimate the sum of the first three electron affinities for nitrogen
using data from your textbook and the following: aluminum nitride crystallizes
in the wurtzite structure and has a heat of formation of –318.0 kJ/mol;
the sublimation energy of aluminum is 330.0 kJ/mol. Answer
2. The unit cell for a simple cubic cell has one sphere at each corner
of the cube. Show that the fraction of filled space is 0.524. Assume that
the spheres touch each other along every direction. Volume of a cube =
a3 where a is the length of a side; volume of a sphere = 4πr3/3
where r is the radius of the sphere. Answer
3. Molecular hydrogen is predicted to become metallic at extremely high
pressures. Explain how this could happen using a simple band picture. Start
with the molecular orbital diagram for gas phase hydrogen and show the
orbital energy diagram as the hydrogen condenses and then is placed under
increasing pressure. Could hydrogen form a semiconductor? Why or why not?
Answer
4. Use Pauling's rules to estimate the pKa values for H2CO3. The reported values for carbonic acid are pKa1 = 6.37 and pKa2 = 10.32. Do the estimated values agree with the experimental values? Why or why not? Answer