edited bv:
exam queftion exchange Using Palladium Hydride To Introduce Crystal Structures David F. Rleck Richard A. Henson Schcol of Science and Technology Salisbury State University. Salisbury, MO 21801
The study of crystal lattice geometries and the relationship between the structure and empirical formula are important components of an inorganic chemistry course. The recent interest in palladium hydrides' provides an excellent means of introducing the topics of interstitial hydrides, nonstoichiometric compounds, and crystal geometries. The following exercise can be used effectively as either a lecture presentation, or an assignment. Problem
Palladium forms an interstitial hydride which may be described as a face-centered cuhic Pd lattice with H occupying octahedral holes only. If a maximum of approximately 70% of these holes can he occupied, how many grams of hydrogen can be contained in 1L of Pd (assume no change in
1034
Journal of Chemical Education
JOHN J. ALEXANDER Univer~ity01 Cincinnati Cincinnati. Ohio 45221
the volume upon absorption of hydrogen)? Compare this value to the density of liquid hydrogen. How many liters of HZgas (measured at STP) could be dissolved into 1L of Pd? Solution
Face-centered cubic with octahedral holes occupied is the NaCl structure; thus-the hydride's formula must be PdHa7. The density of Pd is 12 X 103 g/L, which corresponds to 113 molPd/L. If 0.7 molof hydrogen are present for eachmole of Pd, there will be 79.1 mol of H atoms and thus 79.9 g of hydrogen per liter. The density of liquid hydrogen is 70.8 g per liter. Using the ideal gas law one can determine that the amount of hydrogen that could be contained in 1 L of Pd would correspond to approximately 895 L of Hp gas at STP. While the calculations are not difficult, students seem to be truly excited by the results, especially by the fact that 1L of Pd can contain significantly more hydrogen than is present in 1L of liquid hydrogen.
' Fleischmann, M.; Pons, S. J. Electroanal. Chem. 1989,261,301308.
