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Unformatted text preview: Chem 105 Nielson Recitation 10/25/07
Boiling Point Unit Cell of solids
Denisty of solids 1. Which molecule in each of the pairs has the lower vapor pressure? The higher boiling
point? a. CH3CH2CH2CH2CH3 CH3CH2CH2 CH2CH2CH3 Ans. CH3CH2CH2 CH2CH2CH3 b. NH3 PHS
c. HF LiF Ans. LiF (Yes, its an ionic solid.)
2. Why does water expand when it freezes?
Ans. Water molecules align to produce six-sided channels in ice to maximize hydrogen bonding. The empty volume of the channels results in a lower density in
the solid. 3. Draw the phase diagram for a substance with the following properties: Tb = 90 K, Tm
= 55 K, triple point at 54 K and 40 torr, critical T = 100 K, critical P = 950 torr. What
does it mean when the material is above the critical T and P? Ans. The material has
both liquid and gas properties. The high P causes the molecules to be close together
and the high T causes the molecules to have a lot of energy so that they don’t interact
with neighboring molecules. k .
4. Use the phase diagram for )COz ii? the chapter to answer the following questions. a. What state is C02 in at 13 atm and 0 °C?
Ans. It looks like it is on the liquid-gas phase line. b. What is the name for the phase change of C02(s) ------- > C02(g)
Ans. Sublimation a. What is unique about the triple point?
Ans. It is the point at which the solid, liquid, and gas of C02 exists in equilibrium. 5. For each of the following, indicate whether the solid is ionic, metallic, covalent, or
molecular. a. MgBrz Ans. Ionic: Mg is a metal. Br is a nonmetal. b. Cdng Ans. Metallic: both the elements are metals. 0. Ge Ans. Network: Ge is a group IV metalloid like Si. Group IV elements
such as C, Si, and Ge form covalent bonds throughout their solid structures. d. C12 Ans. Molecular: Cl is nonmetal. The two Cl atoms are covalently bonded to each other and have induced dipole-induced dipole intermolecular
attractions to other C12 molecules. e. SC14 Ans. Molecular: Like C1, these elements are nonmetals and covalently
bond to make a molecule which has weak intermolecular attractions to other
molecules. f. AlNizTi Ans. Metallic: all the elements are metals. . CuSO4 Ans. Ionic: formed from the ions Cu2+ and 8042'. The fact that the 8042' anion is has covalent bonds in it, does not make in a molecular solid. h. AlAs Ans. Network: This one is hard. This compound is isoelectronic with
group IV elements, so the structure is diamond cubic with no identiﬁable molecules.
The electronegativity difference between Aland As is small, so the bonding is
covalent. (IO 6. Iridium has a face- centered cubic (fcc) unit cell and a density of 22.56 g/cm3 . a. How man atoms in a fcc unit cell? Ans. 4 atoms ,
1/ 5 (0595 CW1 c. What IS the length of an edge of the unit cell? Ans. 3. 840 x 10'8 cm Area 33W = [a 3 5, (0635 x1616”? : 335674 ”IO-80M d. What IS the radius 1n angstroms of an iridium atom? Ans. 1. 357 ang strom E g
—1o"°m11+12 w J7. 1 1A 1 = 5L: «(WWW (a ”(3%?” W" 7. Thallium (I) chloride crystallizes 1n either a simple cubicz(sc) or a face- centered cubic: :1 5‘3 74 X ID (fcc) unit cell of C1 ions in the lattice with T1 ions in the lattice holes. The density of the solid is 7.00 g/cm3 and the edge of the unit cell is 385 pm. What is the unit cell 0W
geometry? Ans. Simple cubic like CsCl- structure in the chapter.
If sc, then only 1 TlCl unit ner unit cell. If fcc, then 4 T1Cl units per unit cell. .
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- Spring '07