Name TA Name Lab Sec # ALL work must be shown to receive full credit. Due Due in lecture at 1:30 p.m. Wednesday, January 28th

Transcription

1 Chem 1515 Section 2 Problem Set #2 Spring 1998 Name TA Name Lab Sec # ALL work must be shown to receive full credit. Due Due in lecture at 1:30 p.m. Wednesday, January 28th PS2.1. Indicate the type of crystal (molecular, metallic, extended covalent, or ionic) each of the following would form upon solidification: a) CO 2 ; molecular g) KBr ; ionic b) SiO 2 ; extended covalent h) H 2 O ; molecular c) Si ; extended covalent i) NaOH ; ionic d) CH 4 ; molecular j) U ; metallic e) Ru ; metallic k) PH 3 ; molecular f) I 2 ; molecular l) CaCO 3. ionic PS2.2. A cell of nickel arsenide is shown below. If this cell contains the unit cell for this compound, determine the correct formula for nickel arsenide. There is 1 nickel atom per unit cell as a result of the 1 8 contributions from each of the eight corner nickel atoms. There is one nickel atom per unit cell as a result of the the 1 4 the four edgecentered nickel atoms. The 2 atoms of arsenic are completely contained within the unit cell. Formula is NiAs. There are two formula units of NiAs per unit cell. CHEM Spring 1998

2 PS2.3. Perovskite is a mineral containing calcium, titanium and oxygen. Two different cells are shown below. Support or refute whether or not the two structures contain the same number of atoms? Figure I. Figure II. The cell in Figure I. has; one titanium atom located in the center of the cubic cell; one calcium atom from the eight 1 8 three oxygen atoms from the six 1 2 the six face centered atoms. CaTiO 3 The cell in Figure II. has; one titanium atom from the eight 1 8 one calcium atom located in the center of the cubic cell; three oxygen atoms from the twelve 1 4 contributions from each of the six face centered atoms. CaTiO 3 CHEM Spring 1998

3 PS2.4. A particular solid has a structure in which W atoms are located at cube corners, O atoms at the centers of the cube edges, and Na atoms at cube centers. The cube edge is 3.86 Å. a) What is the formula of this material? (Show some work.) one tungsten atom from the eight 1 8 one sodium atom located in the center of the cubic cell; three oxygen atoms from the twelve 1 4 contributions from each of the six face centered atoms. NaWO 3 b) What is it theoretical density? mass in the unit cell density = = 255 volume of the unit cell x formula units (3.86 x 10-8 cm) 3 g mol = 4.23 x g 5.75 x cm 3 = 7.36 g cm 3 PS2.5. Europium metal crystallizes in a body-centered cubic unit cell. The density of europium is 5.26 g cm3. Calculate the edge length of the unit cell and the atomic radius of europium. volume = 1 cm g 152 g volume = 2.40 x cm 3 edge length = 3 r = x atoms 2.40 x cm 3 = 2.88 x 10-8 cm edge length 2 = 2.88 x 10-8 cm 2 = 1.02 x 10-8 cm 1 unit cell 2 atoms PS2.6. Using radius ratios, suggest a probable structure for each of the following ionic crystals. Describe the structures of these ionic crystals. a) AgCl (Ag Å:Cl 1.67 Å) c) SrCl 2 (Sr Å:Cl 1.67 Å) 1.16 Å 1.67 Å = Å 1.67 Å = The anions are cubic closest The cations are cubic closest packed packed with cations in all of the with anions in all of the tetrahedral octahedral holes holes. CHEM Spring 1998

4 PS2.6. (Continued) b) BeS (Be Å:S Å) d) CsI (Cs Å:I 2.06 Å) 0.41 Å 1.70 Å = Two possible structures, but we only discussed cubic closest packed so that is the description to look for. The anions are cubic closest packed with the cations in half the tetrahedral holes 1.81 Å 2.06 Å = The anions are in a simple cubic structure with th cations in the cubic hole. PS2.7. A superalloy of nickel and aluminum consists of aluminum atoms at the corners of a cube and nickel atoms in the center of each face of the cube. What is the composition of this superalloy? one aluminum atom from the eight 1 8 three nickel atoms from the twelve 1 4 the six face centered atoms. AlNi 3 PS2.8. The compounds K 2 O, CuI and ZrI 4 all can be described as cubic closest packed anions with the cations in tetrahedral holes. What fraction of the tetrahedral holes is occupied in each compound? K 2 O : 4 O 2 in the unit cell. From the formula K 2 O there are 8 K + in the unit cell. Since there are eight tetrahedra holes all of the holes are filled. CuI : 4 I in the unit cell. From the formula CuI there are 4 Cu + in the unit cell. Since there are eight tetrahedra holes half of the holes are filled. ZrI 4 : 4 I in the unit cell. From the formula ZrI 4 there is one Zr 4+ in the unit cell. Since there are eight tetrahedra holes one-eighth of the holes are filled. CHEM Spring 1998

5 PS2.9. In the phase diagram for water shown below; Pressure, mmhg Temperature C a) determine the physical state of water at i) 900 mmhg and 40 C liquid ii) 500 mmhg and 30 C liquid iii) 300 mmhg and 90 C gas b) At 400 mm Hg what is the approximate temperature needed to convert water from a solid to a liquid? 0 C c) What is the approximate pressure at which water changes from a liquid to a gas at 80 C? 350 mmhg CHEM Spring 1998