Spring 2010

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Drexel University *

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220

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Chemistry

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Jan 9, 2024

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ENGR 220: Mid-Term Exam #2 Name: Section Spring 2010 R. Knight (Print) 50 mins. Closed book, closed notes exam. Useful formulae, constants and a periodic table are attached (at the end). You may also use a calculator and ruler. Please show all working for full credit. 1. For the following questions choose the correct answer and/or fill in the blanks, as necessary. i) 3 pts. In covalent molecules such as methane, the bonding is dueto_ | — | between [ so that o hal hal i1) 2 pts. The predominant bonding within the chains of a polymer such as polyethylene is (circle one) A. Ionic B. Hydrogen @. Covalent D. Van der Waals i11) 2 pts. C1n Fe is an example of anlinterstitialkubstitutional (circle one) solid solution. | i [ ] iv) 3 pts. Polymer crystallinity 1s said to occur when | —© v) 4 pts. The coordination number (CN) for octahedral interstitial sites iower (circle one) than that for the tetrahedral sites. There are = tetrahedral sites per FCC unit cell. That number was obtained as follows: - vi) 3 pts. The equation for Fick’s first law of diffusionis [~ and in essence relates oy to =7 o . vil) 3 pts. Interstitial diffusion 1s normally morlower (circle one) than vacancy diffusion because hal viil) 2 pts. For a ceramic compound, which two characteristics of the component i1ons determine the crystal structure? [ ix) 3 pts. Carbon can exist in one/twc four (circle one) polymorphic forms. These forms are: g, x) 2 pts. Graphite 1s often used as a solid lubricant because (explain in terms of the structure and bonding) x1) 2 pts. A syndiotactic polymer has its side groups located all on the same mde/on'alternate' sides/randomly (circle one). xi1) 2 pts. Polymer chains comprising two or more different repeat units are known as [ xii1) 3 pts. The three stages of free radical polymerization are: - _ xiv) 2 pts. Thermoplastic polymercannot (circle one) be remelted because xv) 4 pts. Galvanic corrosion involves a reaction at the and a reaction at th eaction at the | xvi) 3 pts. Metals and polymers 0 not (circle one) interact with their environment. In the case of metals this may occur as or R This study source was downloaded by 100000836502887 from CourseHero.com on 11-08-2021 16:27:42 GMT -06:00 https://www.coursehero.com/file/12257575/ENGR-220-Sample-Mid-Term-Exam-1I-RK-Rev/

ENGR 220: Mid-Term Exam #2 Name: Section Spring 2010 R. Knight (Print) 50 mins. 2. (a) 15 pts. For a steel alloy 1t has been determined that a carburizing heat treatment of 25 hours duration will increase the carbon concentration to 0.55 wt.% at a point 3.0 mm from the surface. Estimate the time required to achieve the same concentration at a position 6.0 mm from the surface for an identical steel and at the same carburizing temperature. (b) 15 pts. Complete the table below and determine the number average molecular weight of the polymer. Is 1t possible to have a PVC homopolymer (repeat unit shown) with the molecular weight data shown below and a degree of polymerization of 1120? Briefly explain why, or why not? Molecular Weight Range Mean M; X; x;M; (g/mol) 8,000-20,000 0.05 H H 20,000-32,000 0.15 | | 32,000-44,000 0.21 —C—C— 44.,000-56,000 0.28 | | 56,000-68,000 0.18 H 68,000-80,000 0.10 80,000-92,000 0.03 /30 This study source was downloaded by 100000836502887 from CourseHero.com on 11-08-2021 16:27:42 GMT -06:00 https://www.coursehero.com/file/12257575/ENGR-220-Sample-Mid-Term-Exam-II-RK-Rev/

ENGR 220: Mid-Term Exam #2 Name: Section Spring 2010 R. Knight (Print) 50 mins. 3. (a) 15 pts. Compute the theoretical density of NaCl, given that is has the rock salt crystal structure. Hint: Assume that the anions and cations are just touching along the cube edges and across the face diagonals. Ana= 22.99 g/mol. Ac; = 35.45 g/mol. rna+ = 0.102 nm, rc. = 0.181 nm. (b) 12 pts. Show that the minimum cation-to-anion radius ration for a coordination number of 3 1s 0.155. /27 This study source was downloaded by 100000836502887 from CourseHero.com on 11-08-2021 16:27:42 GMT -06:00 https://www.coursehero.com/file/12257575/ENGR-220-Sample-Mid-Term-Exam-1I-RK-Rev/

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