exam3fall01 - ENSC 3313, Fall 2001 A~swer alllO Questions....

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g~ ENSC 3313, Fall 2001 TEST 3 Name A~swer alllO Questions. 2 points each. Depicted is a text sketch of a silicon-oxygen compound. What is -the coordination number of the a) silicon, b) oxygen? What would the chemical formula of the compound be? ].ICUI\E 13.10 The arrangement of silicon and oxygen atoms in a unit cell of cristobalite, a polymorph of SiO2- * The coordination number is the number of equivalent nearest neighbors. By inspection, we have Si = 4, O = 2. * From the foregoing, there must be 2x as much. oxygen as silicon, hence SiO2. 1'%\ 0 2- ~ .Si4+ --Alternatively, we can count; we have 16 oxygen in the unit cell and 8 silicon ( 4 fonning an FCC network and 4 additional interior ions). 2 Explain the following with respect to polymers. a) plasticizers A small stable high boiling point organic molecule added to a polymer to lower its glass transition temperature. It fits between chains so facilities chain sliding. It is used in vinyls, for example. b) polydispersivity index pm -Mm /Mn ---the ratio of mass average to number average molecular weight. It is a measure of the uniformity of polymerization. c) tacticity The arrangement of side groups along the polymer chain. The choices are iso-(same side), syndio-(altemating sides) and a-(random). The former two permit crystallization, hence superior
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This note was uploaded on 01/10/2012 for the course ENSC 3313 taught by Professor Staff during the Fall '11 term at Oklahoma State.

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exam3fall01 - ENSC 3313, Fall 2001 A~swer alllO Questions....

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