CH301H2007_ExamII_Key - CH 301H Fall 2007 Exam II CH 301H...

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CH 301H Fall 2007 Exam II 1-8 CH 301H 54785 Fall 2007 October 23, 2007 Professor Campion Exam II 1. What are the physical characteristics of metals? Of non-metals? Describe the characteristics so that I know that you understand them, do not merely list them. (5 pts.) ANS: Metals are shiny (highly reflective), ductile (can be drawn into wires), malleable (can be pounded or flattened into thin sheets), and good conductors of electricity and of heat (current will flow through a wire when a voltage is applied across its ends; heat will flow if there is a temperature difference between two sides of a piece of metal). Nonmetals are poor conductors of electricity and heat, brittle (easily fractured as opposed to stretched or flattened), not shiny (transparent or opaque). 2. The successive ionization energies of an element ( IE 1 through IE 6 ) increase as follows: 6.0 eV, 18.86 eV, 28.4 eV, 120.0 eV, 205 eV, 246.5. Based upon the trend observed, which elements might this be? Based upon the magnitudes of the ionization energies, which of these choices is the most likely possibility? Why? (10 pts.) ANS: The pattern suggests three electrons in an outer shell, because of the large jump between the third and fourth ionization energies. Boron and aluminum would be good possibilities; aluminum is the more likely choice because of the relatively low value of IE 1 . 3. What is electron affinity ( EA )? What is the general trend observed in electron affinities as we move from left to right across each of the first three rows of the periodic table? Use the shell model of the atom to rationalize the trend. (10 pts.) ANS: Electron affinity is the amount of energy released when an electron attached to a neutral atom to form an anion in the gas phase. By convention, electron affinities are positive quantities. Electron affinities increase as we move from left to right across a row in the periodic table. The shell model explains this trend by placing each electron into a shell of a fixed radius; each electron is located at about the same distance from the nucleus. As we move from left to right, however, the nuclear charge Z (or even better, Z eff )
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CH 301H Fall 2007 Exam II 2-8 increases so that the Coulomb attraction, and thus the electron affinity, increases moving in that direction. 4. Let’s assume that we can prepare all of the following as stable molecules in the gas phase. Assuming that the bond lengths are comparable and that the bonds are predominantly ionic, which bonds are the strongest and why? (10 pts.) NaCl KCl MgO CaCl 2 MgF 2 ANS: The MgO bond is the strongest bond because the Coulomb potential energy depends upon the product of the charges on the ions. V ( r ) = q 1 q 2 / 4 πε 0 r If the bond length r is the same for all bonds, then the relative bond strengths (in units of e 2 4 0 ) are NaCl (1), KCl (1), MgO (4), CaCl 2 (2) and MgF 2 (2). 5.
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CH301H2007_ExamII_Key - CH 301H Fall 2007 Exam II CH 301H...

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