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CHEMISTRY 121 1 Department of Chemistry University of British Columbia Problem Set 7 Answers p-Block Elements 1. A zeolite containing alkali metal ions (M) has the formula M n [(AlO 2 ) 2 (SiO 2 ) 4 ]. What is n to preserve electrical neutrality ? n = 2. We can think of M n [(AlO 2 ) 2 (SiO 2 ) 4 ] as a form of SiO 2 where some (1/3) of the Si atoms have been replaced with Al atoms, which have one valence electron less than Si. Thus the [(AlO 2 ) 2 (SiO 2 ) 4 ] unit requires two extra electrons (one for each Al) to satisfy its bonding requirements, and therefore carries an overall -2 charge. Thus two alkali metal cations are requirement to achieve electrical neutrality. 2. Which crown ether would you expect to complex Ca 2+ most strongly: (a) 12-crown-4 (b) 15-crown-5 (c) 18-crown-6 (d) 21-crown-7 The answer is: B Consulting a table of ionic radii (such as Figure 9-8 in the textbook), reveals that the radius of Ca 2+ is 100 pm, very close in size to Na + (99 pm). Therefore, 15- crown-5, which complexes Na + strongly is also expected to complex Ca 2+ strongly. NOTE: This “diagonal relationship” between the size of Ca 2+ and Na + is also found for Li + and Mg 2+ (similar sizes) and, likewise Sr 2+ and K + have similar sizes. 3. For each of the following guests in 18-crown-6 select all of the interactions involved in stabilizing the complex from the following: (1) London forces; (2) hydrogen bonding; (3) dipole-dipole interactions; (4) ion-dipole interactions. (a) water The answers are: 1, 2 and 3 All guests will be stabilized by London or dispersion forces. Water can also hydrogen bond to the oxygen atoms in the crown, and can engage in dipole-dipole interactions since water is polar and the oxygen-carbon bonds in the ring have a dipole moment.
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This note was uploaded on 11/03/2009 for the course ECON 210 taught by Professor James during the Spring '09 term at UBC.

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