Chpt 8 Solutions


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247 CHAPTER 8 BONDING: GENERAL CONCEPTS Questions 15. In H 2 and HF, the bonding is covalent in nature, with the bonding electrons pair shared between the atoms. In H 2 , the two atoms are identical, so the sharing is equal; in HF, the two atoms are different with different electronegativities, so the sharing in unequal, and as a result, the bond is polar covalent. Both these bonds are in marked contrast to the situation in NaF. NaF is an ionic compound, where an electron has been completely transferred from sodium to fluorine, producing separate ions. 16. In Cl 2 the bonding is pure covalent, with the bonding electrons shared equally between the two chlorine atoms. In HCl, there is also a shared pair of bonding electrons, but the shared pair is drawn more closely to the chlorine atom. This is called a polar covalent bond as opposed to the pure covalent bond in Cl 2 . 17. Of the compounds listed, P 2 O 5 is the only compound containing only covalent bonds. (NH 4 ) 2 SO 4 , Ca 3 (PO 4 ) 2 , K 2 O, and KCl are all compounds composed of ions, so they exhibit ionic bonding. The ions in (NH 4 ) 2 SO 4 are NH 4 + and SO 4 2 . Covalent bonds exist between the N and H atoms in NH 4 + and between the S and O atoms in SO 4 2 . Therefore, (NH 4 ) 2 SO 4 contains both ionic and covalent bonds. The same is true for Ca 3 (PO 4 ) 2 . The bonding is ionic between the Ca 2+ and PO 4 3 ions and covalent between the P and O atoms in PO 4 3 . Therefore, (NH 4 ) 2 SO 4 and Ca 3 (PO 4 ) 2 are the compounds with both ionic and covalent bonds. 18. Ionic solids are held together by strong electrostatic forces that are omnidirectional. i. For electrical conductivity, charged species must be free to move. In ionic solids, the charged ions are held rigidly in place. Once the forces are disrupted (melting or dissolution), the ions can move about (conduct). ii. Melting and boiling disrupts the attractions of the ions for each other. Because these electrostatic forces are strong, it will take a lot of energy (high temperature) to accomplish this. iii. If we try to bend a piece of material, the ions must slide across each other. For an ionic solid the following might happen: Just as the layers begin to slide, there will be very strong repulsions causing the solid to snap across a fairly clean plane. strong attraction strong repulsion
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CHAPTER 8 BONDING: GENERAL CONCEPTS 248 iv. Polar molecules are attracted to ions and can break up the lattice. These properties and their correlation to chemical forces will be discussed in detail in Chapters 10 and 11. 19. Electronegativity increases left to right across the periodic table and decreases from top to bottom. Hydrogen has an electronegativity value between B and C in the second row and identical to P in the third row. Going further down the periodic table, H has an electro- negativity value between As and Se (row 4) and identical to Te (row 5). It is important to know where hydrogen fits into the electronegativity trend, especially for rows 2 and 3. If you
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This note was uploaded on 10/27/2010 for the course CHEM 102 taught by Professor Peterpastos during the Spring '08 term at CUNY Hunter.

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