afternoon-notes - CHAPTER 13 Chemical Bonds Ionic bonding...

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CHAPTER 13 ¾ Chemical Bonds 1 Chemical Bonding ¾ Ionic bonding ± Energies associated with ionic bonding ¾ Covalent bonding ± Drawing dot structures 2 Chemical Bonding ¾ So what makes an ionic compound so stable? 3 Formation of Ionic Compounds ¾ Ionic compounds form extended three dimensional arrays of oppositely charged ions. ¾ Ionic compounds have high melting points because the coulomb force, which holds ionic compounds together, is strong. 4 Figure 13.9: The structure of lithium fluoride Extra stability associated with each ion surrounded by opposite charge . 5 Lattice Energy ¾ Salts crystals are formed by the transfer of electrons: ± the mutual attraction of cations and anions bring about the formation of formula units ± the magnitude of the opposite charges ultimately bring about the formation of an ionic crystal. ± The strength of the attraction between opposing charges in the crystal is determined by the lattice energy. 6
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Lattice Energy ¾ Lattice energy is the change in energy that occurs when the separated gaseous ions are packed together to form an ionic solid. energy ) s ( MX ) g ( X ) g ( M + → + + 7 = r Q Q k Energy Lattice 2 1 r = average distance between cations and anions k = a constant for this crystal structure Q 1 , Q 2 = charges on the ions For an ionic compound this will be negative. . energy is RELEASED. 8 Lattice Energy ¾ I E Li = 520 kJ/mol ¾ E A F = -328 kJ/mol ¾ Note that net energy change is still positive – less stable! ¾ Why does LiF form? 9 ¾ Let’s look at all the energies associated with the formation of an ionic compound… Example: The formation of lithium fluoride from its elements. ) s ( LiF ) g ( F 2 / 1 ) s ( Li 2 + Lattice Energy 10 Figure 13.8: The energy changes involved in the formation of solid lithium fluoride from its elements. Sublimation of solid lithium. Ionization of lithium atoms. Dissociation of ½ mole of F 2 molecules. Formation of F - ions. (Energy from electron affinity.) Formation of solid LiF. (Lattice energy). 11 Thought Questions! 1. Lattice Energy will be large for: A. small r (close spacing - permitted by small ions) B. large r (large spacing required by larger ions) A. small r (close spacing - permitted by small ions) 2. Lattice Energy will be large for: A. large Q (highly charged ions (+2,-3 for example) B. small Q (low charged ions (+1, -1 for example) A. large Q (highly charged ions (+2,-3 for example) 12
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Thought Questions! 1. Which would have the larger lattice energy? A. NaCl B. KCl A. NaCl (Na+ smaller than K+) 2. Which would have the larger lattice energy? A. CaO B. KCl A. CaO (Ca 2+ and O 2 have larger charges and have smaller radii than K + and Cl ) 13 Lattice Energy • Consider magnesium oxide (MgO) IE 1 Mg = 735 kJ/mol IE 2 Mg = 1445 kJ/mol EA 1 O = -141 kJ/mol EA 1 O = 878 kJ/mol What is your expectation of the lattice energy – large or small?
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afternoon-notes - CHAPTER 13 Chemical Bonds Ionic bonding...

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