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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