lecture 7sf

lecture 7sf - Introduction to Bonding H 2 H + H requires...

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Unformatted text preview: Introduction to Bonding H 2 H + H requires energy to break covalent H-H bond NaCl Na + + Cl- requires energy to break ionic bond between Na + and Cl- H 2 O( ) H 2 O(g) requires energy to break intermolecular forces in the liquid phase. A bond is formed because of attractive forces. It requires energy to break the bond-- that energy is called the bond energy. Conversely, energy is released when a bond is formed. When a bond is formed, energy is released, and the system goes to a state of lower energy. H + H H 2 energy is released in forming covalent H-H bonds Na + + Cl- NaCl energy is released in forming ionic bonds between Na + and Cl- H 2 O(g) H 2 O( ) energy is released in forming intermolecular forces in the liquid phase An ionic bond is formed due to the electrostatic attraction of oppositely charged ions. When two charges, q 1 , q 2 , are separated by distance r, there is a force between them: F = 2 2 1 r q kq Energy = r q kq 2 1 The force will be attractive (leading to bond formation) if the charges are opposite (one charge positive, the other negative). The force will be repulsive (charges tend to move apart) if the charges are alike (both negative or both positive. There is a closely related equation for the energy of two charges q 1 , q 2 , separated by distance r. Energy = r q kq 2 1 When r = (charges very far apart), the energy = 0. When r < (charges moving together, the energy becomes greater if q 1 and q 2 are like charges (it takes work to bring them together since they repel each other), and the energy becomes less (more negative) when q 1 and q 2 are opposite charges. This lowering of the energy leads to bond formation. Ionic Bonds Ionic bonds are formed by a transfer of electrons from one atom to another. Sodium chloride, NaCl, is a typical example of an ionic compound. Na 1s 2 2s 2 2p 6 3s or [Ne]3s Cl 1s 2 2s 2 2p 6 3s 2 3p 5 or [Ne]3s 2 3p 5 The outer shell 3s electron gets transferred from the Na to the 3p orbital of Cl, resulting in: Na + 1s 2 2s 2 2p 6 Cl- 1s 2 2s 2 2p 6 3s 2 3p 6 The electrostatic attraction of the positive and negative ions results in the ionic bond. This may be represented as: where the 3s electron originally on Na is now on the Cl atom, making the Cl atom into a negative ion, and leaving Na as a positive ion. Lattice Energy An ionic lattice is a long-range array of positive and negative ions, held together by electrostatic attraction. The energy required to break apart this lattice into gaseousions is called the lattice energy. Using NaCl as an example: Energy required NaCl(s) Na + (g) + Cl- (g) 787 mol kJ Would you expect KI to have a higher or lower lattice energy than NaCl?...
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This note was uploaded on 02/20/2012 for the course 160 161 taught by Professor Kim during the Fall '08 term at Rutgers.

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lecture 7sf - Introduction to Bonding H 2 H + H requires...

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