Lecture 17

# Lecture 17 - Which has more energy H2 or H H NaCl or Na...

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Which has more energy? H 2 or H + H NaCl or Na + + Cl - H 2 O( ) or H 2 O(g) 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. All of the above reactions are endothermic, with a positive H, since energy is used to break a bond. The bonds in these examples are either covalent (H 2 ), ionic (NaCl) or intermolecular (between H 2 O molecules in a liquid). They differ in strength, but in all cases, the H is positive, meaning that the heat used to break the bond is stored in the product (with bonds broken). The products (with bonds broken) have more energy. When a bond is formed, energy is released, and the system goes to a state of lower energy. 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.

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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. The potential energy curve for bond formation is shown below. When the atoms are far apart, the energy is zero, or slightly below zero, since there’s a slight attraction, except when r = ∞. As they approach, the energy decreases, reaching a minimum at a certain distance, known as the bond distance. Any closer, and the energy increases due to repulsive forces.
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. Covalent Bonds

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## This note was uploaded on 02/15/2010 for the course CHEMISTRY 160:161 taught by Professor Paulkimmel during the Fall '09 term at Rutgers.

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Lecture 17 - Which has more energy H2 or H H NaCl or Na...

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