Chapter 18 - Oxidation is the loss of electrons by a...

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Unformatted text preview: Oxidation is the loss of electrons by a chemical species. When sodium (Na) forms a compound, Na + is formed, so sodium is oxidized. Reduction is the gain of electrons by a chemical species. Chloride ions (Cl- ) are formed when Cl 2 reacts, so Cl is reduced. An oxidation-reduction reaction , or redox reaction , is one in which electrons are transferred. In a redox reaction, at least one species is oxidized and at least one species is reduced. 2Na(s) + Cl 2 (g) 2NaCl(s) is a redox reaction, because Na is oxidized and Cl is reduced. Oxidizing agent is the reactant that gains electrons, causing an oxidation to occur. The oxidizing agent is reduced in the reaction. Reducing agent is the reactant that loses electrons, causing reduction to occur. The reducing agent is oxidized in the reaction. In the reaction of sodium with chlorine, Na is the reducing agent , and Cl 2 is the oxidizing agent . In a half-reaction either the oxidation or reduction portion of a redox reaction is given, showing the electrons explicitly. Half-reactions emphasize the transfer of electrons in a redox reaction. Na Na + + 1e- oxidation half-reaction Cl 2 + 2e- 2Cl- reduction half-reaction Oxidation state is the charge on a monatomic ion, or the charge on an atom when the shared electrons in covalent bonds are assigned to the more electronegative atom. Electron pairs shared by atoms of the same element are divided equally. Calcium in CaCl 2 , an ionic compound, has an oxidation state of +2. Hydrogen in H 2 O, has an oxidation state of +1. Determine the oxidation state of each atom in formaldehyde O is more electronegative than C, C is more electronegative than H O is assigned 8 electrons; oxidation state = - 2 C is assigned 4 electrons; oxidation state = 0 H is assigned no electrons; oxidation state = +1 C H H O Draw the Lewis structure and determine the oxidation states of each atom in SO 2 Draw the Lewis structure and determine the oxidation states of each atom in SO 2 Answer: S = +4, O = -2 1.Oxidation states for atoms in uncombined elements is 0. 2.The oxidation state of a monatomic ion is equal to the charge of the ion. Group IA is +1, group IIA is +2. 3.In compounds, F is always -1. Other halogens are -1 unless combined with a more electronegative element (oxygen or a halogen above it in the periodic table). 4.In compounds, H is +1 except in metal hydrides where it is -1. 5.In compounds, oxygen is always -2 except for peroxides (O 2 2- ) or combined with F 6.The sum of the oxidation states must equal zero in a neutral compound, or the charge of the ion for polyatomic ions. Assign oxidation states for each atom in K 2 CrO 4 Assign oxidation states to each atom in a) CO b) NH 4 Cl Assign oxidation states to each atom in a) CO b) NH 4 Cl Answer: a) C = +2, O = -2 b) N = -3, H = +1, Cl = -1 Balance the equation Cu + NO 3- Cu 2+ + NO + H 2 O ...
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