Redox Reaction Notes - Lecture #12 Redox Reactions...

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Redox Reactions Chemistry 142 B James B. Callis, Instructor Autumn Quarter, 2004 Lecture #12
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This lecture covers two topics: Assignment of oxidation numbers Balancing of redox equations
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Oxidation - A process by which a substance (reductant) gives up electrons to another substance (oxidizing agent). The reductant is oxidized. Reduction - A process by which a substance (oxidant) accept electrons from another substance (reducing agent). In a chemical reaction, the total number of electrons are conserved; so also are the number of charges. Thus, it is convenient to assign fictitious charges to the atoms in a molecule and call them oxidation numbers. Oxidation numbers are chosen so that (a) conservation laws are obeyed, and (b) in ionic compounds the sum of oxidation numbers on the atoms coincides with the charge on the ion.
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Rules for Assigning Oxidation Numbers: 1. The oxidation numbers of the atoms in a neutral molecule must add up to zero, and those in an ion must add up to the charge on the ion. 2. Alkali metal atoms have oxidation number +1, alkaline earth atoms +2, in their compounds. 3. Fluorine always has oxidation number -1 in its compounds. The other halogens have oxidation number -1 in their compounds except those with oxygen and with other halogens, where they can have positive oxidation number.
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Oxidation Numbers (cont.): 4. Hydrogen is assigned oxidation number +1 in its compounds except in metal hydrides such as LiH, where convention (2) takes precedence and hydrogen has oxidation number -1. 5. Oxygen is assigned oxidation number -2 in compounds. There are two exceptions: in compounds with fluorine, convention 3 takes precedence, and in compounds that contain O - O bonds, conventions 2 and 4 take precedence. Thus the oxidation number of oxygen in OF 2 is +2: in peroxides (such as H 2 O 2 and Na 2 O 2 ) it is -1. In superoxides (such as KO 2 ) oxygen's oxidation number is -1/2.
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Examples: NH 4 NO 3 N = H = N = O = B 2 H 6 B = H =
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Examples (cont.): BaH 2 Ba = H = (S 4 O 6 ) 2- S = O =
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Problem 12-1: Determining the Oxidation Number of an Element in a Compound Problem: Determine the oxidation number (ox. no.) of each element in the following compounds. a)
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Redox Reaction Notes - Lecture #12 Redox Reactions...

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