Lecture 14sf

Lecture 14sf - Oxidation-Reduction Reactions 2Mg + O2 2MgO...

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Oxidation-Reduction Reactions 2Mg + O 2 → 2MgO In this combustion reaction, Mg starts as an element (zero charge) and ends as an ion (2+ charge) in MgO. The Mg atom must lose 2 electrons to go from 0 charge to 2+ charge. In this same reaction, oxygen starts as zero charge (as an element in O 2 ), and becomes 2- charge (as an ion in MgO. Each O atom must gain 2 electrons to go from 0 charge to 2- charge. Whenever an element loses electrons and goes up in charge, we say that element is oxidized . Oxidation: Loss of electrons and increase in charge (oxidation number). Whenever an element gains electrons and goes down in charge, we say that element is reduced . Reduction : Gain of electrons and decrease in charge (oxidation number). Oxidation and reduction always occur together. If an element gets oxidized (gives off electrons) there must be another element getting reduced (gaining electrons). Thus oxidation- reduction reactions are electron transfer reactions. You cannot have oxidation without reduction or reduction without oxidation. They are complementary processes. . Consider the reaction: 2Na + Cl 2 → 2NaCl In this reaction Na starts at 0 charge and becomes Na + in NaCl The sodium atom gives off an electron to become Na + . It gets oxidized. In this reaction Cl begins as an element with zero charge, and becomes Cl - in NaCl. The Cl atom gains an electron to become Cl - . It gets reduced. Consider the reaction between copper metal and silver nitrate solution. Words: Copper + silver nitrate → Silver + copper nitrate Full formula: Cu + 2AgNO 3 → 2Ag + Cu(NO 3 ) 2 Net ionic: Cu(s) + 2Ag + (aq) → 2Ag(s) + Cu 2+ (aq) Cu gives off 2 electrons to become Cu 2+ (gets oxidized) Each Ag + gains one electron to become Ag (gets reduced)
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are a way to keep track of what elements get oxidized or reduced in reactions Some general guidelines on oxidation numbers: Oxidation Number Elements by themselves 0 Simple ions the charge of that ion Oxygen -2 in all compounds except peroxides Hydrogen +1 in compounds except metallic hydrides The sum of all oxidation numbers equals the total charge on the compound or ion Compare oxidation number for nitrogen in a variety of compounds: Compound or ion Oxidation number of N NO 3 - HNO 3 +5 NO 2 N 2 O 4 +4 NO 2 - HNO 2 +3 NO +2 N 2 O +1 N 2 0 NH 3 -3
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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 14sf - Oxidation-Reduction Reactions 2Mg + O2 2MgO...

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