08_-_Electrochemistry

08_-_Electrochemistry - ChE102:ChemistryforEngineers

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ChE 102: Chemistry for Engineers 20.1 – 20.8 – Electrochemistry 
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Oxidation States (Chap 5) Oxidation state (or number) is related to the number of electrons that an atom gains, loses, or otherwise uses when combining with other atoms in a compound The concept of oxidation state is arbitrary it does not necessarily have a fundamental meaning it is useful when understanding oxidation-reduction reactions 2 08 - Electrochemistry
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Examples of Oxidation States NaCl: Na has oxidation state +1 Cl has oxidation state -1 3 08 - Electrochemistry
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NaCl: Na loses 1 electron → Na + Cl gains 1 electron → Cl - 4 08 - Electrochemistry
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Rules for Assigning Oxidation States (O.S.) The O.S. of an individual atom in a free element is 0 Cl, Cl 2 , N 2 , O 2 , O, etc. The total of the O.S. of all atoms in a neutral species is 0 H 2 O (H: +1, O: -2) The total of the O.S. of all atoms in an ion equals the charge of the ion Fe 3+ (Fe: +3), MnO 4 - (Mn: +7, O: -2) Group 1 metals (Li, Na, K, Rb, Cs, Fr) have O.S. = + 1 Group 2 metals (Be, Mg, Ca, Sr, Ba, Ra) have O.S. = +2 F has O.S. = -1 Cl, Br, I, and At have O.S. = -1 except when combined with oxygen and other halogens 5 08 - Electrochemistry
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H has O.S. = +1, except when bonded to metals when its O.S. = -1 (LiH, for instance) O has O.S. = - 2 except for peroxides (H 2 O 2 and N 2 O 2 where O.S. = -1 for O) or when combine with F (OF 2 where O.S. = +2 for O) In binary compounds with metals: group 17 elements (F, Cl, Br, I, At) have O.S. = -1 group 16 elements (O, S, Se, Te, Po) have O.S. = -2 group 15 elements (N, P, As, Sb, Bi) have O.S. = -3 All other oxidation numbers are selected to make the algebraic sum of the oxidation numbers equal to the net charge of the molecule or ion Rules for Assigning Oxidation States (O.S.) 6 08 - Electrochemistry
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Reduction-Oxidation (REDOX) Reactions In a reduction-oxidation reaction, one of the reagents is oxidized while the other is reduced A chemical substance is oxidized when it loses electrons: Zn Zn 2+ + 2e - ( O.S.: 0 +2) A chemical substance is reduced when it gains electrons: Cu 2+ + 2e - Cu ( O.S.: +2 0) Substance that causes the oxidation of another is called the oxidizing agent or oxidant Substance that causes the reduction of another is called the reducing agent or reductant 7 08 - Electrochemistry
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8 08 - Electrochemistry
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Zn ( s ) + 2 H + ( aq ) Zn 2+ ( aq ) + H 2 ( g ) Zn ( s ) Zn 2+ ( aq ) + 2 e - (reducing agent) 2 H + ( aq ) + 2 e - H 2 ( g ) (oxidizing agent) Zn ( s ) + Cu 2+ ( aq ) Zn 2+ ( aq ) + Cu ( s ) Zn ( s ) Zn 2+ ( aq ) + 2 e - (reducing agent) Cu 2+ ( aq ) + 2 e - Cu ( s ) (oxidizing agent) Cu ( s ) + 2 Ag + ( aq ) Cu 2+ ( aq ) + 2 Ag ( s ) Cu ( s ) Cu 2+ ( aq ) + 2 e - (reducing agent)
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This note was uploaded on 09/17/2011 for the course CHE 102 taught by Professor Simon during the Spring '08 term at Waterloo.

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08_-_Electrochemistry - ChE102:ChemistryforEngineers

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