ChE102_Notes_-_Chap_6

ChE102_Notes_-_Chap_6 - ChE102 Div 9 & 10 Fall...

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Unformatted text preview: ChE102 Div 9 & 10 Fall 2009 CHAPTER 6 Electrochemistry Petrucci: 3.4, 5.1-5.6, 20.1-20.8 Oxidation States 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 NaCl: Na has oxidation state +1 Cl has oxidation state -1 NaCl: Na loses 1 electron Na + Cl gains 1 electron Cl- MgBr 2 : Mg loses 2 electrons Mg 2+ Each Br gains 1 electron 2 Br- Course Notes 6 1 ChE102 Div 9 & 10 Fall 2009 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 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 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 . Substance that causes the reduction of another is called the reducing agent or . Course Notes 6 2 [Petrucci Fig 20.2] ChE102 Div 9 & 10 Fall 2009 Examples of REDOX Reactions Zn ( s ) + 2 H + ( aq ) Zn 2+ ( aq ) + H 2 ( g ) Zn ( s ) Zn 2+ ( aq ) + ___ e- (reducing agent) ___ H + ( aq ) + ___ e- H 2 ( g ) (oxidizing agent) Zn ( s ) + Cu 2+ ( aq ) Zn 2+ ( aq ) + Cu ( s ) Zn ( s ) Zn 2+ ( aq ) + ___ e- (reducing agent) Cu 2+ ( aq ) + ___ e- Cu ( s ) (oxidizing agent) Cu ( s ) + 2 Ag + ( aq ) Cu 2+ ( aq ) + 2 Ag ( s ) Cu ( s ) Cu 2+ ( aq ) + ___ e- (reducing agent) __ Ag + ( aq ) + ___ e- ___ Ag ( s ) (oxidizing agent) Zn ( s ) + SnCl 2 ( aq ) Zn 2+ ( aq ) + Sn ( s ) Zn ( s ) Zn 2+ ( aq...
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This note was uploaded on 12/15/2009 for the course ENGINEERIN CHE 102 taught by Professor Soares during the Fall '09 term at Waterloo.

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ChE102_Notes_-_Chap_6 - ChE102 Div 9 & 10 Fall...

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