E_-_102_F11_Notes

E_-_102_F11_Notes - E)Electrochemistry...

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ChE102 Fall 2011 Class Notes ‐ Electrochemistry E‐1 E) Electrochemistry Petrucci et al., 10 th Edition: 3.4, 5.1‐5.2, 5.4‐5.6, 20.1‐20.7 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 loses 1 electron Na + (Na has an oxidation state +1) Cl gains 1 electron Cl (Cl has an oxidation state ‐1) MgBr 2 : Mg loses 2 electrons Mg 2+ Each Br gains 1 electron 2 Br [see Petrucci et al., front cover for periodic table] Rules for Assigning Oxidation States (O.S.) The O.S. of an individual atom in a free element is 0 (zero) Cl, Cl 2 , N 2 , O 2 , O, etc. The total of the O.S. of all atoms in a neutral species is 0 (zero) 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
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ChE102 Fall 2011 Class Notes ‐ Electrochemistry E‐2 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 ______________________. 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 ) + ___ e (reducing agent) Sn 2+ ( aq ) + ___ e Sn ( s ) (oxidizing agent) 2 Zn ( s ) + O 2 ( g ) 2 ZnO ( s ) ___ Zn ( s ) ___ Zn 2+ ( s ) + ___ e (reducing agent) O 2 ( g ) + ____ e 2 O 2‐ ( s ) (oxidizing agent)
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ChE102 Fall 2011 Class Notes ‐ Electrochemistry E‐3 Hydronium Ion (H 3 O + ) Zn ( s ) + 2 H + ( aq ) Zn 2+ ( aq ) + H 2 ( g ) Zn ( s ) Zn 2+ ( aq ) + 2 e 2 H + ( aq ) + 2 e
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E_-_102_F11_Notes - E)Electrochemistry...

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