notes 9 - Unit9Electrochemistry...

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Unit 9 – Electrochemistry Ox idation-Red uction (REDOX) reactions Redox reactions involve the transfer of electrons; the  number of electrons lost must equal the number of electrons  gained. Redox reactions always involve simultaneous  oxidation  reactions and reduction reactions. Oxidation reactions involve the loss of electrons; the  oxidation number of the particle goes up. Reduction reactions involve the gain of electrons; the  oxidation number of the particle goes down. LeO says GeR !! or OiL RiG !! lose electrons   oxidation gain electrons   reduction Oxidation numbers are numbers assigned to an element  based on its apparent charge or electronegativity. Rules for oxidation numbers: Chip Shea 1 11/5/2011
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1) Free elements and H 2 O 2 F 2 Br 2 I 2 N 2 Cl 2  have  oxidation numbers of 0. 2) In a binary molecule, the more electronegative  element is negative. 3) Hydrogen has an oxidation number of +1,  except when it is combined with a metal (in a  hydride).  In a hydride, the oxidation number of  hydrogen is -1. 4) Oxygen has an oxidation number of -2, except  in peroxides (like H 2 O 2 ) and when it is  combined with fluorine.  In peroxides, its  oxidation number is -1.  When combined with  fluorine, its oxidation number is +2. 5) The sum of the oxidation numbers in a  compound is 0. 6) The sum of the oxidation numbers in an ion is  equal to the ion’s charge.  This includes  polyatomic ions! Example: Assign oxidation numbers to 1) O 2 O = 0 5) Al 2 S 3 Al =  3 + 8) H 2 SO 4 H = 1 + S = 2- S = 6 + Chip Shea 2 11/5/2011
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2) H 2 O H = 1 + O = 2 - O = 2 - 6) HNO 3 H = 1 + N = 5 + 9) Fe(NO 3 ) 2 Fe = 2 + 3) Fe Fe = 0 O = 2 - N = 5 + O = 2 - 4) Ca O Ca = 2 + 7) HF H = 1 + O = 2 - F = 1 - 10) He He = 0 Chip Shea 3 11/5/2011
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Identifying Redox Reactions Redox reactions always involve a change in oxidation  numbers! Example: single replacement reactions, combustion  reactions, corrosion of metals, synthesis reactions,  decomposition reactions, biological reactions ox #’s 0 0 1+ 1- rxn 2Na(s ) + Cl 2 (g ) 2NaCl(s ) Nonredox reactions show no change in oxidation  numbers. Example: double replacement reactions, acid/base  reactions ox #’s 1+ 5+ 2- 1+ 1- 1 + 1- 1+ 5+ 2- rxn AgNO 3 (aq ) + NaCl(aq ) AgCl(s ) + NaNO 3 (aq ) Identify as redox reactions: Chip Shea 4 11/5/2011
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1) C(s) + O 2 (g) CO 2 (g) redox 2) NH 3 (g) + HCl(aq) NH 4 + (aq) + Cl - (aq) nonredo x 3) 2H 2 O(l) 2H 2 (g) + O 2 (g) redox 4) 2KClO 3 (s) 2KCl(s) + 3O
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notes 9 - Unit9Electrochemistry...

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