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Unit 1 - Unit 9 Electrochemistry Oxidation-Reduction(REDOX...

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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: 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 + 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) CaO Ca = 2 + 7) HF H = 1 + O = 2 - F = 1 - 10) He He = 0 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: 1) C(s) + O 2 (g) CO 2 (g) redox 2) NH 3 (g) + HCl(aq) NH 4 + (aq) + Cl - (aq) nonredox 3) 2H 2 O(l) 2H 2 (g) + O 2 (g) redox 4) 2KClO 3 (s) 2KCl(s) + 3O 2 (g) redox 5) H 2 SO 4 (aq) + 2KOH(aq) K 2
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