Ch5_C_Redox - EXCHANGE: Precipitation Reactions EXCHANGE:...

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Unformatted text preview: EXCHANGE: Precipitation Reactions EXCHANGE: Oxidation-Reduction Reactions Section 4.10 Thermite reaction Fe2O3(s) + 2 Al(s) ----> 2 Fe(s) + Al 2O3(s) EXCHANGE Gas-Forming Reactions EXCHANGE Acid-Base Reactions REDOX REACTIONS Oxidation— 2 H2(g) + O 2(g) ---> 2 H2O(liq) O(liq) Mg(s) + 2 HCl(aq) --> MgCl2(aq) + H2(g) HCl(aq) aq) All corrosion reactions are oxidations. oxidations. 2 Al(s) + 3 Cu 2+(aq) ---> 2 Al 3+(aq) + 3 Cu(s) aq) aq) REACTIONS REDOX REDOX REACTIONS REACTIONS Reduction— Fe2O3(s) + 2 Al(s) --> 2 Fe(s) + Al 2O3(s) REDOX REDOX REACTIONS REACTIONS But notice that in all reactions if something has been oxidized then something has also been reduced. Cu(s) + 2 Ag+(aq) aq) ---> Cu2+(aq) + 2 Ag(s) aq) Ag(s) Why Study Redox Reactions Why Study Redox Reactions Batteries Batteries Corrosion Corrosion REDOX REACTIONS REDOX REACTIONS Redox reactions are characterized by ELECTRON TRANSFER between an electron donor and electron acceptor. Transfer leads to— Manufacturing metals Manufacturing metals Fuels Fuels 1. increase in oxidation number of some element = OXIDATION 2. decrease in oxidation number of some element = REDUCTION Page 1 OXIDATION NUMBERS OXIDATION NUMBERS The electric charge an element APPEARS to have when electrons are counted by some arbitrary rules: 1. Each atom in free element has ox. no. = 0. Zn O2 I2 S8 2. In simple ions, ox. no. = charge on ion. -1 for Cl+2 for Mg2+ +2 3. 4. OXIDATION NUMBERS OXIDATION NUMBERS O has ox. no. = -2 Ox. no. of H = +1 OXIDATION NUMBERS OXIDATION NUMBERS NH3 ClOH3PO4 MnO4Cr2O72C3H8 N= Cl = Cl P= Mn = Mn Cr = Cr C= Oxidation number of F in HF? (except in peroxides: in H 2O2, O = -1) (except when H is associated with a metal as in NaH where it is -1) 5. Algebraic sum of oxidation numbers = 0 for a compound = overall charge for an ion Recognizing a Redox Reaction Recognizing a Redox Reaction Corrosion of aluminum Recognizing a Redox Reaction Recognizing a Redox Reaction Corrosion of aluminum Recognizing a Redox Reaction Recognizing a Redox Reaction Notice that the 2 half-reactions add up to give the overall reaction if we use 2 mol of Al and 3 mol of Cu2+. 2 Al(s) --> 2 Al 3+(aq) + 6 eaq) 3 Cu2+(aq) + 6 e - --> 3 Cu(s) Cu aq) --> ----------------------------------------------------------2 Al(s) + 3 Cu 2+(aq) ---> 2 Al 3+(aq) + 3 Cu(s) aq) aq) Final eqn. is balanced for mass and charge. eqn. 2 Al(s) + 3 Cu 2+(aq) ---> 2 Al 3+(aq) + 3 Cu(s) aq) aq) Al(s) --> Al 3+(aq) + 3 e aq) • Ox. no. of Al increases as e - are donated are by the metal. • Therefore, Al is OXIDIZED and is the REDUCING AGENT in this balanced halfreaction. 2 Al(s) + 3 Cu 2+(aq) ---> 2 Al 3+(aq) + 3 Cu(s) aq) aq) Cu2+(aq) + 2 e - --> Cu(s) aq) --> • Ox. no. of Cu decreases as e - are are accepted by the ion. • Therefore, Cu is REDUCED and is the OXIDIZING AGENT in this balanced halfreaction. Page 2 Examples of Redox Reactions Examples of Redox Reactions Metal + acid Mg + HCl Mg = reducing agent H+ = oxidizing agent oxidizing Examples of Redox Reactions Nonmetal (P) + Oxygen Metal (Mg) + Oxygen Metal + acid Cu + HNO 3 Cu = reducing agent HNO3 = oxidizing agent oxidizing Metal + halogen 2 Al + 3 Br 2 ---> Al 2Br6 ---> Recognizing a Redox Recognizing a Redox Reaction Reaction See Table 4.3 See Table 4.3 Oxidation Reaction Type Reduction Common Oxidizing and Common Oxidizing and Reducing Agents Reducing Agents See Table 4.2 See Table 4.2 Metals (Cu) are reducing agents Learn to recognize common Learn to recognize common oxidizing and reducing oxidizing and reducing agents. See Table 4.2. agents. See Table 4.2. Metals (Na, K, Na, Mg, Fe) are reducing agents In terms of oxygen In terms of halogen In terms of electrons gain gain loss loss loss gain HNO3 is an is oxidizing agent Cu + HNO3 --> 2 K + 2 H2O --> --> Cu2+ + NO2 2 KOH + H2 NO Page 3 ...
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This note was uploaded on 01/11/2011 for the course ENGINEERIN MAE 107 taught by Professor Pozikrizdis during the Fall '08 term at San Diego.

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