Ch5_C_Redox - Oxidation-Reduction Reactions EXCHANGE:...

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Unformatted text preview: Oxidation-Reduction Reactions EXCHANGE: Precipitation Reactions Section 4.10 Thermite reaction Fe2O3(s) + 2 Al(s) ----> Oxidation— EXCHANGE Gas-Forming Reactions EXCHANGE Acid-Base Reactions REACTIONS Cu(s) + 2 Ag+(aq) ---> Cu2+(aq) + 2 Ag(s) Mg(s) + 2 HCl (aq ) --> MgCl2(aq) + H2(g) All corrosion reactions are oxidations . Reduction— REDOX REDOX REACTIONS REACTIONS But notice that in all reactions if something has been oxidized then something has also been reduced. 2 H2(g) + O 2(g) ---> 2 H2O(liq) 2 Al(s) + 3 Cu 2+(aq) ---> 2 Al 3+(aq) + 3 Cu(s) 2 Fe(s) + Al 2O3(s) REDOX REDOX REACTIONS REACTIONS REDOX REACTIONS Fe2O3(s) + 2 Al(s) --> 2 Fe(s) + Al 2O3(s) Why Study Redox Reactions Why Study Redox Reactions Batteries Batteries Corrosion Corrosion Manufacturing metals Manufacturing metals Fuels Fuels Page 1 REDOX REACTIONS REDOX REACTIONS Redox reactions are characterized by ELECTRON TRANSFER between an electron donor and electron acceptor. Transfer leads to— 1. increase in oxidation number 1. increase of some element = OXIDATION of OXIDATION 2. decrease in oxidation number of some element = REDUCTION of REDUCTION 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+ Recognizing a Redox Reaction Recognizing a Redox Reaction OXIDATION NUMBERS OXIDATION NUMBERS 3. O has ox. no. = -2 (except in peroxides: in H 2O2, O = -1) 4. Ox. no. of H = +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 OXIDATION NUMBERS OXIDATION NUMBERS NH3 N= ClO- Cl = H3PO4 P= MnO4- Mn = Cr2O72- Cr = Cr C3H8 Oxidation number of F in HF? C= = overall charge for an ion Recognizing a Redox Reaction Recognizing a Redox Reaction Corrosion of aluminum Corrosion of aluminum 2 Al(s) + 3 Cu 2+(aq) ---> 2 Al 3+(aq) + 3 Cu(s) 2 Al(s) + 3 Cu 2+(aq) ---> 2 Al 3+(aq) + 3 Cu(s) Al(s) --> Al 3+(aq) + 3 e - Cu2+(aq) + 2 e - --> Cu(s) --> • Ox. no. of Al increases as e - are donated by the metal. • Ox. no. of Cu decreases as e - are accepted by the ion. • Therefore, Al is OXIDIZED and is the Therefore, and REDUCING AGENT in this balanced halfhalfreaction. • Therefore, Cu is REDUCED and is the Therefore, and OXIDIZING AGENT in this balanced halfhalfreaction. Page 2 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 e3 Cu2+(aq) + 6 e - --> 3 Cu(s) --> ----------------------------------------------------------2 Al(s) + 3 Cu 2+(aq) ---> 2 Al 3+(aq) + 3 Cu(s) Final eqn. is balanced for mass and charge. Examples of Redox Reactions Examples of Redox Reactions Examples of Redox Reactions Metal + acid Mg + HCl Mg = reducing agent H+ = oxidizing agent Nonmetal (P) + Oxygen Metal (Mg) + Oxygen Recognizing a Redox Recognizing a Redox Reaction Reaction See Table 4.3 See Table 4.3 Reaction Type Oxidation Reduction In terms of oxygen gain loss In terms of halogen gain loss In terms of electrons loss gain Metal + acid Cu + HNO 3 Cu = reducing agent HNO3 = oxidizing agent oxidizing Common Oxidizing and Common Oxidizing and Reducing Agents 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. See Table 4.2 See Table 4.2 Metals (Cu) are reducing agents Metals (Na, K, Mg, Fe) are reducing agents HNO3 is an oxidizing agent Cu + HNO3 --> 2 K + 2 H2O --> Cu2+ + NO2 2 KOH + H2 Page 3 Metal + halogen 2 Al + 3 Br 2 ---> Al 2Br6 ...
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