(note: each ½ reaction has balanced the number of atoms and the charge.)

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Moses Sparks
5 years ago
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1 Oxidation- Reduction Reactions in Aqueous Solutions Introduction: When a piece of copper wire is immersed in a clear, colorless solution of silver nitrate, silver whiskers grow on the surface of the wire and the solution turns blue, indicating the presence of copper (II) ions. In simple terms the silver comes out of solution as the copper goes into it. On the atomic level, there were electrons transferred from the copper metal to the silver ions; the copper metal was oxidized while the silver ions were reduced: Oxidation: Cu(s) Cu 2+ (aq) + 2 e Reduction: Ag + (aq) + e Ag(s) (note: each ½ reaction has balanced the number of atoms and the charge.) Stoichiometrically, the two processes are related by the number of electrons: Oxidation: Cu(s) Cu 2+ (aq) + 2 e Net transfer of 2 moles of e for each mole of Reduction: 2Ag + (aq) + 2 e 2 Ag(s) reaction Net Ionic Reaction: Cu(s) + 2 Ag + (aq) + 2 e Cu 2+ (aq) + 2 e + 2 Ag(s) Model 1: Reactions in acidic aqueous solution: The ½ reactions of oxidizing or reducing agents in aqueous solution may be complicated by the inclusion of an acid/base reaction with the electron transfer process. If the reaction is under acidic aqueous conditions, both hydrogen ions and water are present as major species in the solution and may be included in the reaction, if needed. For example, the five electron reduction of the manganese in the permanganate ion (MnO 4 ) under acidic conditions proceeds as: Mn(+7) + 5 e Mn(+2) reduction 4 O H + (aq) + 4 H 2 O(l) acid/base MnO 4 (aq) + 8 H + (aq) + 5 e Mn 2+ (aq) + 4 H 2 O(l) Net reaction The ½ reaction may be balanced systematically by following a simple process 1 MnO 4 (aq) 1 Mn 2+ (aq) 1 MnO 4 (aq) 1 Mn 2+ (aq) + 4 H 2 O(l) 1 MnO 4 (aq) + 8 H + (aq) 1 Mn 2+ (aq) + 4 H 2 O(l) D) Balance the charges of the species with electrons 1 MnO 4 (aq) + 8 H + (aq) + 5 e 1 Mn 2+ (aq) + 4 H 2 O(l) Oxidation-Reduction Reaction in Aqueous Solutions page 1 of 5

2 Apply the process to the following net reaction in acidic media Cr 2 O 7 2 (aq) + H 2 S (aq) Cr 3+ (aq) + SO 4 2 (aq) 1. Identify the oxidizing and reducing agents. 2. Identify the major species in the unbalanced oxidation and reduction ½ reactions. oxidation: reduction: 3. For the reduction ½ reaction: A. Balance the atoms other than hydrogen and oxygen; B. Balance the oxygen atoms with water; C. Balance the hydrogen atoms with hydrogen ions D. Balance the charges of the species with electrons 4. For the oxidation ½ reaction: A. Balance the atoms other than hydrogen and oxygen; B. Balance the oxygen atoms with water; C. Balance the hydrogen atoms with hydrogen ions D. Balance the charges of the species with elections 5. Write the balanced reaction of dichromate ion with hydrosulfuric acid in acidic media: Oxidation-Reduction Reaction in Aqueous Solutions page 2 of 5

3 Model 2: Basic Aqueous Media If the reaction is under basic aqueous conditions, both hydroxide ions and water are present as major species in the solution and may be included in the reaction, if needed. In contrast to the previous model, the strongest acid present as a major species is water, which forms hydroxide after the donation of the proton. For example, the three electron reduction of the manganese in the permanganate ion (MnO 4 ) under basic conditions proceeds as: Mn(+7) + 3 e Mn(+4) reduction 2 O H 2 O(l) + 4 OH (aq) acid/base MnO 4 (aq) + 2 H 2 O(l) + 3 e MnO 2 (s) + 4 OH (aq) Net reaction The ½ reaction may be balanced systematically by following a simple process 1 MnO 4 (aq) 1 MnO 2 (s) 1 MnO 4 (aq) 1 MnO 2 (s) + 2 H 2 O(l) 1 MnO 4 (aq) + 4 H + (aq) 1 MnO 2 (s) + 2 H 2 O(l) D) Correct to account for water as the source of H + 1 MnO 4 (aq) + 4 H 2 O(l) 1 MnO 2 (s) + 2 H 2 O(l) + 4 OH E) Balance the charges of the species with electrons and simplify 1 MnO 4 (aq) + 2 H 2 O(l) + 3 e 1 MnO 2 (s) + 4 OH Apply the process to the following net reaction in basic media NO 2 (aq) + Al(s) NH 3 (aq) + AlO 2 (aq) 6. Identify the oxidizing and reducing agents 7. Identify the major species in the unbalanced oxidation and reduction ½ reactions. oxidation: reduction: Oxidation-Reduction Reaction in Aqueous Solutions page 3 of 5

4 8. For the reduction ½ reaction: D) Correct to account for water as source of protons, not H+ E) Balance the charges of the species with elections 9. For the oxidation ½ reaction: D) Correct to account for water as source of protons, not H+ E) Balance the charges of the species with electrons 10. Write the balanced reaction of aluminum solid with the nitrite ion in basic media: Oxidation-Reduction Reaction in Aqueous Solutions page 4 of 5

5 Exercises 1. C 2 H 5 OH(aq) + Ce 4+ (aq) CO 2 (g) + Ce 3+ (aq) (Acidic) 2. (Hg 2 ) 3 (PO 4 ) 2 (s) + Au(s) Hg(l) + AuCl 4 (aq) + H 3 PO 4 (aq) (aqueous HCl) 3. Pb 3 O 4 (s) Pb 2+ (aq) + PbO 2 (s) (Acidic) 4. Cr(NCS) 6 4 (aq) + MnO 4 (aq) Cr 3+ + NO 3 (aq) + CO 2 (g) + HSO 4 (aq) + Mn 2+ (aq) (Acidic) 5. H 2 CO(aq) + Ag(NH 3 ) 2 + (aq) HCO 3 (aq) + Ag(s) + NH 3 (aq) (Basic) 6. H 2 BO 3 (aq) + Al(s) BH 4 (aq) + H 2 AlO 3 (aq) (Basic) Oxidation-Reduction Reaction in Aqueous Solutions page 5 of 5

Activity Balancing Oxidation-Reduction Reactions

Activity Balancing Oxidation-Reduction Reactions Activity 201 3 Balancing Oxidation-Reduction Reactions Directions: This GLA worksheet goes over the half-reaction method of balancing oxidation-reduction (redox) reactions. Part A introduces the oxidation