Supplemental Instruction 1060 Hixson-Lied Student Success Center 294-6624 www.si.iastate.edu Chapter 20: Oxidation and Reduction Supplemental Instruction Iowa State University Leader: Kelsey Course: Chemistry 178 Instructor: Verkade Date: 11/09/2010 1. Circle the appropriate answers, in an oxidation-reduction (Redox) reaction: a. Oxidation involves gain/loss of elections OIL (Oxidized goes up in oxidation state) b. Reduction involves gain/loss of elections RIG (Reduced goes down in oxidation state) c. A reducing agent is oxidized/reduced, and an oxidizing agent in oxidized/reduced. 2. What is the oxidation number of every element? 0 3. What is the oxidation number of a monatomic ion? (Hint: not the same number each time, it varies with the ion). The oxidation number is the same as its charge. 4. What is the sum of individual oxidation numbers in these compounds? a. HCl 0 b. BO33- -3 c. ClO4- -1 d. H2O 0 e. H2CO4- -1 5. What are the oxidation numbers in this reaction? 0 +1 +2 0 6. Balancing Oxidation-Reduction Reactions a. Consider the titration of an acidic solution of Na2C2O4 with KMnO4. MnO4- is reduced to Mn2+ and C2O42- is oxidized to CO2. i. Write down the two incomplete half reactions: 1. 2. ii. Balance each half reaction 1. First, balance elements other than H and O a) b) 2. Then balance O by adding water a) b) 3. Then balance H by adding H+ a) b) iii. Balance charge by adding electrons 1.2. iv. Multiply each half reaction to make the number of electrons equal 1. 2. v. Add the reactions and simplify 1. vi. You’re done! 7. Balance the following redox reactions, in each case: (a) give the balanced half-reactions; identify the oxidation half-reaction and the reduction half-reaction. (b) give the balanced net reaction. (c) identify the oxidizing agent and the reducing agent. Cl2(g) + S2O32-(aq) Cl-(aq) + SO42-(aq) (a) S2O32-(aq) + 5 H20 2 SO42-(aq) + 10 H+(aq) + 8 e- (oxidation half-reaction – LEO); Cl2(g) + 2 e- 2 Cl-(aq) (reduction half-reaction – GER). (b) S2O32-(aq) + 5 H20 + 4 Cl2(g) 2 SO42-(aq) + 10 H+(aq) + 8 Cl-(aq) (c) S2O32-(aq) is the reducing agent; Cl2(g) is the oxidizing agent. O3(g) + Br-(aq) O2(g) + BrO-(aq) Answers: (a) Br - (aq) + H20 + 2 OH-(aq) BrO-(aq) + 2H2O + 2 e- or, after simplifying, Br - (aq) + 2 OH-(aq) BrO-(aq) + H2O + 2 e- (oxidation half-reaction – LEO); O3(g) + 2 H2O + 2 e- O2(g) + H2O + 2 OH-(aq) or, after simplifying, O3(g) + H2O + 2 e- O2(g) + 2 OH-(aq) (reduction half-reaction – GER). (b) Br - (aq) + O3(g) BrO-(aq) + O2(g) (c) Br - (aq) is the reducing agent; O3(g) is the oxidizing
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