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HalfReactionBalancing

HalfReactionBalancing - Module 16 Half-Reaction Balancing...

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Module 16: Half-Reaction Balancing ©2009 ChemReview.net v. k1 Page 380 Redox: Half-Reaction Balancing: Self-Study Assignment You will have a QUIZ on the attached pages on _____________________ . Your assignment is: READ the pages attached. WORK the examples in the lesson. Complete the pages as homework. To work the examples, use a sheet of paper to cover below the * * * * * line, try the problem on your paper, then check your answer below the * * * * * line. Start early. This assignment will require 2-4 hours of work outside of class. * * * * *
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Module 16: Half-Reaction Balancing ©2009 ChemReview.net v. k1 Page 381 Module 16: Half-Reaction Balancing Timing : Module 16 covers redox balancing using half-reactions . Some courses assign this topic after oxidation numbers. Others assign half-reactions as part of electrochemistry later in the course. Do this unit when half- reactions are assigned in your course. Prerequisite : If you cannot solve these problems easily, review Lessons 15A, 15B, and 15D before starting Module 16. Answers are at the end of Lesson 16A. 1. Define reduction. 2. Balance this half-reaction: F 2 Æ F 3. What is the oxidation number of a. Each Cl atom in Cl 2 O 5 ? b. The Mn atom in MnO 4 ? 4. Label each reactant as an oxidizing agent (OA) or reducing agent (RA). Circle the reactant being oxidized. a. Sn 4 + + Co Æ Co 2 + + Sn 2 + b. Ca + 2 H + Æ H 2 + Ca 2 + * * * * * Lesson 16A: Balancing Half-Reactions: The CA-WHe! Method Balancing Half-Reactions That Include Acid and Water Many redox reactions occur in aqueous solutions, and water is often a term in the reactants or products. Many redox reactions carried out in aqueous solutions need acidic conditions for the desired reaction to take place. In these cases, both the redox half-reactions and the overall reaction may include H + ions and H 2 O in the reactants and/or products. The steps for balancing half-reactions which include H + and H 2 O are the same as for balancing other half-reactions. First add coefficients to balance atoms , then add electrons to balance charge . Using those two steps, solve the following problem. Q . Balance this half-reaction. MnO 4 + H + Æ Mn 2 + + H 2 O * * * * * (the * * * means: cover below write your answer, then check below.) Both sides have one Mn atom: Mn is balanced. The left has 4 oxygens, so the right must have a coefficient of 4 for water. That gives 8 H atoms on the right, so the H + coefficient on the left must be 8 for the H atoms to balance. The atoms are now balanced: MnO 4 + 8 H + Æ Mn 2 + + 4 H 2 O but the charge is not balanced. Add electrons to the equation to balance the charges. * * * * *
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Module 16: Half-Reaction Balancing ©2009 ChemReview.net v. k1 Page 382 Since the left charges total 7+ and the right 2+, we must add 5 electrons to the left in order for the charges to balance. 5 e + MnO 4 + 8 H + Æ Mn 2 + + 4 H 2 O The net charge is now +2 on both sides. Atoms and charge are now balanced, so the half-reaction is balanced.
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