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Chem1202PReviewCh20 - Chemistry 1202 Review/Preview Chapter...

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Chemistry 1202 Review/Preview Chapter Twenty Review Guide Dr. Saundra McGuire Spring 2007 Director, Center for Academic Success Adj. Prof., Dept. of Chemistry Electrochemistry IMPORTANT NOTE: Chapter 20 assumes mastery of the material on oxidation- reduction (redox) reactions in Chapter 4, especially section 4.4 (pages 137-139). The section on oxidation-reduction reactions in the Chapter 4 review guide should be thoroughly reviewed before beginning the study of Chapter 20. I. Oxidation States When electrons are transferred between the reactants in a chemical reaction, the element that loses electrons is said to be oxidized and the element that gains electrons is said to be reduced. Oxidation states are compared to tell if a reaction is a redox reaction. The substance containing the element that is oxidized is called the reducing agent (or reductant). The substance containing the element being reduced is called the oxidizing agent (or oxidant). A. Assigning Oxidation States (Numbers) Refer to the rules presented in Section 4.4 of the textbook, and rememorize these rules if you have forgotten any of them! (Work through exercise 20.1 for practice.) II. Balancing Redox Equations in Acidic or Basic Solution A. Half-Reactions An oxidation-reduction reaction can be broken into two half-reactions, one showing the oxidation process, and the other showing the reduction process. In the overall reaction, the number of electrons lost during oxidation must equal the number of electrons gained during reduction. (We can’t have extra electrons floating out in space, and we can’t grab electrons from space!) The electrons gained can only be produced by the substance that is losing electrons. When each half- reaction has the same number of electrons involved, we can add them together to get the overall oxidation-reduction reaction. B. Balancing Equations by the Method of Half-Reactions Important note: I do not recommend assigning oxidation numbers first (as the textbook does). It is not generally necessary to determine what is being oxidized and what is being reduced before you can write the half- reactions. It does not hurt to assign the oxidation numbers, but it is usually not necessary. Dr. McGuire’s recommended steps for balancing by the half-reaction method 1. Write the unbalanced ionic equation. 2. Write the individual half-reactions by doing the following: a) Balance all atoms other than H and O b) Balance the oxygens by adding water to the side deficient in oxygens c) Balance the hydrogens by adding H+ to the side deficient in hydrogens d) Balance the charges by adding electrons to the more positive side of the equation 3. Multiply one or both half-reactions by the appropriate number(s) to make the number of electrons transferred equal. 4. Add the two half-reactions, canceling species that appear on both sides of
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the equation), and check that the overall equation is balanced.
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Chem1202PReviewCh20 - Chemistry 1202 Review/Preview Chapter...

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