CHAPTER 21 - Chapter 21 Electrochemistry 1 Oxidation-Reduction Reactions Zn added to HCl yields the spontaneous reaction Zn(s 2H(aq Zn2(aq H2(g The

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1 Chapter 21: Electrochemistry
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2 Oxidation-Reduction Reactions Oxidation-Reduction Reactions Zn added to HCl yields the spontaneous reaction Zn( s ) + 2H + ( aq ) Zn 2+ ( aq ) + H 2 ( g ). The oxidation number of Zn has increased from 0 to 2+. The oxidation number of H has reduced from 1+ to 0. Therefore, Zn is oxidized to Zn 2+ while H + is reduced to H 2 . H + causes Zn to be oxidized and is the oxidizing agent. Zn causes H + to be reduced and is the reducing agent. Note that the reducing agent is oxidized and the
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3 Balancing Oxidation-Reduction Balancing Oxidation-Reduction Equations Equations Law of conservation of mass : the amount of each element present at the beginning of the reaction must be present at the end. Conservation of charge : electrons are not lost in a chemical reaction. In complicated redox reactions, we need to look at the transfer of electrons carefully. Half-Reactions Half-Reactions Half-reactions are a convenient way of separating oxidation and reduction reactions.
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4 Balancing Oxidation-Reduction Balancing Oxidation-Reduction Equations Equations Half-Reactions Half-Reactions The half-reactions for Sn 2+ ( aq ) + 2Fe 3+ ( aq ) Sn 4+ ( aq ) + 2Fe 2+ ( aq ) are Sn 2+ ( aq ) Sn 4+ ( aq ) +2e - 2Fe 3+ ( aq ) + 2e - 2Fe 2+ ( aq ) Oxidation: electrons are products. Reduction: electrons are reagents.
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5 Balancing Equations by the Method of Balancing Equations by the Method of Half-Reactions Half-Reactions Consider the titration of an acidic solution of Na 2 C 2 O 4 (sodium oxalate, colorless) with KMnO 4 (deep purple). MnO 4 - is reduced to Mn 2+ (pale pink) while the C 2 O 4 2- is oxidized to CO 2 . The equivalence point is given by the presence of a pale pink color. If more KMnO 4 is added, the solution turns purple due to the excess KMnO .
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6 Balancing Equations by the Method of Balancing Equations by the Method of Half-Reactions Half-Reactions
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7 Balancing Equations by the Method of Balancing Equations by the Method of Half-Reactions Half-Reactions What is the balanced chemical equation? 1. Write down the two half reactions. 2. Balance each half reaction: a. First with elements other than H and O. b. Then balance O by adding water. c. Then balance H by adding H + . d. Finish by balancing charge by adding electrons.
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8 Balancing Equations by the Method of Balancing Equations by the Method of Half-Reactions Half-Reactions 3. Multiply each half reaction to make the number of electrons equal. 4. Add the reactions and simplify. 5. Check! For KMnO 4 + Na 2 C 2 O 4 :
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9 Balancing Equations by the Method of Balancing Equations by the Method of Half-Reactions Half-Reactions 1. The two incomplete half reactions are MnO 4 - ( aq ) Mn 2+ ( aq ) C 2 O 4 2- ( aq ) 2CO 2 ( g ) 2. Adding water and H + yields 8H + + MnO 4 - ( aq ) Mn 2+ ( aq ) + 4H 2 O There is a charge of 7+ on the left and 2+ on the right. Therefore, 5 electrons need to be added to the left: 5e - + 8H + + MnO - ( aq ) Mn 2+ ( aq ) + 4H O
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This note was uploaded on 04/09/2008 for the course CHM 111 taught by Professor Jamesadams during the Spring '08 term at Greenville Technical College.

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CHAPTER 21 - Chapter 21 Electrochemistry 1 Oxidation-Reduction Reactions Zn added to HCl yields the spontaneous reaction Zn(s 2H(aq Zn2(aq H2(g The

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