CHAPTER 21 - Chapter 21: Electrochemistry 1...

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1Chapter 21: Electrochemistry
2Oxidation-Reduction ReactionsOxidation-Reduction ReactionsZn added to HCl yields the spontaneous reactionZn(s) + 2H+(aq) Zn2+(aq) + H2(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 Zn2+while H+is reduced to H2.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 oxidizing agent is reduced.
3Balancing Oxidation-Reduction Balancing Oxidation-Reduction EquationsEquationsLaw 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-ReactionsHalf-ReactionsHalf-reactions are a convenient way of separating oxidation and reduction reactions.
4Balancing Oxidation-Reduction Balancing Oxidation-Reduction EquationsEquationsHalf-ReactionsHalf-ReactionsThe half-reactions forSn2+(aq) + 2Fe3+(aq) Sn4+(aq) + 2Fe2+(aq)areSn2+(aq) Sn4+(aq) +2e-2Fe3+(aq) + 2e-2Fe2+(aq)Oxidation: electrons are products.Reduction: electrons are reagents.
5Balancing Equations by the Method of Balancing Equations by the Method of Half-ReactionsHalf-ReactionsConsider the titration of an acidic solution of Na2C2O4(sodium oxalate, colorless) with KMnO4(deep purple).MnO4-is reduced to Mn2+(pale pink) while the C2O42-is oxidized to CO2.The equivalence point is given by the presence of a pale pink color.If more KMnO4is added, the solution turns purple due to the excess KMnO4.
6Balancing Equations by the Method of Balancing Equations by the Method of Half-ReactionsHalf-Reactions
7Balancing Equations by the Method of Balancing Equations by the Method of Half-ReactionsHalf-ReactionsWhat 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.
8Balancing Equations by the Method of Balancing Equations by the Method of Half-ReactionsHalf-Reactions3. Multiply each half reaction to make the number of electrons equal.4. Add the reactions and simplify.5. Check!For KMnO4+ Na2C2O4:
9Balancing Equations by the Method of Balancing Equations by the Method of Half-ReactionsHalf-Reactions1. The two incomplete half reactions areMnO4-(aq) Mn2+(aq)C2O42-(aq) 2CO2(g)2. Adding water and H+yields 8H++ MnO4-(aq) Mn2+(aq) + 4H2OThere 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++ MnO4-(aq) Mn2+(aq) + 4H2O
10Balancing Equations by the Method of Balancing Equations by the Method of Half-ReactionsHalf-Reactions

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Term
Spring
Professor
JamesAdams
Tags
Electrochemistry, Oxidation Number, Oxidation Reduction Reactions, standard reduction potentials, Cell EMF, Zn2

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