Chapter 20 Electrochemistry：20章电化学

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1、ElectrochemistryChapter 20ElectrochemistryChemistry, The Central Science, 10th editionTheodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. BurstenJohn D. BookstaverSt. Charles Community CollegeSt. Peters, MO 2006, Prentice Hall, Inc.ElectrochemistryAPRIL 8 REDOX REACTIONS HOW TO ASSIGN OXIDATION NU

2、MBERS HOW TO BALANCE REACTIONS IN ACID AND BASIC MEDIUM USING THE HALF REACTION METHOD HW : 13,15, 17,19 ElectrochemistryElectrochemical ReactionsIn electrochemical reactions, electrons are transferred from one species to another. Metals tend to lose electrons and are oxidized, non metals tend to ga

3、in electrons and are reduced.ElectrochemistryElectrochemistryElectrochemistryElectrochemistryOxidation NumbersIn order to keep track of what loses electrons and what gains them, we assign oxidation numbers.ElectrochemistryAssigning Oxidation NumbersElements in their elemental form have an ON= 0.The

4、oxidation number of a monatomic ion is its charge.Nonmetals tend to have negative oxidation numbers, although some are positive in certain compounds or ions. Oxygen has an oxidation number of 2, except in the peroxide ion in which it has an oxidation number of 1. Hydrogen is +1 except in metal hydri

5、des when is 1 . Fluorine always has an oxidation number of 1.ElectrochemistryAssigning Oxidation Numbers The other halogens have an oxidation number of 1 when they are negative; they can have positive oxidation numbers, however, most notably in oxyanionsThe sum of the oxidation numbers in a polyatom

6、ic ion is the charge on the ion.The sum of the oxidation numbers in a neutral compound is 0.ElectrochemistryOxidation and Reduction A species is oxidized when it loses electrons.Here, zinc loses two electrons to go from neutral zinc metal to the Zn2+ ion.ElectrochemistryOxidation and Reduction A spe

7、cies is reduced when it gains electrons.Here, each of the H+ gains an electron and they combine to form H2.ElectrochemistryOxidation and Reduction What is reduced is the oxidizing agent.H+ oxidizes Zn by taking electrons from it. What is oxidized is the reducing agent.Zn reduces H+ by giving it elec

8、trons.Electrochemistry Zn 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. Zn is oxidized to Zn2+ while H+ is reduced to H2. H+ causes Zn to be oxidized and is the

9、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.Electrochemistry Law of conservation of mass: the amount of each element present at the beginning of the reaction must be present at the end. Conservatio

10、n of charge: electrons are not lost in a chemical reaction.Half Reactions Half-reactions are a convenient way of separating oxidation and reduction reactions.ElectrochemistryHalf Reactions The half-reactions forSn2+(aq) + 2Fe3+(aq) Sn4+(aq) + 2Fe2+(aq)areSn2+(aq) Sn4+(aq) +2e-2Fe3+(aq) + 2e- 2Fe2+(a

11、q) Oxidation: electrons are products. Reduction: electrons are reactants. Loss of Gain of Electrons isElectrons is OxidationReductionElectrochemistryBalancing Equations by the Method of Half Reactions Consider the titration of an acidic solution of Na2C2O4 (sodium oxalate, colorless) with KMnO4 (dee