电极过程动力学 (9).pdf

ElectrochemistryElectrochemistryThe study of the exchange of chemical The study of the exchange of chemical and electrical energy.and electrical energy.Oxidation-Reduction reactions take Oxidation-Reduction reactions take place in place in electrochemical cells.electrochemical cells.Each electrochemical cell consists of Each electrochemical cell consists of two solid conductors called electrodes.two solid conductors called electrodes.Two types of electrochemical Two types of electrochemical cells:cells:Voltaic CellVoltaic CellElectrolytic CellElectrolytic CellVoltaic CellVoltaic CellReactions are spontaneous(Table J)Reactions are spontaneous(Table J)RedoxRedox reactions produce electrical reactions produce electrical energyenergyLets look at an example:Lets look at an example:Cu Cu+2+2 (aqaq)+Zn +Zn(s)(s)-Cu -Cu(s)(s)+Zn+Zn+2+2 (aqaq)Each Voltaic cell is composed of Each Voltaic cell is composed of two half cells.two half cells.Zinc electrode Copper electrodeThe anode,is the electrode where the oxidation of zinc occurs:Zn(s)-Zn+2(aq)+2e-As zinc is oxidized,the zinc electrode will slowly dissolve to produce zinc ions,which enter into the solution.Anode (negative)The cathode is the electrode where the reduction of copper occurs:Cu+2(aq)+2e-Cu(s)As copper is reduced,copper atoms accumulate on the surface of the solid copper electrode.Cathode(positive)Looking at the two cells,can a Looking at the two cells,can a redoxredox reaction take place?reaction take place?The reaction The reaction will not occurwill not occur.There is no way for the electrons lost by There is no way for the electrons lost by Zinc to get over to the Copper ions and Zinc to get over to the Copper ions and reduce them to Copper metal.reduce them to Copper metal.What if we provide a path for the electrons What if we provide a path for the electrons released by the oxidation of the zinc to get released by the oxidation of the zinc to get over to the copper?over to the copper?Now that the electrons have a path to the Cu/Cu2+side it would appear that the reaction can proceed,but it does not.Why?As the As the redoxredox reaction proceeds:reaction proceeds:There is a build up of ZnThere is a build up of Zn2+2+ions in the solution ions in the solution where the Zinc is being oxidizedwhere the Zinc is being oxidizedAt the same time,CuAt the same time,Cu2+2+ions are removed from ions are removed from solution where the Copper ion is being reducedsolution where the Copper ion is being reducedAs a result,there is a build up of a positive charge As a result,there is a build up of a positive charge in the zinc solution,and a negative charge in the in the zinc solution,and a negative charge in the copper solution.These charges do not allow copper solution.These charges do not allow electrons to flow.electrons to flow.The positive charge in the zinc solution makes it The positive charge in the zinc solution makes it harder for the negative electrons to leave and the harder for the negative electrons to leave and the negative charge in the copper solution repels the negative charge in the copper solution repels the electrons that are trying to come over from the electrons that are trying to come over from the zinc sidezinc sideWe need a way to neutralize the We need a way to neutralize the charge build-up in the solutions charge build-up in the solutions due to the change in ion due to the change in ion concentrationconcentrationWhat if we had a tube filled with aqueous solution that connected the two redox reactions?This tube containsan electrolyte and is called a salt bridge.Electron flow The salt bridge allows the electrons to flow,keeping the overall concentration of ions in keeping the overall concentration of ions in balance between the two sides.balance between the two sides.By connecting the salt bridge we have movement of both types of ions:anions are going into the oxidation side and cations are going into the reduction sideStandard Cell potentialStandard Cell potential Electrical energy produced by a Electrical energy produced by a redoxredox reaction is measured as voltagereaction is measured as voltageSome standard electrode potentials are listed Some standard electrode potentials are listed below.below.Electrode Process Electrode Process E Eo o/V/V Li+(aqLi+(aq)+e-)+e-Li(sLi(s)-3.03)-3.03 K+(aqK+(aq)+e-)+e-K(sK(s)-2.92)-2.92 Zn2+(aq)+2e-Zn2+(aq)+2e-Zn(sZn(s)-0.76)-0.76 Cu2+(aq)+2e-Cu2+(aq)+2e-Cu(sCu(s)+0.34)+0.34StandardStandard Cell Potential for Zn and Cu Cell Potential for Zn and Cu is 1.1 voltsis 1.1 voltsElectrolytic CellElectrolytic CellReactions are non spontaneousReactions are non spontaneous RedoxRedox reactions require electrical reactions require electrical energy to occurenergy to occurAnions are attracted towards the Anions are attracted towards the anode where they undergo oxidation.anode where they undergo oxidation.Electrons flow from the anode Electrons flow from the anode to the cathode where to the cathode where cationscations undergo reduction.undergo reduction.Voltaic Cell Voltaic Cell vsvs Electrolytic Cell Electrolytic CellUse of Electrochemical CellUse of Electrochemical CellA common dryA common dry cell battery.cell battery.Use of Electrolytic CellUse of Electrolytic CellElectroplatingElectroplating Electrolysis of water Electrolysis of water