Chapter 6 Electrochemistry-20100414.ppt
Chapter 6Electrochemistry School of Materials Science and EngineeringThere is such a chemical reaction:if it occurs at constant temperature and constant pressure(298.15K,po),40.0kJ heat is released.If it is accomplished via a reversible cell,it absorbs 4kJ heat.(1)Calculate rSm of the reaction;(2)When the reaction proceeds spontaneously(no electrical work is done),calculate the Senv.and Siso.;(3)What is the maximum“useful”work the system can do?IntroductionQuestionQuestion：(1)Since entropy is state function and its change is independent of the process,so we can use the heat exchange in the reversible process to calculate the entropy change,so:(2)When the reaction proceeds spontaneously,for the environment:(3)The maximum the system can do is:So,the maximum amount of“useful”work the system can do is 44kJ.IntroductionAt constant T and P,for a reaction:At constant T and P,for a reaction:Sign,p3,read Sign,p3,read(3.8,p71)(3.8,p71)P149,exampleP149,exampleEnergy Energy converstionconverstion:combustion,:combustion,heat engine,highest efficiency heat engine,highest efficiency limitationlimitation，Carnot theoremCarnot theoremEnergy conversion:electron flow,electrical Energy conversion:electron flow,electrical work,electrochemical cellwork,electrochemical cell6.1 Electrochemical cellA load is applied on the external circuit:A load is applied on the external circuit:electrical work is done,energy conversion electrical work is done,energy conversion is accomplished,fuel cell is accomplished,fuel cell P151 read together6.2 Nomenclature/Conventionslanode:the electrode at which oxidation takes place;cathode:the electrode at which reduction takes place l In a primary cell:electrons flow from anode to cathode;electric current flows in opposite directionl Anions migrate toward the anode;cations migrate toward the cathode6.5 Half cell reactions&Electromotive forceStandard hydrogen electrode:Standard hydrogen electrode:Standard state:pStandard state:pH2 H2=1atm,=1atm,a aH H+=1=1(concentration,p155)(concentration,p155)P154,table 6.1,standard oxidation potential:standard hydrogen electrode as reference P155,calculation of P155,calculation of standard electromotive force,standard electromotive force,E Eo oThermodynamics of cell reaction-Nernst equationFor the cell reaction,at constant T and P:F F2 2 -What if the reaction happens What if the reaction happens irreversibly?irreversibly?F F2-F F Faraday constant,96500C(the electrical charge on one mole of electrons)Thermodynamics of cell reaction-Nernst equationAt constant T and P,for a reaction:At constant T and P,for a reaction:Similarly:Similarly:As was learnt in Chapter 5:As was learnt in Chapter 5:so:so:-Nernst equation-Nernst equationF F-zF F-zF=F=-zF F-zzFzFPourbaix diagramNernst equation:Diagrams which describe the variation of the stability of a metal and/or its compounds with the pH change of an aqueous solution.Phase diagrams for corrosion scientists!Pourbaix diagramPourbaix diagram for water,hydrogen,and oxygen;PH2 and PO2=1Similarly:P158.Figure 6.1,Pourbaix diagram for water/water/oxygenPourbaixPourbaix diagram for water,diagram for water,hydrogen,andhydrogen,and oxygen;P oxygen;PH2H2 and P and PO2O2=1=1pqrstpqrH2O produced,water stableSimilarly:srtNo H2/H2O producedH2 produced,water decomposedH2O produced,water stableNo O2/H2O producedO2 produced,water decomposedPourbaix diagram for Fe-H2O system(1)Vertical lineThe pH position of the vertical line varies with the concentration of Fe3+,or the concentration of the of Fe3+of the vertical line depends on pH value of the system“(A)”line:l Vertical lines separate species that are in acid/alkali equilibrium,i.e.reactions involving H+/OH-Pourbaix diagram for Fe-H2O systemThe electric potential is related to the concentration of the ions,not pH value.Above and below (B)?l Horizontal lines separate reduction/oxidation equilibrium species not involving H+or OH-Line“(B)”:Pourbaix diagram for Fe-H2O system“(C)”line:Strong oxidants(forms of the metal)occur at the top of the diagram.Strong reductant at the bottom.Pourbaix diagram for Fe-H2O system“(D)”line:l Sloping lines separate reduction/oxidation equilibrium species involving H+/OH-ions -pH diagram for Fe-H2O system(different ion concentration)-pH diagram for Fe-H2O system(ion concentration(10-6mol/l)-pH diagram for metals When the ion concentration in the solution is lower than 10-6mol/l,the metal/alloy can be considered immune to corrosion Corrosion/Protection Predict the corrosion behavior of metals:Fe example“A、C”:immune to corrosion“A”:Fe corroded,H2 evolved“B”：Fe corroded,no H2 producedTo protect the metal,the potential must be sufficiently low-pH diagram for Ce-H2 2O system(A):(B)(C)(D)(E)-pH diagram for Ce-H2 2O systemRare earth oxides are in a mixture.All the hydroxides of RE elements(with valance 3+)are soluble in dilute nitric acid,except Ce,which has the state of Ce(OH)4 in addition to Ce(OH)3,and Ce(OH)4 is not soluble in dilute nitric acid.This attribute of Ce make it possible to separate Ce from other rare earth elements!pH=8 10 oK!Bubble some oxygen and Concentration cells-Nernst equation(the standard state for the reaction is the same for both sides)So:Application:determine the activity of an ion in the solution;Overall:determine the activity of an element in an alloyOxygen pressure determinationMeasure E of the cell,PO2 can be obtained,and the concentration of O2 in a gaseous mixture or the concentration of O in a molten metal bath can be obtained(solution assumption should be made).Temperature dependence of voltageEntropy change and enthalpy change at standard state?
