Presentation on theme: "Basic Concepts of Electrochemical Cells"— Presentation transcript:
1Basic Concepts of Electrochemical Cells Electrifying!AnodeCathode
2CHEMICAL CHANGE ---> ELECTRIC CURRENT With time, Cu plates out onto Zn metal strip, and Zn strip “disappears.”Zn is oxidized and is the reducing agent Zn(s) ---> Zn2+(aq) + 2e-Cu2+ is reduced and is the oxidizing agent Cu2+(aq) + 2e- ---> Cu(s)
3CHEMICAL CHANGE ---> ELECTRIC CURRENT Electrons are transferred from Zn to Cu2+, but there is no useful electric current.Oxidation: Zn(s) ---> Zn2+(aq) + 2e-Reduction: Cu2+(aq) + 2e- ---> Cu(s)Cu2+(aq) + Zn(s) ---> Zn2+(aq) + Cu(s)
4CHEMICAL CHANGE ---> ELECTRIC CURRENT To obtain a useful current, we separate the oxidizing and reducing agents so that electron transfer occurs thru an external wire.This is accomplished in a GALVANIC or VOLTAIC cell.A group of such cells is called a battery.
5Zn --> Zn2+ + 2e-Cu2+ + 2e- --> CuOxidationAnodeNegativeReductionCathodePositive<--AnionsCations-->•Electrons travel thru external wire.Salt bridge allows anions and cations to move between electrode compartments.
6The Cu|Cu2+ and Ag|Ag+ Cell Electrons move from anode to cathode in the wire.Anions & cations move thru the salt bridge.
7Anode, site of oxidation, negativeCathode, site of reduction, positive
8CELL POTENTIAL, E1.10 V1.0 MZn and Zn2+,anodeCu and Cu2+,cathodeElectrons are “driven” from anode to cathode by an electromotive force or emf.For Zn/Cu cell, this is indicated by a voltage of 1.10 V at 25 ˚C and when [Zn2+] and [Cu2+] = 1.0 M.
9CELL POTENTIAL, EFor Zn/Cu cell, potential is V at 25 ˚C and when [Zn2+] and [Cu2+] = 1.0 M.This is the STANDARD CELL POTENTIAL, Eo—a quantitative measure of the tendency of reactants to proceed to products when all are in their standard states at 25 ˚C.This means pure solids or in solution at a concentration of 1M!!!!
10Calculating Cell Voltage Balanced half-reactions can be added together to get overall, balanced equation.Zn(s) ---> Zn2+(aq) + 2e-Cu2+(aq) + 2e- ---> Cu(s)Cu2+(aq) + Zn(s) ---> Zn2+(aq) + Cu(s)If we know Eo for each half-reaction, we could get Eo for net reaction.Lets revisit my haiku!
11Oxidation Haiku!Lost an electronBut now feeling positiveOxidized is cool!What is that? You want a reduction Haiku?
12Reduction Haiku!!! Gained some electrons Gave me a negative mood! Now I can say Ger!Thank you… Enjoy the buffet… Don’t eat the chemicals or furniture kids!
132 H+(aq, 1 M) + 2e- <----> H2(g, 1 atm) CELL POTENTIALS, EoCan’t measure 1/2 reaction Eo directly. Therefore, measure it relative to a STANDARD HYDROGEN CELL, SHE.2 H+(aq, 1 M) e- <----> H2(g, 1 atm)Eo = 0.0 V
14Supplier of electrons Acceptor of electrons Zn/Zn2+ half-cell hooked to a SHE.Eo for the cell = VNegative electrodePositive electrodeSupplier of electronsAcceptor of electronsZn --> Zn2+ + 2e-OxidationAnode2 H+ + 2e- --> H2ReductionCathode
24Standard Redox Potentials, Eo Cu2++ 2e Cu+0.34+2 H+ 2e H20.00Zn+ 2e Zn-0.76Any substance on the right will reduce any substance higher than it on the left.Northwest-southeast rule: product-favored reactions occur between reducing agent at southeast corner (anode) and oxidizing agent at northwest corner (cathode).
25Standard Redox Potentials, Eo Any substance on the right will reduce any substance higher than it on the left.Zn can reduce H+ and Cu2+.H2 can reduce Cu2+ but not Zn2+Cu cannot reduce H+ or Zn2+.
26Using Standard Potentials, Eo Table 20.1 In which direction do the following reactions go?Cu(s) Ag+(aq) ---> Cu2+(aq) Ag(s)2 Fe2+(aq) + Sn2+(aq) ---> 2 Fe3+(aq) + Sn(s)What is Eonet for the overall reaction?
27Standard Redox Potentials, Eo E˚net = “distance” from “top” half-reaction (cathode) to “bottom” half-reaction (anode)E˚net = E˚cathode - E˚anodeEonet for Cu/Ag+ reaction = V
28All ingredients are present. Which way does reaction proceed? Eo for a Voltaic CellCd --> Cd2+ + 2e-orCd2+ + 2e- --> CdFe --> Fe2+ + 2e-orFe2+ + 2e- --> FeAll ingredients are present. Which way does reaction proceed?
29Eo for a Voltaic Cell From the table, you see • Fe is a better reducing agent than Cd• Cd2+ is a better oxidizing agent than Fe2+Overall reactionFe + Cd > Cd + Fe2+Eo = E˚cathode - E˚anode= (-0.40 V) - (-0.44 V)= V
30More About Calculating Cell Voltage Assume I- ion can reduce water.2 H2O e- ---> H OH Cathode2 I > I e Anode2 I H2O --> I OH H2Assuming reaction occurs as written,E˚net = E˚cathode - E˚anode= ( V) - ( V) = VMinus E˚ means rxn. occurs in opposite direction