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# Ch21_4 - CELL POTENTIAL E CELL POTENTIAL E 1 2 CELL...

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Unformatted text preview: CELL POTENTIAL, E CELL POTENTIAL, E 1 2 CELL POTENTIAL, E CELL POTENTIAL, E Zn and Zn2+, anode • For Zn/Cu cell, voltage is 1.10 V at 25 ˚C and when [Zn2+] and [Cu2+] = 1.0 M. • This is the STANDARD CELL This STANDARD Cu and Cu2+, cathode • Electrons are “driven” from anode to cathode by an electromotive force or • —a quantitative measure of the tendency of reactants to proceed to products when all are in their standard states at 25 ˚C. • For Zn/Cu cell, this is indicated by a voltage of 1.10 V at 25 ˚C and when [Zn 2+] and [Cu2+] = 1.0 M. 4 CELL POTENTIALS, Eo CELL POTENTIALS, Eo Can’t measure 1/2 reaction E o directly. Therefore, measure it relative to a STANDARD HYDROGEN CELL, SHE. 2 H+((aq, 1 M) + 2e- <----> H 22(g, 1 atm ) 2 H+ aq, 1 M) + 2e- <----> H (g, 1 atm) Eo = 0.0 V Calculating Cell Voltage Calculating Cell Voltage • Balanced half-reactions can be added together to get overall, balanced equation. Zn(s) ---> Zn 2+((aq) + 2eZn(s) ---> Zn 2+ aq) + 2eCu2+((aq) + 2e- ---> Cu(s) Cu2+ aq) + 2e- ---> Cu(s) --------------------------------------------------------------------------------------Cu2+((aq) + Zn(s) ---> Zn 2+((aq) + Cu(s) Cu2+ aq) + Zn Zn(s) ---> Zn 2+ aq) + Cu(s) POTENTIAL, Eo emf. emf 3 If we know Eo for each half-reaction, we could get Eo for net reaction. 5 Zn/Zn2+ half-cell hooked to a SHE. Zn/Zn2+ half-cell hooked to a SHE. Eo ffor the cell = +0.76 V Eo or the cell = +0.76 V Negative electrode Zn Volts - + Positive electrode Salt Bridge H2 Supplier of electrons H+ Zn2+ Zn2+ Zn --> 2+ + 2eZn Zn + 2eOxidation Oxidation OXIDATION Anode ANODE H+ Acceptor of electrons 2 + 2e- --> H2 2 H+ + 2eH Reduction 2 REDUCTION Cathode CATHODE Page 1 6 Volts Zn - + Salt Bridge H2 Zn2+ Zn Zn2+ + 2eOXIDATION ANODE H+ 2 H+ + 2eH2 REDUCTION CATHODE Overall reaction is reduction of H + by Zn metal. Zn(s) + 2 H+ (aq) --> Zn2+ + H2(g) Eo = +0.76 V Therefore, Eo for Zn ---> Zn2+ (aq) + 2e- is +0.76 V Zn +0.76 Zn is a (better) (poorer) reducing agent than H 2. (better) 7 Cu/Cu2+ and H2/H+ Cell Cu/Cu2+ and H2/H+ Cell Cu/Cu2+ and H2/H+ Cell Cu/Cu2+ and H2/H+ Cell 8 Zn/Cu Electrochemical Cell Zn/Cu Electrochemical Cell wire Volts Eo = +0.34 V Positive Cu Acceptor of electrons - Negative + 2e- --> Cu Cu2+ + 2eCu Reduction REDUCTION Cathode CATHODE Supplier of electrons H+ Overall reaction is reduction of by H2 gas. Cu2+ (aq) + H2(g) ---> Cu(s) + 2 H +(aq) Measured Eo = +0.34 V Therefore, Eo for Cu2+ + 2e- ---> Cu is H2 --> 2 + 2eH2 Oxidation + 2e2 H+ OXIDATION Anode ANODE +0.34 V 10 ele c t rons Zn2+ ions salt bridge TABLE OF STANDARD TABLE OF STANDARD REDUCTION POTENTIALS REDUCTION POTENTIALS oxidizing ability of ion wire Zn H2 2 H+ + 2eOXIDATION ANODE Cu 2+ H+ Uses of Eo Values Eo • Organize halfreactions by relative ability to act as oxidizing agents • Table 21.1 • Use this to predict cell potentials and direction of redox direction redox reactions. Anode, negative, source of electrons H+ Cu2+ + 2eCu REDUCTION CATHODE H2 Cu2+ ele c t rons Salt Bridge Cu2+ Salt Bridge Cu2+ + Zn H2 Volts + Cu Cu Cu2+ ions Cu2+ + 2e2 H+ + 2eZn2+ + 2e- Eo (V) salt bridge Zn2+ ions H2 Zn +0.34 0.00 -0.76 reducing ability of element Page 2 + Cu Cu2+ ions Cathode, positive, sink for electrons Zn(s) ---> Zn 2+(aq) + 2eEo = +0.76 V Cu2+(aq) + 2e- ---> Cu(s) Eo = +0.34 V --------------------------------------------------------------Cu2+(aq) + Zn(s) ---> Zn 2+(aq) + Cu(s) Eo (calc’d) = +1.10 V 11 12 Standard Redox Potentials, Eoo Standard Redox Potentials, E oxidizing ability of ion Eo (V) Cu2+ + 2e- Cu 9 Cu +0.34 2 H+ + 2e- H2 0.00 Zn -0.76 Zn2+ + 2e- reducing ability of element Any substance on the right will reduce any substance higher than it on the left. • Zn can reduce H+ and Cu2+. • H2 can reduce Cu 2+ but not Zn2+ • Cu cannot reduce H + or Zn2+. Using Standard Potentials, Eo Using 13 Table 21.1 Eo for a Voltaic Cell Eo for a Voltaic Cell Eo for a Voltaic Cell Eo for a Voltaic Cell Volts • Which is the best oxidizing agent: O2, H2O2, or Cl2? _________________ Cd Fe Salt Bridge Volts • Which is the best reducing agent: Cd Hg, Al, or Sn? ____________________ Cd2+ • In which direction does the following Cd --> Cd2+ + 2eor Cd2+ + 2e- --> Cd reaction go? Cu(s) + 2 Ag +(aq) ---> Cu 2+(aq) + 2 Ag(s) E˚ and G? Fe --> Fe2+ + 2eor Fe2+ + 2e- --> Fe YES! • Balanced half-reactions can be added together to get overall, balanced equation. Fe Salt Bridge Fe2+ 16 Calculating Cell Voltage Calculating Cell Voltage 14 Cd2+ Fe2+ From the table, you see • Fe is a better reducing agent than Cd • Cd2+ is a better oxidizing agent than Fe2+ Overall reaction Fe + Cd2+ ---> Cd + Fe2+ ---> Eo = +0.04 V Michael Faraday Michael Faraday 1791-1867 1791-1867 Originated the terms anode, cathode, anion, cation, electrode. Discoverer of • electrolysis • magnetic props. of matter • electromagnetic induction • benzene and other organic chemicals Was a popular lecturer. 2 II- ------> II2 + 2e> 2 + 2e2 2 H22O + 2e- ------> 2 OH-- + H22 > 2 OH + H 2 H O + 2e-------------------------------------------------------------------------------------------------- + 2 H O - -> I + 2 OH-- + H 2 II + 2 H2O - -> I2 + 2 OH + H2 2 2 2 2 If we know Eo for each half-reaction, we could get Eo for net reaction. Page 3 15 18 19 Eo and G o Eo and G o Eo is related to Go, the free energy change for the reaction. G o = - n F Eo where F = Faraday constant where = 9.6485 x 10 4 J/V•mol 9.6485 and n is the number of moles of Michael Faraday electrons transferred 1791-1867 Eo and G o Eo and G o Go = - n F E o For a product-favored reaction For product-favored Reactants ----> Products Reactants Go < 0 and so Eo > 0 Eo is positive For a reactant-favored reaction For reactant-favored Reactants <---- Products Reactants Go > 0 and so Eo < 0 Eo is negative Page 4 20 ...
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