Week 2 - Electrochemistry.pdf;JSESSIONID=KyvgrKcbSwGKFy1wzrddG2pCTmJyX5jNPfL9LqYnJ3B1zwvPL8mZ!-27772

Week 2- - 1 INDC 2003 – Instrumental Analysis Electroanalytical Chemistry Direct Potentiometry and Ion Selective Electrodes Maria Skyllas-Kazacos

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Unformatted text preview: 1 INDC 2003 – Instrumental Analysis Electroanalytical Chemistry: Direct Potentiometry and Ion Selective Electrodes Maria Skyllas-Kazacos School of Chemical Sciences and Engineering Electrochemical Cells • Oxidation (loss of electrons) at the anode • Reduction (gain of electrons) at the cathode • Galvanic cells involve spontaneous reactions (negative ∆ G reaction ) • Electrolytic cell require external source of energy (positive ∆ G reaction ) • Battery charging is electrolytic process - requires external source of energy • Battery discharging is a galvanic process - releases energy 2 Cell Voltage • ∆ G o = - nFE o cell • E o cell = E o cathode- E o anode (where E o is standard reduction potential) • ie E o cell > 0 for galvanic cell E o cell < 0 for electrolytic cell Electrode Potentials • Electrochemical cells made up of 2 half-cell reactions • Each half-cell reaction has a particular chemical potential or driving force • Cannot measure half-cell potentials, only differences- therefore measure all half-cell potentials relative to arbitrary reference ---> standard hydrogen electrode (SHE)- ie 2¡H + + 2 e -------> H 2(g) where E SHE = 0 V at all T’s • I.U.P.A.C. convention:- all half-reactions written as reductions (eg Al 3+ + 3e -> Al)- Resultant half-cell potentials vs SHE will have: (a) positive sign if acts as oxidiser relative to SHE - ie reaction goes as written vs SHE (b) negative sign if acts as reducer relative to SHE 3 Electromotive force series 3.06 F 2 (g) + 2H + + 2 e − ⇌ 2HF(aq) 1.223 O 2 (g) + 4H + (aq) + 4 e − ⇌ 2H 2 O 0.69 Fe(CN) 6 3 − (aq) + e − ⇌ Fe(CN) 6 4 − (aq) 0.337 Cu 2+ (aq) + 2 e − ⇌ Cu(s) 0.222 AgCl(s) + e − ⇌ Ag(s) + Cl − (aq) 0.246 Hg 2 Cl 2 (s) + 2 e − ⇌ 2Hg(s) + 2Cl − (sat aq KCl) 2H + (aq) + 2 e − ⇌ H 2 (g) − 0.763 Zn 2+ (aq) + 2 e − ⇌ Zn(s) − 3.045 Li + (aq) + e − ⇌ Li(s) E °/ V Electrode reaction Reduction Potentials • Standard reduction potential, E o refers to conditions:- one molar concentrations for all ions- one atmosphere pressure for all gases • Concentration effects adjusted with Nernst Equation: eg a Ox + n e <===> b Red RT [ Red] b E = E o- nF ln [ Ox ] a where E o = standard reduction potential n = no of equivalents (or moles of electrons) R = Gas constant = 8.314 V coulombs K-1 mole-1 T = o K F = Faraday’s constant = 96,493 coulombs /equiv 4 Effect of Passage of Current • Potential calculated from standard reduction potentials and Nernst equation also known as reversible potential . This is thermodynamic potential for reaction and is potential needed to keep electrode or cell reaction at equilibrium - ie no net oxidation or reduction occurring in cell....
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This note was uploaded on 04/20/2010 for the course CEIC 2003 taught by Professor Alice during the Three '10 term at University of New South Wales.

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Week 2- - 1 INDC 2003 – Instrumental Analysis Electroanalytical Chemistry Direct Potentiometry and Ion Selective Electrodes Maria Skyllas-Kazacos

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