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Lecture 10


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McMaster University
David Brock

CHEM 1AO3 LECTURE 10 – ENERGYAND ELECTROCHEMISTRY th October 30 2012 Electrochemical cells: Used to harness the energy transfers in spontaneous reactions. Example: Copper-Silver cells • Two interconnected half-cells. Half reactions are separated yet electrically connected by a salt bridge. • Solid metals make up the electrodes. (Cu = anode, oxidation) (Ag = cathode, reduction) • Movement of electrons from anode to cathode generates voltage. • Movement of cations and anions through the salt bride maintains electroneutrality in solution. Oxidation (Anode): Surface of electrode erodes as cations are produced. Electrons travel to the cathode. Reduction (Cathode): Cathode gains mass as cations gain electrons and form solid metals. Overall cell reaction: Cu (s) Ag (aq) Cu 2(aq) 2 Ag (s) Galvanic (voltaic) Cells: Results from spontaneous chemical reactions. Electrolytic cell: Uses electricity to accomplish non-spontaneous chemical change. Cell Diagrams: Standard Electrode Potentials • Absolute half-cell potentials cannot be measured • All potentials are measured relative to the standard hydrogen electrode (SHE), assigned to a potential of 0V. E = exactly 0V o 2 H+ (a=1) + 2e-  H (g21 bar) E = 0V. Occurs on Pt |H (s) 2(g, 1atm)+ (aq, 1M) o a =1 becomes [H+] = 1M; 1 bar = 1atm  STANDARD CONDITIONS • Standard electrode potentials (E ) report tendency for reduction to occur (E ored o Positive (+) values are most easily reduced  oxidizing agents o Negative values are most easily oxidized  reducing agents Standard Cell Potentials 0 E cellvoltage of a cell formed from 2 standard electrodes. • Can be expressed as E (cathode(right)) – anode (left)) o o o Alternatively, for a given half-cell we can say that E ox = -E red Note: standard potential is an intensive property – does not depend on the amount of substance reacting. DO NOT multiply Ered or Eox by a factor even if we multiply a half- reaction when balancing. • Magnitude of cell potential indicates driving force for cell: Higher E cell = greater tendency to reach completion. Spontaneity: E°cell and ∆G° - Gibb’s Free Energy Electromotive force of cell potential (E )Jcells = volt x coulomb Welec = zFE , where work = -∆G cell z = # e- transferred, F = 965485 C/mol*e- (Faraday Constant) R = 8.314 J/K (gas constant) ∆G = -zFE cell Non-standard conditions ∆G = -zFE ocell Standard conditions: reactants, products in standard states. o ∆G = -RT ln(K) K>1 –
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