CHEM 1AO3 LECTURE 10 – ENERGYAND ELECTROCHEMISTRY
October 30 2012
Used to harness the energy transfers in spontaneous reactions. Example: Copper-Silver
• 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
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.
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
E cellvoltage of a cell formed from 2 standard electrodes.
• Can be expressed as E (cathode(right)) – anode (left))
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
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.
∆G = -RT ln(K) K>1 –