Concentration CellsConcentration Cells  Alternate voltaic cell  Both electrodes contain the same chemical compound/metal  Half-cell solutions are the SAME but DIFFERENT concentrations.  cell potential based on difference in concentrations  Concentration difference between the solutions of the half-cells generates potential difference for electrochemical cell ----textbook definition.

Concentration cells (cont.)Concentration cells (cont.)  Higher solution concentration—diluted as ions removed from solution to electrode.  Lower solution concentration—becomes more concentrated as ions removed from electrode to solution.  Cell potential (Ecell) POSITIVE if [anode] < [cathode]  [anode] increases as reaction progresses  [cathode] decreases as reaction progresses

Free Energy ( Δ G)Free Energy ( Δ G)  Maximum amount of energy available to do work  Work can be in the form of electrical energy  NEGATIVE Δ G = spontaneous process

Free Energy and Voltaic Cells  Work from electrochemical cells is comprised of 3 components  1) Cell Voltage  2) # of moles of electrons transferred  3) electric charge per mole of electrons  w elec = nFEcell (electrical work from eletrochemical cell)  - Δ G = w elec = nFEcell  Δ G = -nFEcell  Δ G° = nFE°cell (standard state conditions)

Guidelines for Spontaneity of Redox Reactions 1)Forward Reaction SPONTANEOUS  Positive cell potential  Negative free energy 2)Forward Reaction NONSPONTANEOUS/Reverse reaction spontaneous  Negative cell potential  Positive free energy 3)Ecell = 0  Equilibrium established  Both cell potential and free energy = 0

Example 1:Example 1:  Will copper metal displace silver ions from the solution? Does this reaction occur spontaneously left to right?  Cu (s) + 2Ag +  Cu Ag (s)

Electrochemistry and Equilibrium (K)  E° cell = ( V/n) lnK  Δ G° = -RTlnK  Electochemistry is a good source for free energy and equilibrium constant values.

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