lec5_2011 - Concentration Cells M | M+(aq, L) | M+(aq, R) |...

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Concentration Cells M | M + (aq, L) || M + (aq, R) | M Cell reaction: M + (aq, R) → M + (aq, L) E = since Δ r G θ = 0 (why?) E = Q vF RT vF G r ln - - θ R L b b vF RT ln -
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The cell emf The standard emf of a cell can be calculated by the difference of the standard potentials of the two electrodes. Consider: Ag(s)|Ag + (aq) || Cl - (aq) |AgCl(s)| Ag(s) The overall potential of the cell is: E θ = E θ (AgCl/Ag, Cl - ) – E θ (Ag + /Ag) In a general form: E θ = E θ (right) – E θ (left) Example: Calculate the standard voltage for the cell: Mn +2 |Mn +3 ||Fe +3 |Fe +2 Solution: First, separate the cell into two half reactions Reduction: Fe +3 (aq) + e - → Fe +2 (aq) Reduction: Mn +3 (aq) + e - Mn +2 (s) To compute the cell voltage, simply use the right-hand electrode subtract the left-hand electrode. E θ = E θ right - E θ left = 0.769 - (1.51) = - 0.741 V (???)
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Calculating equilibrium constant from the standard emf Example: Evaluate the solubility constant of silver chloride, AgCl, from cell potential data at 298.15K. Solution: AgCl(s) Ag + (aq) + Cl - (aq) Establish the electrode combination: Right: AgCl + e - Ag(s) + Cl - (aq) E θ = 0.22V Left: Ag + (aq) + e - → Ag(s) E θ = 0.80V The standard emf is : E θ (right) – E θ (Left) = - 0.58V ln K = = 0.58/0.025693 K = 1.6x10 -10 RT vFE θ
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The measurement of standard potentials The potential of standard hydrogen electrode: Pt(s)|H 2 (g)|H + (aq) is defined as 0 at all temperatures. The standard potential of other electrodes can be obtained by constructing an electrochemical cell, in which hydrogen electrode is employed as the left-hand electrode (i.e. anode) Example: the standard potential of the AgCl/Ag couple is the standard emf of the following cell: Pt(s)|H 2 (g)|H + (aq), Cl - (aq)|AgCl(s)|Ag(s) or Pt(s)|H 2 (g)|H + (aq) || Cl - (aq)|AgCl(s)|Ag(s) with the cell reaction is: ½ H 2 (g) + AgCl(s) → H + (aq) + Cl - (aq) + Ag(s)
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Δ r G = Δ r G θ + RTln According to Nernst equation:
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This note was uploaded on 02/14/2011 for the course SCIENCE 321 taught by Professor Aaa during the Spring '11 term at Windsor.

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lec5_2011 - Concentration Cells M | M+(aq, L) | M+(aq, R) |...

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