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Chapter 19 Electrochemistry Week 2 2009

# Chapter 19 Electrochemistry Week 2 2009 - Section 19.5 Cell...

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Section 19.5. Cell Potentials Under Non-Standard Conditions Concs or pressures are rarely going to be standard ones (1 M or 1 atm) - even if they start that way, they will change with time during the reaction. ** Non-standard conditions mean cell potential is E cell not E° cell ** Since G = G° + RT ln Q and G° = -nFE° cell , we can get E cell = E° cell - nF RT ln Q NERNST EQUATION E cell = E° cell - n V 0592 . 0 log Q (at 25°C) If Q < 1 log Q < 0 E cell > E° cell Q > 1 log Q > 0 E cell < E° cell Q = 1 log Q = 0 E cell = E° cell Example 1: Voltaic cell: Zn (s)/ Zn 2+ anode and H + /H 2 (gas) cathode. Calc E cell if [Zn 2+ ] = 0.010 M, [H + ] = 2.5 M, p(H 2 ) = 0.30 atm at 25 °C Answer : Calculate E° cell and Q, and use E cell = E° cell - n V 0592 . 0 log Q Zn (s) + 2 H + (aq) Zn 2+ (aq) + H 2 (g) ( n = 2) cell = 0.00 V - (-0.76 V) = 0.76 V Q = 2 2 2 ] H [ ) H ( p ] Zn [ + + = 2 ) 5 . 2 ( ) 30 . 0 )( 010 . 0 ( = 4.8 x 10 -4 E cell = 0.76 V - 2 V 0592 . 0 log Q = 0.86 V E cell = 0.86 V (compare with E° cell = 0.76 V) 11

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Example 2: Voltaic cell: Fe (s) + Cu 2+ (aq) Fe 2+ (aq) + Cu (s) Calculate E° cell . Then, if [Cu 2+ ] = 0.30 M, calculate [Fe 2+ ] needed to give E cell that is 0.25 V bigger than E° cell . Answer : cell = E° cat - E° ano = 0.34 V – (- 0.44 V) = 0.78 V Need [Fe 2+ ] to give E cell = E° cell + 0.25 V = 1.03 V E cell = E° cell - n 0592 . 0 log Q = E° cell - 2 0592 . 0 log ] Cu [ ] Fe [ 2 2 + + 1.03 V = 0.78 V - 2 0592 . 0 log ] Cu [ ] Fe [ 2 2 + + 2 0592 . 0 log ] Cu [ ] Fe [ 2 2 + + = -0.25 V log ] Cu [ ] Fe [ = -8.446 ] Cu [ ] Fe [ 2 2 + + = 3.6 x 10 -9 [Fe 2+ ] = 1.1 x 10 -9 M E cell can be both Smaller and Bigger than E° cell Consider Zn (s) + Cu 2+ (aq) Zn 2+ (aq) + Cu (s) Q = ] Cu [ ] Zn [ 2 2 + + [ Remember: do not include M (s) ] If Q < 1 then E cell > E° cell Q = 1 then E cell = E° cell (standard conditions) Q > 1, then E cell < E° cell Note : 1) At equilibrium, Q = K and E cell = 0.00 V. We say the voltaic cell has fully discharged, or the battery is dead or flat. 2) If [Cu 2+ ] = [Zn 2+ ], then Q = 1 and E cell = E° cell even though concentrations are not necessarily standard ones (1 M). Thus, they could both be 0.2 M, 1.6 M, 7 M, etc, etc, and E cell = E° cell 12
because Q = 1. Concentration Cells

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Chapter 19 Electrochemistry Week 2 2009 - Section 19.5 Cell...

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