AP Chemistry Unit 9.8 Cell Potential and Free Energy

Cell Potential and Free Energy

  • Cell potential or electromotive force: the “pull” or driving force on the electrons
    • Substance being reduced “pulls” the electrons thru the wire
    • Substance being oxidized “pushes” the electrons
  • Ecell is measured in Volts (V) using a voltmeter: measures the voltage (electricity)→ how much electricity is flowing thru the wire
  • The total cell potential is the sum of the potential at each electrode
  • Ex:
    • “Ecell = Oxidation – Reduction”
    • One of the reactions must be reversed, so the sign must be changed
  • How to know which half-reaction should be reversed: For a galvanic (voltaic) cell, the overall cell potential will be POSITIVE → have to flip the one with more negative E° (the oxidation-half reaction)
    • Key word = voltaic → E° must be (+)

Standard Hydrogen Electrode

  • E° = 0
  • When changing the amount of electrons by multiplying an integer, DO NOT change/multiply E°
  • Substance with most favorable reduction potential (stronger reducing power/agent) = more negative E° red
  • Substance with the most favorable oxidation potential (stronger oxidizing power/agent) = more positive E° red

Description of a Galvanic Cell

3 things for a complete description:

  1. Cell potential (+) and balanced cell reaction
  2. Direction of electron flow
  3. Designation of anode and cathode
    • Whichever half-reaction that flipped/reversed = anode

Favorability of a Galvanic Cell

  • The reaction always run spontaneously in the direction that produces a positive (total) cell potential
  • If E° < 0, then ΔG° > 0 → nonspontaneous
  • If E° > 0, then ΔG° < 0 → spontaneous

Free Energy and Cell Potential

  • ΔG° = –nFE° (on RFS)
    • N = number of moles = coefficient of the number of electrons that you canceled out
    • F = Faraday’s constant = 96, 485 coulombs per mole of electrons
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