Potential, Free Energy, Equilibrium

Introduction

  • If the calculated standard cell potential (E°cell) of a redox reaction is positive, then the reaction is spontaneous at standard conditions.
  • This is why E°cell for galvanic cells is always positive.
  • If the standard potential (E°cell) of a redox reaction is negative, then the reaction is nonspontaneous at standard conditions. 

Relationship Between E°, ΔG° and K

  • E°, ΔG°, and K are all measures of reaction spontaneity. 

  • A redox reaction is spontaneous if,

    • E°cell is positive
    • ΔG° is negative
    • K is greater than 1

  • E° and ΔG°

    • ΔG° = -nFE°
    • ° indicates standard conditions (gases at 1 atm (or 1 bar); aqueous species at1 M)
    • ΔG° = the standard free energy change 
    • E°cell = the standard cell potential
    • n = the number of moles of electrons transferred in the balanced redox reaction. 
    • F = the Faraday constant; this is the charge of a mole of electrons; 9.648 × 104 J/mol . V
    • Notice that ΔG° and E° have opposite signs.

Effect of Concentration on Potential

  • Since reaction conditions influence ΔG, potential (E) must also be influenced by the conditions. 
  • The equation for  “E”, the cell potential for ANY value of Q, is  E=E°- 0.0257/n Ln Q.
  • Note that if Q < 1, then E is GREATER than E° 
    • This makes sense, since we know that a decrease in Q favors the forward reaction.
  • Voltage will increase and the reaction will become more spontaneous if
    • The concentration of a reactant is increased
    • The concentration of a product is decreased     (  smaller Q)
  • Voltage will decrease and the reaction will become less spontaneous if
    • The concentration of reactant is decreased
    • The concentration of product is increased   ( larger Q)