Standard Electrode Potentials

Standard Electrode Potentials

1. Under standard conditions (pressure of hydrogen gas 1 atm, acid concentration of 1 mol L-1), the potential for the reduction reaction below is assigned a value of zero vaults

   • 2H+(aq) + 2e- <--> H2(g) Eo = 0.00V

2. For any redox couple, the standard electrode (reduction) potential is the voltage obtained under standard conditions when that half-cell is connected to the standard hydrogen electrode

   • Experimentally, the more positive terminal is always where reduction is occurring in a spontaneous reaction

3. Example

   • The cell potential for the electrochemical cell below is +0.76V: Zn(s) | Zn2+(aq) || H+(aq) | H2(g) | Pt(s)
     • The Zn electrode is the negative and the standard reduction potential for the Zn2+/Zn half cell is -0.76V
       • If the Zn2+/Zn electrode is replaced with a Cu2+/Cu electrode, the Cu2+ is reduced and the H2 gas is oxidised
         • The Cu electrode is positive and the Cu2+ | Cu couple will have a positive reduction potential
           • The voltmeter has a reading of +0.34V: Pt(s) | H+(aq) | H2(g) || Cu2+(aq) | Cu(s)
   • When comparing standard reduction potentials, the species on the left in the couple with the most positive reduction potential will be the strongest oxidising agent or oxidant
     • As the electrode potential decreases, the strength of oxidant species decreases, while strength of reductant species increases

4. When comparing standard reduction potentials, the species on the left in the couple with the most positive reduction potential will be the strongest oxidising agent or oxidant

   • As the electrode potential decreases, the strength of the oxidant species decreases, while strength of reductant species increases

5. Using reduction potentials to determine Eo cell

   • In any electrochemical cell, Eo cell = Eo (reduction half-cell) - Eo (ocidation half-cell) or Eo cell = Eo (cathode) - Eo (anode)
     • The couple with the more positive reduction potential will be the reduction half-cell (cathode)
   • A more reactive metal will reduce the cation of a less reactive metal, as the Eo cell value for the reaction will be positive (the more reactive metal is the reductant in the redox couple with the lower Eo value
   • Despite a positive standard cell potential, a reaction may be very slow due to a high activation energy
     • A reaction may not proceed if the surface of the metal is coated with an oxide