16A2.Redox equlibria

• Electrochemical cell


  A device for producing an electric current from chemical reactions, constructed from two half-cells
• Reactions in an electrochemical cell

  1. At negative electrode: Zn(s) → Zn2+(aq) + 2e-

  2. At positive electrode: Cu2+(aq) + 2e- → Cu(s)

  3. Overall cell reaction: Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
• Cell diagram


  Shorthand notation to represent half-cells
• Examples of half-cell representations

  • Zn2+(aq) | Zn(s) E* = -0.76 V
  • Mg2+(aq) | Mg(s) E* = -2.37 V
  • Cu2+(aq) | Cu(s) E* = +0.34 V
  • Al3+(aq) | Al(s) E* = -1.66 V
  • Ag+(aq) | Ag(s) E* = +0.80 V
• Ions flow in the salt bridge, not electrons
• Standard electrode potential


  Measure of how easily a species loses electrons, relative to the standard hydrogen electrode
• Zn2+(aq) | Zn(s) half-cell

  Has more negative E* value than Cu2+(aq) | Cu(s) half-cell
• Determining if a reaction is thermodynamically feasible

  1. Compare E* values of relevant half-cell reactions

  2. If E* of one half-cell is more negative, that equilibrium will shift left, releasing electrons

  3. If E* of other half-cell is more positive, that equilibrium will shift right, accepting electrons

  4. Therefore, the overall reaction is thermodynamically feasible
• Zn(s) + Cu2+(aq)


  Reaction is thermodynamically feasible as Zn2+(aq) | Zn(s) E* is more negative than Cu2+(aq) | Cu(s) E*
• Zn(s) + 2H+(aq)


  Reaction is thermodynamically feasible as Zn2+(aq) | Zn(s) E* is more negative than 2H+(aq) | H2(g) E* = 0.00 V
• Cu(s) + 2H+(aq)


  Reaction is not thermodynamically feasible as Cu2+(aq) | Cu(s) E* is more positive than 2H+(aq) | H2(g) E* = 0.00 V
• Reverse reaction Cu2+(aq) + H2(g) → Cu(s) + 2H+(aq) is thermodynamically feasible but kinetically stable
• Changing the sign of an electrode potential when reversing a half-reaction is incorrect
• E[Cu2+(aq) | Cu(s)] is more positive than E[2H+(aq) | H2(g)]


  The reaction is not thermodynamically feasible because copper will not release electrons to hydrogen ions
• The two equilibria tend to move in the opposite directions to those required for reaction
• The reverse reaction: Cu2+(aq) + H2(g) → Cu(s) + 2H+(aq)


  Is thermodynamically feasible
• No reaction takes place when hydrogen is bubbled into an aqueous solution containing copper(II) ions because the activation energy for the reaction is very large
• The reactants are therefore kinetically stable
• Calculating a value for Ecell for a reaction is meaningless because a reaction taking place in a reaction vessel, such as a test tube, does not generate an emf
• Alternative method to predict the thermodynamic feasibility of a reaction using standard electrode (redox) potentials

  1. Reverse one of the half-equations and change the sign of the electrode potential

  2. Calculate a value for Ecell for the proposed reaction
• This alternative method does produce a correct answer regarding the feasibility of the reaction, despite the imperfect science
• The standard electrode potentials suggest that the reaction between manganese(IV) oxide and hydrochloric acid is not feasible
• Increasing the concentration of hydrochloric acid


  Shifts the position of equilibrium 1 to the right and the position of equilibrium 2 to the left, making the reaction feasible
• Disproportionation reaction


  A reaction in which an element in a species is both oxidised and reduced in the same reaction at the same time
• Cu+(aq) + Cu+(aq) → Cu2+(aq) + Cu(s)


  The Cu+ ions disproportionate into Cu2+ ions and Cu atoms
• Relationship between total entropy and Ecell


  ΔStotal ∝ nFEcell
• Relationship between equilibrium constant and Ecell

  ln K ∝ nFEcell/RT